// SPDX-License-Identifier: GPL-2.0+
/*
 * fat.c
 *
 * R/O (V)FAT 12/16/32 filesystem implementation by Marcus Sundberg
 *
 * 2002-07-28 - rjones@nexus-tech.net - ported to ppcboot v1.1.6
 * 2003-03-10 - kharris@nexus-tech.net - ported to uboot
 */

#include <common.h>
#include <blk.h>
#include <config.h>
#include <exports.h>
#include <fat.h>
#include <fs.h>
#include <log.h>
#include <asm/byteorder.h>
#include <part.h>
#include <malloc.h>
#include <memalign.h>
#include <asm/cache.h>
#include <linux/compiler.h>
#include <linux/ctype.h>

/*
 * Convert a string to lowercase.  Converts at most 'len' characters,
 * 'len' may be larger than the length of 'str' if 'str' is NULL
 * terminated.
 */
static void downcase(char *str, size_t len)
{
        while (*str != '\0' && len--) {
                *str = tolower(*str);
                str++;
        }
}

static struct blk_desc *cur_dev;
static struct disk_partition cur_part_info;

#define DOS_BOOT_MAGIC_OFFSET   0x1fe
#define DOS_FS_TYPE_OFFSET      0x36
#define DOS_FS32_TYPE_OFFSET    0x52

static int disk_read(__u32 block, __u32 nr_blocks, void *buf)
{
        ulong ret;

        if (!cur_dev)
                return -1;

        ret = blk_dread(cur_dev, cur_part_info.start + block, nr_blocks, buf);

        if (ret != nr_blocks)
                return -1;

        return ret;
}

int fat_set_blk_dev(struct blk_desc *dev_desc, struct disk_partition *info)
{
        ALLOC_CACHE_ALIGN_BUFFER(unsigned char, buffer, dev_desc->blksz);

        cur_dev = dev_desc;
        cur_part_info = *info;

        /* Make sure it has a valid FAT header */
        if (disk_read(0, 1, buffer) != 1) {
                cur_dev = NULL;
                return -1;
        }

        /* Check if it's actually a DOS volume */
        if (memcmp(buffer + DOS_BOOT_MAGIC_OFFSET, "\x55\xAA", 2)) {
                cur_dev = NULL;
                return -1;
        }

        /* Check for FAT12/FAT16/FAT32 filesystem */
        if (!memcmp(buffer + DOS_FS_TYPE_OFFSET, "FAT", 3))
                return 0;
        if (!memcmp(buffer + DOS_FS32_TYPE_OFFSET, "FAT32", 5))
                return 0;

        cur_dev = NULL;
        return -1;
}

int fat_register_device(struct blk_desc *dev_desc, int part_no)
{
        struct disk_partition info;

        /* First close any currently found FAT filesystem */
        cur_dev = NULL;

        /* Read the partition table, if present */
        if (part_get_info(dev_desc, part_no, &info)) {
                if (part_no != 0) {
                        printf("** Partition %d not valid on device %d **\n",
                                        part_no, dev_desc->devnum);
                        return -1;
                }

                info.start = 0;
                info.size = dev_desc->lba;
                info.blksz = dev_desc->blksz;
                info.name[0] = 0;
                info.type[0] = 0;
                info.bootable = 0;
#if CONFIG_IS_ENABLED(PARTITION_UUIDS)
                info.uuid[0] = 0;
#endif
        }

        return fat_set_blk_dev(dev_desc, &info);
}

/*
 * Extract zero terminated short name from a directory entry.
 */
static void get_name(dir_entry *dirent, char *s_name)
{
        char *ptr;

        memcpy(s_name, dirent->nameext.name, 8);
        s_name[8] = '\0';
        ptr = s_name;
        while (*ptr && *ptr != ' ')
                ptr++;
        if (dirent->lcase & CASE_LOWER_BASE)
                downcase(s_name, (unsigned)(ptr - s_name));
        if (dirent->nameext.ext[0] && dirent->nameext.ext[0] != ' ') {
                *ptr++ = '.';
                memcpy(ptr, dirent->nameext.ext, 3);
                if (dirent->lcase & CASE_LOWER_EXT)
                        downcase(ptr, 3);
                ptr[3] = '\0';
                while (*ptr && *ptr != ' ')
                        ptr++;
        }
        *ptr = '\0';
        if (*s_name == DELETED_FLAG)
                *s_name = '\0';
        else if (*s_name == aRING)
                *s_name = DELETED_FLAG;
}

static int flush_dirty_fat_buffer(fsdata *mydata);

#if !CONFIG_IS_ENABLED(FAT_WRITE)
/* Stub for read only operation */
int flush_dirty_fat_buffer(fsdata *mydata)
{
        (void)(mydata);
        return 0;
}
#endif

