/*
 * Compressed rom filesystem for Linux.
 *
 * Copyright (C) 1999 Linus Torvalds.
 *
 * This file is released under the GPL.
 */

/*
 * These are the VFS interfaces to the compressed rom filesystem.
 * The actual compression is based on zlib, see the other files.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/module.h>
#include <linux/fs.h>
#include <linux/file.h>
#include <linux/pagemap.h>
#include <linux/pfn_t.h>
#include <linux/ramfs.h>
#include <linux/init.h>
#include <linux/string.h>
#include <linux/blkdev.h>
#include <linux/mtd/mtd.h>
#include <linux/mtd/super.h>
#include <linux/fs_context.h>
#include <linux/slab.h>
#include <linux/vfs.h>
#include <linux/mutex.h>
#include <uapi/linux/cramfs_fs.h>
#include <linux/uaccess.h>

#include "internal.h"

/*
 * cramfs super-block data in memory
 */
struct cramfs_sb_info {
        unsigned long magic;
        unsigned long size;
        unsigned long blocks;
        unsigned long files;
        unsigned long flags;
        void *linear_virt_addr;
        resource_size_t linear_phys_addr;
        size_t mtd_point_size;
};

static inline struct cramfs_sb_info *CRAMFS_SB(struct super_block *sb)
{
        return sb->s_fs_info;
}

static const struct super_operations cramfs_ops;
static const struct inode_operations cramfs_dir_inode_operations;
static const struct file_operations cramfs_directory_operations;
static const struct file_operations cramfs_physmem_fops;
static const struct address_space_operations cramfs_aops;

static DEFINE_MUTEX(read_mutex);


/* These macros may change in future, to provide better st_ino semantics. */
#define OFFSET(x)       ((x)->i_ino)

static unsigned long cramino(const struct cramfs_inode *cino, unsigned int offset)
{
        if (!cino->offset)
                return offset + 1;
        if (!cino->size)
                return offset + 1;

        /*
         * The file mode test fixes buggy mkcramfs implementations where
         * cramfs_inode->offset is set to a non zero value for entries
         * which did not contain data, like devices node and fifos.
         */
        switch (cino->mode & S_IFMT) {
        case S_IFREG:
        case S_IFDIR:
        case S_IFLNK:
                return cino->offset << 2;
        default:
                break;
        }
        return offset + 1;
}

static struct inode *get_cramfs_inode(struct super_block *sb,
        const struct cramfs_inode *cramfs_inode, unsigned int offset)
{
        struct inode *inode;
        static struct timespec64 zerotime;

        inode = iget_locked(sb, cramino(cramfs_inode, offset));
        if (!inode)
                return ERR_PTR(-ENOMEM);
        if (!(inode->i_state & I_NEW))
                return inode;

        switch (cramfs_inode->mode & S_IFMT) {
        case S_IFREG:
                inode->i_fop = &generic_ro_fops;
                inode->i_data.a_ops = &cramfs_aops;
                if (IS_ENABLED(CONFIG_CRAMFS_MTD) &&
                    CRAMFS_SB(sb)->flags & CRAMFS_FLAG_EXT_BLOCK_POINTERS &&
                    CRAMFS_SB(sb)->linear_phys_addr)
                        inode->i_fop = &cramfs_physmem_fops;
                break;
        case S_IFDIR:
                inode->i_op = &cramfs_dir_inode_operations;
                inode->i_fop = &cramfs_directory_operations;
                break;
        case S_IFLNK:
                inode->i_op = &page_symlink_inode_operations;
                inode_nohighmem(inode);
                inode->i_data.a_ops = &cramfs_aops;
                break;
        default:
                init_special_inode(inode, cramfs_inode->mode,
                                old_decode_dev(cramfs_inode->size));
        }

        inode->i_mode = cramfs_inode->mode;
        i_uid_write(inode, cramfs_inode->uid);
        i_gid_write(inode, cramfs_inode->gid);

        /* if the lower 2 bits are zero, the inode contains data */
        if (!(inode->i_ino & 3)) {
                inode->i_size = cramfs_inode->size;
                inode->i_blocks = (cramfs_inode->size - 1) / 512 + 1;
        }