/*
 * Get the entry at index 'entry' in a FAT (12/16/32) table.
 * On failure 0x00 is returned.
 */
static __u32 get_fatent(fsdata *mydata, __u32 entry)
{
        __u32 bufnum;
        __u32 offset, off8;
        __u32 ret = 0x00;

        if (CHECK_CLUST(entry, mydata->fatsize)) {
                printf("Error: Invalid FAT entry: 0x%08x\n", entry);
                return ret;
        }

        switch (mydata->fatsize) {
        case 32:
                bufnum = entry / FAT32BUFSIZE;
                offset = entry - bufnum * FAT32BUFSIZE;
                break;
        case 16:
                bufnum = entry / FAT16BUFSIZE;
                offset = entry - bufnum * FAT16BUFSIZE;
                break;
        case 12:
                bufnum = entry / FAT12BUFSIZE;
                offset = entry - bufnum * FAT12BUFSIZE;
                break;

        default:
                /* Unsupported FAT size */
                return ret;
        }

        debug("FAT%d: entry: 0x%08x = %d, offset: 0x%04x = %d\n",
               mydata->fatsize, entry, entry, offset, offset);

        /* Read a new block of FAT entries into the cache. */
        if (bufnum != mydata->fatbufnum) {
                __u32 getsize = FATBUFBLOCKS;
                __u8 *bufptr = mydata->fatbuf;
                __u32 fatlength = mydata->fatlength;
                __u32 startblock = bufnum * FATBUFBLOCKS;

                /* Cap length if fatlength is not a multiple of FATBUFBLOCKS */
                if (startblock + getsize > fatlength)
                        getsize = fatlength - startblock;

                startblock += mydata->fat_sect; /* Offset from start of disk */

                /* Write back the fatbuf to the disk */
                if (flush_dirty_fat_buffer(mydata) < 0)
                        return -1;

                if (disk_read(startblock, getsize, bufptr) < 0) {
                        debug("Error reading FAT blocks\n");
                        return ret;
                }
                mydata->fatbufnum = bufnum;
        }

        /* Get the actual entry from the table */
        switch (mydata->fatsize) {
        case 32:
                ret = FAT2CPU32(((__u32 *) mydata->fatbuf)[offset]);
                break;
        case 16:
                ret = FAT2CPU16(((__u16 *) mydata->fatbuf)[offset]);
                break;
        case 12:
                off8 = (offset * 3) / 2;
                /* fatbut + off8 may be unaligned, read in byte granularity */
                ret = mydata->fatbuf[off8] + (mydata->fatbuf[off8 + 1] << 8);

                if (offset & 0x1)
                        ret >>= 4;
                ret &= 0xfff;
        }
        debug("FAT%d: ret: 0x%08x, entry: 0x%08x, offset: 0x%04x\n",
               mydata->fatsize, ret, entry, offset);

        return ret;
}

/*
 * Read at most 'size' bytes from the specified cluster into 'buffer'.
 * Return 0 on success, -1 otherwise.
 */
static int
get_cluster(fsdata *mydata, __u32 clustnum, __u8 *buffer, unsigned long size)
{
        __u32 startsect;
        int ret;

        if (clustnum > 0) {
                startsect = clust_to_sect(mydata, clustnum);
        } else {
                startsect = mydata->rootdir_sect;
        }

        debug("gc - clustnum: %d, startsect: %d\n", clustnum, startsect);

        if ((unsigned long)buffer & (ARCH_DMA_MINALIGN - 1)) {
                ALLOC_CACHE_ALIGN_BUFFER(__u8, tmpbuf, mydata->sect_size);

                debug("FAT: Misaligned buffer address (%p)\n", buffer);

                while (size >= mydata->sect_size) {
                        ret = disk_read(startsect++, 1, tmpbuf);
                        if (ret != 1) {
                                debug("Error reading data (got %d)\n", ret);
                                return -1;
                        }

                        memcpy(buffer, tmpbuf, mydata->sect_size);
                        buffer += mydata->sect_size;
                        size -= mydata->sect_size;
                }
        } else if (size >= mydata->sect_size) {
                __u32 bytes_read;
                __u32 sect_count = size / mydata->sect_size;

                ret = disk_read(startsect, sect_count, buffer);
                if (ret != sect_count) {
                        debug("Error reading data (got %d)\n", ret);
                        return -1;
                }
                bytes_read = sect_count * mydata->sect_size;
                startsect += sect_count;
                buffer += bytes_read;
                size -= bytes_read;
        }
        if (size) {
                ALLOC_CACHE_ALIGN_BUFFER(__u8, tmpbuf, mydata->sect_size);

                ret = disk_read(startsect, 1, tmpbuf);
                if (ret != 1) {
                        debug("Error reading data (got %d)\n", ret);
                        return -1;
                }

                memcpy(buffer, tmpbuf, size);
        }

        return 0;
}

/**
 * get_contents() - read from file
 *
 * Read at most 'maxsize' bytes from 'pos' in the file associated with 'dentptr'
 * into 'buffer'. Update the number of bytes read in *gotsize or return -1 on
 * fatal errors.
 *
 * @mydata:     file system description
 * @dentprt:    directory entry pointer
 * @pos:        position from where to read
 * @buffer:     buffer into which to read
 * @maxsize:    maximum number of bytes to read
 * @gotsize:    number of bytes actually read
 * Return:      -1 on error, otherwise 0
 */
static int get_contents(fsdata *mydata, dir_entry *dentptr, loff_t pos,
                        __u8 *buffer, loff_t maxsize, loff_t *gotsize)
{
        loff_t filesize = FAT2CPU32(dentptr->size);
        unsigned int bytesperclust = mydata->clust_size * mydata->sect_size;
        __u32 curclust = START(dentptr);
        __u32 endclust, newclust;
        loff_t actsize;

        *gotsize = 0;
        debug("Filesize: %llu bytes\n", filesize);

        if (pos >= filesize) {
                debug("Read position past EOF: %llu\n", pos);
                return 0;
        }

        if (maxsize > 0 && filesize > pos + maxsize)
                filesize = pos + maxsize;

        debug("%llu bytes\n", filesize);

        actsize = bytesperclust;

        /* go to cluster at pos */
        while (actsize <= pos) {
                curclust = get_fatent(mydata, curclust);
                if (CHECK_CLUST(curclust, mydata->fatsize)) {
                        debug("curclust: 0x%x\n", curclust);
                        printf("Invalid FAT entry\n");
                        return -1;
                }
                actsize += bytesperclust;
        }

        /* actsize > pos */
        actsize -= bytesperclust;
        filesize -= actsize;
        pos -= actsize;

        /* align to beginning of next cluster if any */
        if (pos) {
                __u8 *tmp_buffer;

                actsize = min(filesize, (loff_t)bytesperclust);
                tmp_buffer = malloc_cache_aligned(actsize);
                if (!tmp_buffer) {
                        debug("Error: allocating buffer\n");
                        return -1;
                }

                if (get_cluster(mydata, curclust, tmp_buffer, actsize) != 0) {
                        printf("Error reading cluster\n");
                        free(tmp_buffer);
                        return -1;
                }
                filesize -= actsize;
                actsize -= pos;
                memcpy(buffer, tmp_buffer + pos, actsize);
                free(tmp_buffer);
                *gotsize += actsize;
                if (!filesize)
                        return 0;
                buffer += actsize;

                curclust = get_fatent(mydata, curclust);
                if (CHECK_CLUST(curclust, mydata->fatsize)) {
                        debug("curclust: 0x%x\n", curclust);
                        printf("Invalid FAT entry\n");
                        return -1;
                }
        }

        actsize = bytesperclust;
        endclust = curclust;

        do {
                /* search for consecutive clusters */
                while (actsize < filesize) {
                        newclust = get_fatent(mydata, endclust);
                        if ((newclust - 1) != endclust)
                                goto getit;
                        if (CHECK_CLUST(newclust, mydata->fatsize)) {
                                debug("curclust: 0x%x\n", newclust);
                                printf("Invalid FAT entry\n");
                                return -1;
                        }
                        endclust = newclust;
                        actsize += bytesperclust;
                }

                /* get remaining bytes */
                actsize = filesize;
                if (get_cluster(mydata, curclust, buffer, (int)actsize) != 0) {
                        printf("Error reading cluster\n");
                        return -1;
                }
                *gotsize += actsize;
                return 0;
getit:
                if (get_cluster(mydata, curclust, buffer, (int)actsize) != 0) {
                        printf("Error reading cluster\n");
                        return -1;
                }
                *gotsize += (int)actsize;
                filesize -= actsize;
                buffer += actsize;

                curclust = get_fatent(mydata, endclust);
                if (CHECK_CLUST(curclust, mydata->fatsize)) {
                        debug("curclust: 0x%x\n", curclust);
                        printf("Invalid FAT entry\n");
                        return -1;
                }
                actsize = bytesperclust;
                endclust = curclust;
        } while (1);
}

/*
 * Extract the file name information from 'slotptr' into 'l_name',
 * starting at l_name[*idx].
 * Return 1 if terminator (zero byte) is found, 0 otherwise.
 */
static int slot2str(dir_slot *slotptr, char *l_name, int *idx)
{
        int j;

        for (j = 0; j <= 8; j += 2) {
                l_name[*idx] = slotptr->name0_4[j];
                if (l_name[*idx] == 0x00)
                        return 1;
                (*idx)++;
        }
        for (j = 0; j <= 10; j += 2) {
                l_name[*idx] = slotptr->name5_10[j];
                if (l_name[*idx] == 0x00)
                        return 1;
                (*idx)++;
        }
        for (j = 0; j <= 2; j += 2) {
                l_name[*idx] = slotptr->name11_12[j];
                if (l_name[*idx] == 0x00)
                        return 1;
                (*idx)++;
        }

        return 0;
}

/* Calculate short name checksum */
static __u8 mkcksum(struct nameext *nameext)
{
        int i;
        u8 *pos = (void *)nameext;

        __u8 ret = 0;

        for (i = 0; i < 11; i++)
                ret = (((ret & 1) << 7) | ((ret & 0xfe) >> 1)) + pos[i];

        return ret;
}

/*
 * Read boot sector and volume info from a FAT filesystem
 */
static int
read_bootsectandvi(boot_sector *bs, volume_info *volinfo, int *fatsize)
{
        __u8 *block;
        volume_info *vistart;
        int ret = 0;

        if (cur_dev == NULL) {
                debug("Error: no device selected\n");
                return -1;
        }

        block = malloc_cache_aligned(cur_dev->blksz);
        if (block == NULL) {
                debug("Error: allocating block\n");
                return -1;
        }

        if (disk_read(0, 1, block) < 0) {
                debug("Error: reading block\n");
                goto fail;
        }

        memcpy(bs, block, sizeof(boot_sector));
        bs->reserved = FAT2CPU16(bs->reserved);
        bs->fat_length = FAT2CPU16(bs->fat_length);
        bs->secs_track = FAT2CPU16(bs->secs_track);
        bs->heads = FAT2CPU16(bs->heads);
        bs->total_sect = FAT2CPU32(bs->total_sect);