        /* Struct copy intentional */
        inode->i_mtime = inode->i_atime = inode->i_ctime = zerotime;
        /* inode->i_nlink is left 1 - arguably wrong for directories,
           but it's the best we can do without reading the directory
           contents.  1 yields the right result in GNU find, even
           without -noleaf option. */

        unlock_new_inode(inode);

        return inode;
}

/*
 * We have our own block cache: don't fill up the buffer cache
 * with the rom-image, because the way the filesystem is set
 * up the accesses should be fairly regular and cached in the
 * page cache and dentry tree anyway..
 *
 * This also acts as a way to guarantee contiguous areas of up to
 * BLKS_PER_BUF*PAGE_SIZE, so that the caller doesn't need to
 * worry about end-of-buffer issues even when decompressing a full
 * page cache.
 *
 * Note: This is all optimized away at compile time when
 *       CONFIG_CRAMFS_BLOCKDEV=n.
 */
#define READ_BUFFERS (2)
/* NEXT_BUFFER(): Loop over [0..(READ_BUFFERS-1)]. */
#define NEXT_BUFFER(_ix) ((_ix) ^ 1)

/*
 * BLKS_PER_BUF_SHIFT should be at least 2 to allow for "compressed"
 * data that takes up more space than the original and with unlucky
 * alignment.
 */
#define BLKS_PER_BUF_SHIFT      (2)
#define BLKS_PER_BUF            (1 << BLKS_PER_BUF_SHIFT)
#define BUFFER_SIZE             (BLKS_PER_BUF*PAGE_SIZE)

static unsigned char read_buffers[READ_BUFFERS][BUFFER_SIZE];
static unsigned buffer_blocknr[READ_BUFFERS];
static struct super_block *buffer_dev[READ_BUFFERS];
static int next_buffer;

/*
 * Populate our block cache and return a pointer to it.
 */
static void *cramfs_blkdev_read(struct super_block *sb, unsigned int offset,
                                unsigned int len)
{
        struct address_space *mapping = sb->s_bdev->bd_inode->i_mapping;
        struct page *pages[BLKS_PER_BUF];
        unsigned i, blocknr, buffer;
        unsigned long devsize;
        char *data;

        if (!len)
                return NULL;
        blocknr = offset >> PAGE_SHIFT;
        offset &= PAGE_SIZE - 1;

        /* Check if an existing buffer already has the data.. */
        for (i = 0; i < READ_BUFFERS; i++) {
                unsigned int blk_offset;

                if (buffer_dev[i] != sb)
                        continue;
                if (blocknr < buffer_blocknr[i])
                        continue;
                blk_offset = (blocknr - buffer_blocknr[i]) << PAGE_SHIFT;
                blk_offset += offset;
                if (blk_offset > BUFFER_SIZE ||
                    blk_offset + len > BUFFER_SIZE)
                        continue;
                return read_buffers[i] + blk_offset;
        }

        devsize = mapping->host->i_size >> PAGE_SHIFT;

        /* Ok, read in BLKS_PER_BUF pages completely first. */
        for (i = 0; i < BLKS_PER_BUF; i++) {
                struct page *page = NULL;

                if (blocknr + i < devsize) {
                        page = read_mapping_page(mapping, blocknr + i, NULL);
                        /* synchronous error? */
                        if (IS_ERR(page))
                                page = NULL;
                }
                pages[i] = page;
        }

        for (i = 0; i < BLKS_PER_BUF; i++) {
                struct page *page = pages[i];

                if (page) {
                        wait_on_page_locked(page);
                        if (!PageUptodate(page)) {
                                /* asynchronous error */
                                put_page(page);
                                pages[i] = NULL;
                        }
                }
        }

        buffer = next_buffer;
        next_buffer = NEXT_BUFFER(buffer);
        buffer_blocknr[buffer] = blocknr;
        buffer_dev[buffer] = sb;

        data = read_buffers[buffer];
        for (i = 0; i < BLKS_PER_BUF; i++) {
                struct page *page = pages[i];

                if (page) {
                        memcpy(data, kmap(page), PAGE_SIZE);
                        kunmap(page);
                        put_page(page);
                } else
                        memset(data, 0, PAGE_SIZE);
                data += PAGE_SIZE;
        }
        return read_buffers[buffer] + offset;
}

/*
 * Return a pointer to the linearly addressed cramfs image in memory.
 */
static void *cramfs_direct_read(struct super_block *sb, unsigned int offset,
                                unsigned int len)
{
        struct cramfs_sb_info *sbi = CRAMFS_SB(sb);

        if (!len)
                return NULL;
        if (len > sbi->size || offset > sbi->size - len)
                return page_address(ZERO_PAGE(0));
        return sbi->linear_virt_addr + offset;
}