        /* FAT32 entries */
        if (bs->fat_length == 0) {
                /* Assume FAT32 */
                bs->fat32_length = FAT2CPU32(bs->fat32_length);
                bs->flags = FAT2CPU16(bs->flags);
                bs->root_cluster = FAT2CPU32(bs->root_cluster);
                bs->info_sector = FAT2CPU16(bs->info_sector);
                bs->backup_boot = FAT2CPU16(bs->backup_boot);
                vistart = (volume_info *)(block + sizeof(boot_sector));
                *fatsize = 32;
        } else {
                vistart = (volume_info *)&(bs->fat32_length);
                *fatsize = 0;
        }
        memcpy(volinfo, vistart, sizeof(volume_info));

        if (*fatsize == 32) {
                if (strncmp(FAT32_SIGN, vistart->fs_type, SIGNLEN) == 0)
                        goto exit;
        } else {
                if (strncmp(FAT12_SIGN, vistart->fs_type, SIGNLEN) == 0) {
                        *fatsize = 12;
                        goto exit;
                }
                if (strncmp(FAT16_SIGN, vistart->fs_type, SIGNLEN) == 0) {
                        *fatsize = 16;
                        goto exit;
                }
        }

        debug("Error: broken fs_type sign\n");
fail:
        ret = -1;
exit:
        free(block);
        return ret;
}

static int get_fs_info(fsdata *mydata)
{
        boot_sector bs;
        volume_info volinfo;
        int ret;

        ret = read_bootsectandvi(&bs, &volinfo, &mydata->fatsize);
        if (ret) {
                debug("Error: reading boot sector\n");
                return ret;
        }

        if (mydata->fatsize == 32) {
                mydata->fatlength = bs.fat32_length;
                mydata->total_sect = bs.total_sect;
        } else {
                mydata->fatlength = bs.fat_length;
                mydata->total_sect = (bs.sectors[1] << 8) + bs.sectors[0];
                if (!mydata->total_sect)
                        mydata->total_sect = bs.total_sect;
        }
        if (!mydata->total_sect) /* unlikely */
                mydata->total_sect = (u32)cur_part_info.size;

        mydata->fats = bs.fats;
        mydata->fat_sect = bs.reserved;

        mydata->rootdir_sect = mydata->fat_sect + mydata->fatlength * bs.fats;

        mydata->sect_size = (bs.sector_size[1] << 8) + bs.sector_size[0];
        mydata->clust_size = bs.cluster_size;
        if (mydata->sect_size != cur_part_info.blksz) {
                printf("Error: FAT sector size mismatch (fs=%hu, dev=%lu)\n",
                                mydata->sect_size, cur_part_info.blksz);
                return -1;
        }
        if (mydata->clust_size == 0) {
                printf("Error: FAT cluster size not set\n");
                return -1;
        }
        if ((unsigned int)mydata->clust_size * mydata->sect_size >
            MAX_CLUSTSIZE) {
                printf("Error: FAT cluster size too big (cs=%u, max=%u)\n",
                       (unsigned int)mydata->clust_size * mydata->sect_size,
                       MAX_CLUSTSIZE);
                return -1;
        }

        if (mydata->fatsize == 32) {
                mydata->data_begin = mydata->rootdir_sect -
                                        (mydata->clust_size * 2);
                mydata->root_cluster = bs.root_cluster;
        } else {
                mydata->rootdir_size = ((bs.dir_entries[1]  * (int)256 +
                                         bs.dir_entries[0]) *
                                         sizeof(dir_entry)) /
                                         mydata->sect_size;
                mydata->data_begin = mydata->rootdir_sect +
                                        mydata->rootdir_size -
                                        (mydata->clust_size * 2);

                /*
                 * The root directory is not cluster-aligned and may be on a
                 * "negative" cluster, this will be handled specially in
                 * fat_next_cluster().
                 */
                mydata->root_cluster = 0;
        }

        mydata->fatbufnum = -1;
        mydata->fat_dirty = 0;
        mydata->fatbuf = malloc_cache_aligned(FATBUFSIZE);
        if (mydata->fatbuf == NULL) {
                debug("Error: allocating memory\n");
                return -1;
        }

        debug("FAT%d, fat_sect: %d, fatlength: %d\n",
               mydata->fatsize, mydata->fat_sect, mydata->fatlength);
        debug("Rootdir begins at cluster: %d, sector: %d, offset: %x\n"
               "Data begins at: %d\n",
               mydata->root_cluster,
               mydata->rootdir_sect,
               mydata->rootdir_sect * mydata->sect_size, mydata->data_begin);
        debug("Sector size: %d, cluster size: %d\n", mydata->sect_size,
              mydata->clust_size);