/*
 * Returns a pointer to a buffer containing at least LEN bytes of
 * filesystem starting at byte offset OFFSET into the filesystem.
 */
static void *cramfs_read(struct super_block *sb, unsigned int offset,
                         unsigned int len)
{
        struct cramfs_sb_info *sbi = CRAMFS_SB(sb);

        if (IS_ENABLED(CONFIG_CRAMFS_MTD) && sbi->linear_virt_addr)
                return cramfs_direct_read(sb, offset, len);
        else if (IS_ENABLED(CONFIG_CRAMFS_BLOCKDEV))
                return cramfs_blkdev_read(sb, offset, len);
        else
                return NULL;
}

/*
 * For a mapping to be possible, we need a range of uncompressed and
 * contiguous blocks. Return the offset for the first block and number of
 * valid blocks for which that is true, or zero otherwise.
 */
static u32 cramfs_get_block_range(struct inode *inode, u32 pgoff, u32 *pages)
{
        struct cramfs_sb_info *sbi = CRAMFS_SB(inode->i_sb);
        int i;
        u32 *blockptrs, first_block_addr;

        /*
         * We can dereference memory directly here as this code may be
         * reached only when there is a direct filesystem image mapping
         * available in memory.
         */
        blockptrs = (u32 *)(sbi->linear_virt_addr + OFFSET(inode) + pgoff * 4);
        first_block_addr = blockptrs[0] & ~CRAMFS_BLK_FLAGS;
        i = 0;
        do {
                u32 block_off = i * (PAGE_SIZE >> CRAMFS_BLK_DIRECT_PTR_SHIFT);
                u32 expect = (first_block_addr + block_off) |
                             CRAMFS_BLK_FLAG_DIRECT_PTR |
                             CRAMFS_BLK_FLAG_UNCOMPRESSED;
                if (blockptrs[i] != expect) {
                        pr_debug("range: block %d/%d got %#x expects %#x\n",
                                 pgoff+i, pgoff + *pages - 1,
                                 blockptrs[i], expect);
                        if (i == 0)
                                return 0;
                        break;
                }
        } while (++i < *pages);

        *pages = i;
        return first_block_addr << CRAMFS_BLK_DIRECT_PTR_SHIFT;
}

#ifdef CONFIG_MMU

/*
 * Return true if the last page of a file in the filesystem image contains
 * some other data that doesn't belong to that file. It is assumed that the
 * last block is CRAMFS_BLK_FLAG_DIRECT_PTR | CRAMFS_BLK_FLAG_UNCOMPRESSED
 * (verified by cramfs_get_block_range() and directly accessible in memory.
 */
static bool cramfs_last_page_is_shared(struct inode *inode)
{
        struct cramfs_sb_info *sbi = CRAMFS_SB(inode->i_sb);
        u32 partial, last_page, blockaddr, *blockptrs;
        char *tail_data;

        partial = offset_in_page(inode->i_size);
        if (!partial)
                return false;
        last_page = inode->i_size >> PAGE_SHIFT;
        blockptrs = (u32 *)(sbi->linear_virt_addr + OFFSET(inode));
        blockaddr = blockptrs[last_page] & ~CRAMFS_BLK_FLAGS;
        blockaddr <<= CRAMFS_BLK_DIRECT_PTR_SHIFT;
        tail_data = sbi->linear_virt_addr + blockaddr + partial;
        return memchr_inv(tail_data, 0, PAGE_SIZE - partial) ? true : false;
}

static int cramfs_physmem_mmap(struct file *file, struct vm_area_struct *vma)
{
        struct inode *inode = file_inode(file);
        struct cramfs_sb_info *sbi = CRAMFS_SB(inode->i_sb);
        unsigned int pages, max_pages, offset;
        unsigned long address, pgoff = vma->vm_pgoff;
        char *bailout_reason;
        int ret;

        ret = generic_file_readonly_mmap(file, vma);
        if (ret)
                return ret;

        /*
         * Now try to pre-populate ptes for this vma with a direct
         * mapping avoiding memory allocation when possible.
         */