        return 0;
}

/**
 * struct fat_itr - directory iterator, to simplify filesystem traversal
 *
 * Implements an iterator pattern to traverse directory tables,
 * transparently handling directory tables split across multiple
 * clusters, and the difference between FAT12/FAT16 root directory
 * (contiguous) and subdirectories + FAT32 root (chained).
 *
 * Rough usage
 *
 * .. code-block:: c
 *
 *     for (fat_itr_root(&itr, fsdata); fat_itr_next(&itr); ) {
 *         // to traverse down to a subdirectory pointed to by
 *         // current iterator position:
 *         fat_itr_child(&itr, &itr);
 *     }
 *
 * For a more complete example, see fat_itr_resolve().
 */
struct fat_itr {
        /**
         * @fsdata:             filesystem parameters
         */
        fsdata *fsdata;
        /**
         * @start_clust:        first cluster
         */
        unsigned int start_clust;
        /**
         * @clust:              current cluster
         */
        unsigned int clust;
        /**
         * @next_clust:         next cluster if remaining == 0
         */
        unsigned int next_clust;
        /**
         * @last_cluster:       set if last cluster of directory reached
         */
        int last_cluster;
        /**
         * @is_root:            is iterator at root directory
         */
        int is_root;
        /**
         * @remaining:          remaining directory entries in current cluster
         */
        int remaining;
        /**
         * @dent:               current directory entry
         */
        dir_entry *dent;
        /**
         * @dent_rem:           remaining entries after long name start
         */
        int dent_rem;
        /**
         * @dent_clust:         cluster of long name start
         */
        unsigned int dent_clust;
        /**
         * @dent_start:         first directory entry for long name
         */
        dir_entry *dent_start;
        /**
         * @l_name:             long name of current directory entry
         */
        char l_name[VFAT_MAXLEN_BYTES];
        /**
         * @s_name:             short 8.3 name of current directory entry
         */
        char s_name[14];
        /**
         * @name:               l_name if there is one, else s_name
         */
        char *name;
        /**
         * @block:              buffer for current cluster
         */
        u8 block[MAX_CLUSTSIZE] __aligned(ARCH_DMA_MINALIGN);
};

static int fat_itr_isdir(fat_itr *itr);

/**
 * fat_itr_root() - initialize an iterator to start at the root
 * directory
 *
 * @itr: iterator to initialize
 * @fsdata: filesystem data for the partition
 * @return 0 on success, else -errno
 */
static int fat_itr_root(fat_itr *itr, fsdata *fsdata)
{
        if (get_fs_info(fsdata))
                return -ENXIO;

        itr->fsdata = fsdata;
        itr->start_clust = fsdata->root_cluster;
        itr->clust = fsdata->root_cluster;
        itr->next_clust = fsdata->root_cluster;
        itr->dent = NULL;
        itr->remaining = 0;
        itr->last_cluster = 0;
        itr->is_root = 1;

        return 0;
}

/**
 * fat_itr_child() - initialize an iterator to descend into a sub-
 * directory
 *
 * Initializes 'itr' to iterate the contents of the directory at
 * the current cursor position of 'parent'.  It is an error to
 * call this if the current cursor of 'parent' is pointing at a
 * regular file.
 *
 * Note that 'itr' and 'parent' can be the same pointer if you do
 * not need to preserve 'parent' after this call, which is useful
 * for traversing directory structure to resolve a file/directory.
 *
 * @itr: iterator to initialize
 * @parent: the iterator pointing at a directory entry in the
 *    parent directory of the directory to iterate
 */
static void fat_itr_child(fat_itr *itr, fat_itr *parent)
{
        fsdata *mydata = parent->fsdata;  /* for silly macros */
        unsigned clustnum = START(parent->dent);

        assert(fat_itr_isdir(parent));

        itr->fsdata = parent->fsdata;
        itr->start_clust = clustnum;
        if (clustnum > 0) {
                itr->clust = clustnum;
                itr->next_clust = clustnum;
                itr->is_root = 0;
        } else {
                itr->clust = parent->fsdata->root_cluster;
                itr->next_clust = parent->fsdata->root_cluster;
                itr->start_clust = parent->fsdata->root_cluster;
                itr->is_root = 1;
        }
        itr->dent = NULL;
        itr->remaining = 0;
        itr->last_cluster = 0;
}

/**
 * fat_next_cluster() - load next FAT cluster
 *
 * The function is used when iterating through directories. It loads the
 * next cluster with directory entries
 *
 * @itr:        directory iterator
 * @nbytes:     number of bytes read, 0 on error
 * Return:      first directory entry, NULL on error
 */
void *fat_next_cluster(fat_itr *itr, unsigned int *nbytes)
{
        int ret;
        u32 sect;
        u32 read_size;

        /* have we reached the end? */
        if (itr->last_cluster)
                return NULL;

        if (itr->is_root && itr->fsdata->fatsize != 32) {
                /*
                 * The root directory is located before the data area and
                 * cannot be indexed using the regular unsigned cluster
                 * numbers (it may start at a "negative" cluster or not at a
                 * cluster boundary at all), so consider itr->next_clust to be
                 * a offset in cluster-sized units from the start of rootdir.
                 */
                unsigned sect_offset = itr->next_clust * itr->fsdata->clust_size;
                unsigned remaining_sects = itr->fsdata->rootdir_size - sect_offset;
                sect = itr->fsdata->rootdir_sect + sect_offset;
                /* do not read past the end of rootdir */
                read_size = min_t(u32, itr->fsdata->clust_size,
                                  remaining_sects);
        } else {
                sect = clust_to_sect(itr->fsdata, itr->next_clust);
                read_size = itr->fsdata->clust_size;
        }

        log_debug("FAT read(sect=%d), clust_size=%d, read_size=%u\n",
                  sect, itr->fsdata->clust_size, read_size);