        /* Could COW work here? */
        bailout_reason = "vma is writable";
        if (vma->vm_flags & VM_WRITE)
                goto bailout;

        max_pages = (inode->i_size + PAGE_SIZE - 1) >> PAGE_SHIFT;
        bailout_reason = "beyond file limit";
        if (pgoff >= max_pages)
                goto bailout;
        pages = min(vma_pages(vma), max_pages - pgoff);

        offset = cramfs_get_block_range(inode, pgoff, &pages);
        bailout_reason = "unsuitable block layout";
        if (!offset)
                goto bailout;
        address = sbi->linear_phys_addr + offset;
        bailout_reason = "data is not page aligned";
        if (!PAGE_ALIGNED(address))
                goto bailout;

        /* Don't map the last page if it contains some other data */
        if (pgoff + pages == max_pages && cramfs_last_page_is_shared(inode)) {
                pr_debug("mmap: %s: last page is shared\n",
                         file_dentry(file)->d_name.name);
                pages--;
        }

        if (!pages) {
                bailout_reason = "no suitable block remaining";
                goto bailout;
        }

        if (pages == vma_pages(vma)) {
                /*
                 * The entire vma is mappable. remap_pfn_range() will
                 * make it distinguishable from a non-direct mapping
                 * in /proc/<pid>/maps by substituting the file offset
                 * with the actual physical address.
                 */
                ret = remap_pfn_range(vma, vma->vm_start, address >> PAGE_SHIFT,
                                      pages * PAGE_SIZE, vma->vm_page_prot);
        } else {
                /*
                 * Let's create a mixed map if we can't map it all.
                 * The normal paging machinery will take care of the
                 * unpopulated ptes via cramfs_readpage().
                 */
                int i;
                vma->vm_flags |= VM_MIXEDMAP;
                for (i = 0; i < pages && !ret; i++) {
                        vm_fault_t vmf;
                        unsigned long off = i * PAGE_SIZE;
                        pfn_t pfn = phys_to_pfn_t(address + off, PFN_DEV);
                        vmf = vmf_insert_mixed(vma, vma->vm_start + off, pfn);
                        if (vmf & VM_FAULT_ERROR)
                                ret = vm_fault_to_errno(vmf, 0);
                }
        }

        if (!ret)
                pr_debug("mapped %s[%lu] at 0x%08lx (%u/%lu pages) "
                         "to vma 0x%08lx, page_prot 0x%llx\n",
                         file_dentry(file)->d_name.name, pgoff,
                         address, pages, vma_pages(vma), vma->vm_start,
                         (unsigned long long)pgprot_val(vma->vm_page_prot));
        return ret;

bailout:
        pr_debug("%s[%lu]: direct mmap impossible: %s\n",
                 file_dentry(file)->d_name.name, pgoff, bailout_reason);
        /* Didn't manage any direct map, but normal paging is still possible */
        return 0;
}

#else /* CONFIG_MMU */

static int cramfs_physmem_mmap(struct file *file, struct vm_area_struct *vma)
{
        return vma->vm_flags & (VM_SHARED | VM_MAYSHARE) ? 0 : -ENOSYS;
}

static unsigned long cramfs_physmem_get_unmapped_area(struct file *file,
                        unsigned long addr, unsigned long len,
                        unsigned long pgoff, unsigned long flags)
{
        struct inode *inode = file_inode(file);
        struct super_block *sb = inode->i_sb;
        struct cramfs_sb_info *sbi = CRAMFS_SB(sb);
        unsigned int pages, block_pages, max_pages, offset;

        pages = (len + PAGE_SIZE - 1) >> PAGE_SHIFT;
        max_pages = (inode->i_size + PAGE_SIZE - 1) >> PAGE_SHIFT;
        if (pgoff >= max_pages || pages > max_pages - pgoff)
                return -EINVAL;
        block_pages = pages;
        offset = cramfs_get_block_range(inode, pgoff, &block_pages);
        if (!offset || block_pages != pages)
                return -ENOSYS;
        addr = sbi->linear_phys_addr + offset;
        pr_debug("get_unmapped for %s ofs %#lx siz %lu at 0x%08lx\n",
                 file_dentry(file)->d_name.name, pgoff*PAGE_SIZE, len, addr);
        return addr;
}

static unsigned int cramfs_physmem_mmap_capabilities(struct file *file)
{
        return NOMMU_MAP_COPY | NOMMU_MAP_DIRECT |
               NOMMU_MAP_READ | NOMMU_MAP_EXEC;
}