        /*
         * NOTE: do_fat_read_at() had complicated logic to deal w/
         * vfat names that span multiple clusters in the fat16 case,
         * which get_dentfromdir() probably also needed (and was
         * missing).  And not entirely sure what fat32 didn't have
         * the same issue..  We solve that by only caring about one
         * dent at a time and iteratively constructing the vfat long
         * name.
         */
        ret = disk_read(sect, read_size, itr->block);
        if (ret < 0) {
                debug("Error: reading block\n");
                return NULL;
        }

        *nbytes = read_size * itr->fsdata->sect_size;
        itr->clust = itr->next_clust;
        if (itr->is_root && itr->fsdata->fatsize != 32) {
                itr->next_clust++;
                if (itr->next_clust * itr->fsdata->clust_size >=
                    itr->fsdata->rootdir_size) {
                        debug("nextclust: 0x%x\n", itr->next_clust);
                        itr->last_cluster = 1;
                }
        } else {
                itr->next_clust = get_fatent(itr->fsdata, itr->next_clust);
                if (CHECK_CLUST(itr->next_clust, itr->fsdata->fatsize)) {
                        debug("nextclust: 0x%x\n", itr->next_clust);
                        itr->last_cluster = 1;
                }
        }

        return itr->block;
}

static dir_entry *next_dent(fat_itr *itr)
{
        if (itr->remaining == 0) {
                unsigned nbytes;
                struct dir_entry *dent = fat_next_cluster(itr, &nbytes);

                /* have we reached the last cluster? */
                if (!dent) {
                        /* a sign for no more entries left */
                        itr->dent = NULL;
                        return NULL;
                }

                itr->remaining = nbytes / sizeof(dir_entry) - 1;
                itr->dent = dent;
        } else {
                itr->remaining--;
                itr->dent++;
        }

        /* have we reached the last valid entry? */
        if (itr->dent->nameext.name[0] == 0)
                return NULL;

        return itr->dent;
}

static dir_entry *extract_vfat_name(fat_itr *itr)
{
        struct dir_entry *dent = itr->dent;
        int seqn = itr->dent->nameext.name[0] & ~LAST_LONG_ENTRY_MASK;
        u8 chksum, alias_checksum = ((dir_slot *)dent)->alias_checksum;
        int n = 0;

        while (seqn--) {
                char buf[13];
                int idx = 0;

                slot2str((dir_slot *)dent, buf, &idx);

                if (n + idx >= sizeof(itr->l_name))
                        return NULL;

                /* shift accumulated long-name up and copy new part in: */
                memmove(itr->l_name + idx, itr->l_name, n);
                memcpy(itr->l_name, buf, idx);
                n += idx;

                dent = next_dent(itr);
                if (!dent)
                        return NULL;
        }

        /*
         * We are now at the short file name entry.
         * If it is marked as deleted, just skip it.
         */
        if (dent->nameext.name[0] == DELETED_FLAG ||
            dent->nameext.name[0] == aRING)
                return NULL;

        itr->l_name[n] = '\0';

        chksum = mkcksum(&dent->nameext);

        /* checksum mismatch could mean deleted file, etc.. skip it: */
        if (chksum != alias_checksum) {
                debug("** chksum=%x, alias_checksum=%x, l_name=%s, s_name=%8s.%3s\n",
                      chksum, alias_checksum, itr->l_name, dent->nameext.name,
                      dent->nameext.ext);
                return NULL;
        }

        return dent;
}

/**
 * fat_itr_next() - step to the next entry in a directory
 *
 * Must be called once on a new iterator before the cursor is valid.
 *
 * @itr: the iterator to iterate
 * @return boolean, 1 if success or 0 if no more entries in the
 *    current directory
 */
static int fat_itr_next(fat_itr *itr)
{
        dir_entry *dent;

        itr->name = NULL;

        /*
         * One logical directory entry consist of following slots:
         *                              name[0] Attributes
         *   dent[N - N]: LFN[N - 1]    N|0x40  ATTR_VFAT
         *   ...
         *   dent[N - 2]: LFN[1]        2       ATTR_VFAT
         *   dent[N - 1]: LFN[0]        1       ATTR_VFAT
         *   dent[N]:     SFN                   ATTR_ARCH
         */

        while (1) {
                dent = next_dent(itr);
                if (!dent) {
                        itr->dent_start = NULL;
                        return 0;
                }
                itr->dent_rem = itr->remaining;
                itr->dent_start = itr->dent;
                itr->dent_clust = itr->clust;
                if (dent->nameext.name[0] == DELETED_FLAG)
                        continue;

                if (dent->attr & ATTR_VOLUME) {
                        if ((dent->attr & ATTR_VFAT) == ATTR_VFAT &&
                            (dent->nameext.name[0] & LAST_LONG_ENTRY_MASK)) {
                                /* long file name */
                                dent = extract_vfat_name(itr);
                                /*
                                 * If succeeded, dent has a valid short file
                                 * name entry for the current entry.
                                 * If failed, itr points to a current bogus
                                 * entry. So after fetching a next one,
                                 * it may have a short file name entry
                                 * for this bogus entry so that we can still
                                 * check for a short name.

                                 */
                                if (!dent)
                                        continue;
                                itr->name = itr->l_name;
                                break;
                        } else {
                                /* Volume label or VFAT entry, skip */
                                continue;
                        }
                }

                /* short file name */
                break;
        }

        get_name(dent, itr->s_name);
        if (!itr->name)
                itr->name = itr->s_name;

        return 1;
}

/**
 * fat_itr_isdir() - is current cursor position pointing to a directory
 *
 * @itr: the iterator
 * @return true if cursor is at a directory
 */
static int fat_itr_isdir(fat_itr *itr)
{
        return !!(itr->dent->attr & ATTR_DIR);
}

/*
 * Helpers:
 */

#define TYPE_FILE 0x1
#define TYPE_DIR  0x2
#define TYPE_ANY  (TYPE_FILE | TYPE_DIR)

/**
 * fat_itr_resolve() - traverse directory structure to resolve the
 * requested path.
 *
 * Traverse directory structure to the requested path.  If the specified
 * path is to a directory, this will descend into the directory and
 * leave it iterator at the start of the directory.  If the path is to a
 * file, it will leave the iterator in the parent directory with current
 * cursor at file's entry in the directory.
 *
 * @itr: iterator initialized to root
 * @path: the requested path
 * @type: bitmask of allowable file types
 * @return 0 on success or -errno
 */
static int fat_itr_resolve(fat_itr *itr, const char *path, unsigned type)
{
        const char *next;

        /* chomp any extra leading slashes: */
        while (path[0] && ISDIRDELIM(path[0]))
                path++;

        /* are we at the end? */
        if (strlen(path) == 0) {
                if (!(type & TYPE_DIR))
                        return -ENOENT;
                return 0;
        }

        /* find length of next path entry: */
        next = path;
        while (next[0] && !ISDIRDELIM(next[0]))
                next++;

        if (itr->is_root) {
                /* root dir doesn't have "." nor ".." */
                if ((((next - path) == 1) && !strncmp(path, ".", 1)) ||
                    (((next - path) == 2) && !strncmp(path, "..", 2))) {
                        /* point back to itself */
                        itr->clust = itr->fsdata->root_cluster;
                        itr->next_clust = itr->fsdata->root_cluster;
                        itr->start_clust = itr->fsdata->root_cluster;
                        itr->dent = NULL;
                        itr->remaining = 0;
                        itr->last_cluster = 0;

                        if (next[0] == 0) {
                                if (type & TYPE_DIR)
                                        return 0;
                                else
                                        return -ENOENT;
                        }

                        return fat_itr_resolve(itr, next, type);
                }
        }

        while (fat_itr_next(itr)) {
                int match = 0;
                unsigned n = max(strlen(itr->name), (size_t)(next - path));

                /* check both long and short name: */
                if (!strncasecmp(path, itr->name, n))
                        match = 1;
                else if (itr->name != itr->s_name &&
                         !strncasecmp(path, itr->s_name, n))
                        match = 1;

                if (!match)
                        continue;

                if (fat_itr_isdir(itr)) {
                        /* recurse into directory: */
                        fat_itr_child(itr, itr);
                        return fat_itr_resolve(itr, next, type);
                } else if (next[0]) {
                        /*
                         * If next is not empty then we have a case
                         * like: /path/to/realfile/nonsense
                         */
                        debug("bad trailing path: %s\n", next);
                        return -ENOENT;
                } else if (!(type & TYPE_FILE)) {
                        return -ENOTDIR;
                } else {
                        return 0;
                }
        }

        return -ENOENT;
}

int file_fat_detectfs(void)
{
        boot_sector bs;
        volume_info volinfo;
        int fatsize;
        char vol_label[12];

        if (cur_dev == NULL) {
                printf("No current device\n");
                return 1;
        }

        if (IS_ENABLED(CONFIG_HAVE_BLOCK_DEVICE)) {
                printf("Interface:  %s\n", blk_get_if_type_name(cur_dev->if_type));
                printf("  Device %d: ", cur_dev->devnum);
                dev_print(cur_dev);
        }

        if (read_bootsectandvi(&bs, &volinfo, &fatsize)) {
                printf("\nNo valid FAT fs found\n");
                return 1;
        }

        memcpy(vol_label, volinfo.volume_label, 11);
        vol_label[11] = '\0';
        volinfo.fs_type[5] = '\0';

        printf("Filesystem: %s \"%s\"\n", volinfo.fs_type, vol_label);

        return 0;
}

int fat_exists(const char *filename)
{
        fsdata fsdata;
        fat_itr *itr;
        int ret;

        itr = malloc_cache_aligned(sizeof(fat_itr));
        if (!itr)
                return 0;
        ret = fat_itr_root(itr, &fsdata);
        if (ret)
                goto out;

        ret = fat_itr_resolve(itr, filename, TYPE_ANY);
        free(fsdata.fatbuf);
out:
        free(itr);
        return ret == 0;
}