#endif /* CONFIG_MMU */

static const struct file_operations cramfs_physmem_fops = {
        .llseek                 = generic_file_llseek,
        .read_iter              = generic_file_read_iter,
        .splice_read            = generic_file_splice_read,
        .mmap                   = cramfs_physmem_mmap,
#ifndef CONFIG_MMU
        .get_unmapped_area      = cramfs_physmem_get_unmapped_area,
        .mmap_capabilities      = cramfs_physmem_mmap_capabilities,
#endif
};

static void cramfs_kill_sb(struct super_block *sb)
{
        struct cramfs_sb_info *sbi = CRAMFS_SB(sb);

        if (IS_ENABLED(CONFIG_CRAMFS_MTD) && sb->s_mtd) {
                if (sbi && sbi->mtd_point_size)
                        mtd_unpoint(sb->s_mtd, 0, sbi->mtd_point_size);
                kill_mtd_super(sb);
        } else if (IS_ENABLED(CONFIG_CRAMFS_BLOCKDEV) && sb->s_bdev) {
                kill_block_super(sb);
        }
        kfree(sbi);
}

static int cramfs_reconfigure(struct fs_context *fc)
{
        sync_filesystem(fc->root->d_sb);
        fc->sb_flags |= SB_RDONLY;
        return 0;
}

static int cramfs_read_super(struct super_block *sb, struct fs_context *fc,
                             struct cramfs_super *super)
{
        struct cramfs_sb_info *sbi = CRAMFS_SB(sb);
        unsigned long root_offset;
        bool silent = fc->sb_flags & SB_SILENT;

        /* We don't know the real size yet */
        sbi->size = PAGE_SIZE;

        /* Read the first block and get the superblock from it */
        mutex_lock(&read_mutex);
        memcpy(super, cramfs_read(sb, 0, sizeof(*super)), sizeof(*super));
        mutex_unlock(&read_mutex);

        /* Do sanity checks on the superblock */
        if (super->magic != CRAMFS_MAGIC) {
                /* check for wrong endianness */
                if (super->magic == CRAMFS_MAGIC_WEND) {
                        if (!silent)
                                errorfc(fc, "wrong endianness");
                        return -EINVAL;
                }

                /* check at 512 byte offset */
                mutex_lock(&read_mutex);
                memcpy(super,
                       cramfs_read(sb, 512, sizeof(*super)),
                       sizeof(*super));
                mutex_unlock(&read_mutex);
                if (super->magic != CRAMFS_MAGIC) {
                        if (super->magic == CRAMFS_MAGIC_WEND && !silent)
                                errorfc(fc, "wrong endianness");
                        else if (!silent)
                                errorfc(fc, "wrong magic");
                        return -EINVAL;
                }
        }

        /* get feature flags first */
        if (super->flags & ~CRAMFS_SUPPORTED_FLAGS) {
                errorfc(fc, "unsupported filesystem features");
                return -EINVAL;
        }

        /* Check that the root inode is in a sane state */
        if (!S_ISDIR(super->root.mode)) {
                errorfc(fc, "root is not a directory");
                return -EINVAL;
        }
        /* correct strange, hard-coded permissions of mkcramfs */
        super->root.mode |= 0555;

        root_offset = super->root.offset << 2;
        if (super->flags & CRAMFS_FLAG_FSID_VERSION_2) {
                sbi->size = super->size;
                sbi->blocks = super->fsid.blocks;
                sbi->files = super->fsid.files;
        } else {
                sbi->size = 1<<28;
                sbi->blocks = 0;
                sbi->files = 0;
        }
        sbi->magic = super->magic;
        sbi->flags = super->flags;
        if (root_offset == 0)
                infofc(fc, "empty filesystem");
        else if (!(super->flags & CRAMFS_FLAG_SHIFTED_ROOT_OFFSET) &&
                 ((root_offset != sizeof(struct cramfs_super)) &&
                  (root_offset != 512 + sizeof(struct cramfs_super))))
        {
                errorfc(fc, "bad root offset %lu", root_offset);
                return -EINVAL;
        }

        return 0;
}

static int cramfs_finalize_super(struct super_block *sb,
                                 struct cramfs_inode *cramfs_root)
{
        struct inode *root;