/**
 * fat2rtc() - convert FAT time stamp to RTC file stamp
 *
 * @date:       FAT date
 * @time:       FAT time
 * @tm:         RTC time stamp
 */
static void __maybe_unused fat2rtc(u16 date, u16 time, struct rtc_time *tm)
{
        tm->tm_mday = date & 0x1f;
        tm->tm_mon = (date & 0x1e0) >> 4;
        tm->tm_year = (date >> 9) + 1980;

        tm->tm_sec = (time & 0x1f) << 1;
        tm->tm_min = (time & 0x7e0) >> 5;
        tm->tm_hour = time >> 11;

        rtc_calc_weekday(tm);
        tm->tm_yday = 0;
        tm->tm_isdst = 0;
}

int fat_size(const char *filename, loff_t *size)
{
        fsdata fsdata;
        fat_itr *itr;
        int ret;

        itr = malloc_cache_aligned(sizeof(fat_itr));
        if (!itr)
                return -ENOMEM;
        ret = fat_itr_root(itr, &fsdata);
        if (ret)
                goto out_free_itr;

        ret = fat_itr_resolve(itr, filename, TYPE_FILE);
        if (ret) {
                /*
                 * Directories don't have size, but fs_size() is not
                 * expected to fail if passed a directory path:
                 */
                free(fsdata.fatbuf);
                ret = fat_itr_root(itr, &fsdata);
                if (ret)
                        goto out_free_itr;
                ret = fat_itr_resolve(itr, filename, TYPE_DIR);
                if (!ret)
                        *size = 0;
                goto out_free_both;
        }

        *size = FAT2CPU32(itr->dent->size);
out_free_both:
        free(fsdata.fatbuf);
out_free_itr:
        free(itr);
        return ret;
}

int file_fat_read_at(const char *filename, loff_t pos, void *buffer,
                     loff_t maxsize, loff_t *actread)
{
        fsdata fsdata;
        fat_itr *itr;
        int ret;

        itr = malloc_cache_aligned(sizeof(fat_itr));
        if (!itr)
                return -ENOMEM;
        ret = fat_itr_root(itr, &fsdata);
        if (ret)
                goto out_free_itr;

        ret = fat_itr_resolve(itr, filename, TYPE_FILE);
        if (ret)
                goto out_free_both;

        debug("reading %s at pos %llu\n", filename, pos);

        /* For saving default max clustersize memory allocated to malloc pool */
        dir_entry *dentptr = itr->dent;

        ret = get_contents(&fsdata, dentptr, pos, buffer, maxsize, actread);

out_free_both:
        free(fsdata.fatbuf);
out_free_itr:
        free(itr);
        return ret;
}

int file_fat_read(const char *filename, void *buffer, int maxsize)
{
        loff_t actread;
        int ret;

        ret =  file_fat_read_at(filename, 0, buffer, maxsize, &actread);
        if (ret)
                return ret;
        else
                return actread;
}

int fat_read_file(const char *filename, void *buf, loff_t offset, loff_t len,
                  loff_t *actread)
{
        int ret;

        ret = file_fat_read_at(filename, offset, buf, len, actread);
        if (ret)
                printf("** Unable to read file %s **\n", filename);

        return ret;
}

typedef struct {
        struct fs_dir_stream parent;
        struct fs_dirent dirent;
        fsdata fsdata;
        fat_itr itr;
} fat_dir;

int fat_opendir(const char *filename, struct fs_dir_stream **dirsp)
{
        fat_dir *dir;
        int ret;

        dir = malloc_cache_aligned(sizeof(*dir));
        if (!dir)
                return -ENOMEM;
        memset(dir, 0, sizeof(*dir));

        ret = fat_itr_root(&dir->itr, &dir->fsdata);
        if (ret)
                goto fail_free_dir;

        ret = fat_itr_resolve(&dir->itr, filename, TYPE_DIR);
        if (ret)
                goto fail_free_both;

        *dirsp = (struct fs_dir_stream *)dir;
        return 0;

fail_free_both:
        free(dir->fsdata.fatbuf);
fail_free_dir:
        free(dir);
        return ret;
}

int fat_readdir(struct fs_dir_stream *dirs, struct fs_dirent **dentp)
{
        fat_dir *dir = (fat_dir *)dirs;
        struct fs_dirent *dent = &dir->dirent;

        if (!fat_itr_next(&dir->itr))
                return -ENOENT;

        memset(dent, 0, sizeof(*dent));
        strcpy(dent->name, dir->itr.name);
        if (CONFIG_IS_ENABLED(EFI_LOADER)) {
                dent->attr = dir->itr.dent->attr;
                fat2rtc(le16_to_cpu(dir->itr.dent->cdate),
                        le16_to_cpu(dir->itr.dent->ctime), &dent->create_time);
                fat2rtc(le16_to_cpu(dir->itr.dent->date),
                        le16_to_cpu(dir->itr.dent->time), &dent->change_time);
                fat2rtc(le16_to_cpu(dir->itr.dent->adate),
                        0, &dent->access_time);
        }
        if (fat_itr_isdir(&dir->itr)) {
                dent->type = FS_DT_DIR;
        } else {
                dent->type = FS_DT_REG;
                dent->size = FAT2CPU32(dir->itr.dent->size);
        }

        *dentp = dent;

        return 0;
}

void fat_closedir(struct fs_dir_stream *dirs)
{
        fat_dir *dir = (fat_dir *)dirs;
        free(dir->fsdata.fatbuf);
        free(dir);
}

void fat_close(void)
{
}

int fat_uuid(char *uuid_str)
{
        boot_sector bs;
        volume_info volinfo;
        int fatsize;
        int ret;
        u8 *id;

        ret = read_bootsectandvi(&bs, &volinfo, &fatsize);
        if (ret)
                return ret;

        id = volinfo.volume_id;
        sprintf(uuid_str, "%02X%02X-%02X%02X", id[3], id[2], id[1], id[0]);

        return 0;
}