        /* Set it all up.. */
        sb->s_flags |= SB_RDONLY;
        sb->s_time_min = 0;
        sb->s_time_max = 0;
        sb->s_op = &cramfs_ops;
        root = get_cramfs_inode(sb, cramfs_root, 0);
        if (IS_ERR(root))
                return PTR_ERR(root);
        sb->s_root = d_make_root(root);
        if (!sb->s_root)
                return -ENOMEM;
        return 0;
}

static int cramfs_blkdev_fill_super(struct super_block *sb, struct fs_context *fc)
{
        struct cramfs_sb_info *sbi;
        struct cramfs_super super;
        int i, err;

        sbi = kzalloc(sizeof(struct cramfs_sb_info), GFP_KERNEL);
        if (!sbi)
                return -ENOMEM;
        sb->s_fs_info = sbi;

        /* Invalidate the read buffers on mount: think disk change.. */
        for (i = 0; i < READ_BUFFERS; i++)
                buffer_blocknr[i] = -1;

        err = cramfs_read_super(sb, fc, &super);
        if (err)
                return err;
        return cramfs_finalize_super(sb, &super.root);
}

static int cramfs_mtd_fill_super(struct super_block *sb, struct fs_context *fc)
{
        struct cramfs_sb_info *sbi;
        struct cramfs_super super;
        int err;

        sbi = kzalloc(sizeof(struct cramfs_sb_info), GFP_KERNEL);
        if (!sbi)
                return -ENOMEM;
        sb->s_fs_info = sbi;

        /* Map only one page for now.  Will remap it when fs size is known. */
        err = mtd_point(sb->s_mtd, 0, PAGE_SIZE, &sbi->mtd_point_size,
                        &sbi->linear_virt_addr, &sbi->linear_phys_addr);
        if (err || sbi->mtd_point_size != PAGE_SIZE) {
                pr_err("unable to get direct memory access to mtd:%s\n",
                       sb->s_mtd->name);
                return err ? : -ENODATA;
        }

        pr_info("checking physical address %pap for linear cramfs image\n",
                &sbi->linear_phys_addr);
        err = cramfs_read_super(sb, fc, &super);
        if (err)
                return err;

        /* Remap the whole filesystem now */
        pr_info("linear cramfs image on mtd:%s appears to be %lu KB in size\n",
                sb->s_mtd->name, sbi->size/1024);
        mtd_unpoint(sb->s_mtd, 0, PAGE_SIZE);
        err = mtd_point(sb->s_mtd, 0, sbi->size, &sbi->mtd_point_size,
                        &sbi->linear_virt_addr, &sbi->linear_phys_addr);
        if (err || sbi->mtd_point_size != sbi->size) {
                pr_err("unable to get direct memory access to mtd:%s\n",
                       sb->s_mtd->name);
                return err ? : -ENODATA;
        }

        return cramfs_finalize_super(sb, &super.root);
}

static int cramfs_statfs(struct dentry *dentry, struct kstatfs *buf)
{
        struct super_block *sb = dentry->d_sb;
        u64 id = 0;

        if (sb->s_bdev)
                id = huge_encode_dev(sb->s_bdev->bd_dev);
        else if (sb->s_dev)
                id = huge_encode_dev(sb->s_dev);

        buf->f_type = CRAMFS_MAGIC;
        buf->f_bsize = PAGE_SIZE;
        buf->f_blocks = CRAMFS_SB(sb)->blocks;
        buf->f_bfree = 0;
        buf->f_bavail = 0;
        buf->f_files = CRAMFS_SB(sb)->files;
        buf->f_ffree = 0;
        buf->f_fsid.val[0] = (u32)id;
        buf->f_fsid.val[1] = (u32)(id >> 32);
        buf->f_namelen = CRAMFS_MAXPATHLEN;
        return 0;
}

/*
 * Read a cramfs directory entry.
 */
static int cramfs_readdir(struct file *file, struct dir_context *ctx)
{
        struct inode *inode = file_inode(file);
        struct super_block *sb = inode->i_sb;
        char *buf;
        unsigned int offset;

        /* Offset within the thing. */
        if (ctx->pos >= inode->i_size)
                return 0;
        offset = ctx->pos;
        /* Directory entries are always 4-byte aligned */
        if (offset & 3)
                return -EINVAL;

        buf = kmalloc(CRAMFS_MAXPATHLEN, GFP_KERNEL);
        if (!buf)
                return -ENOMEM;

        while (offset < inode->i_size) {
                struct cramfs_inode *de;
                unsigned long nextoffset;
                char *name;
                ino_t ino;
                umode_t mode;
                int namelen;

                mutex_lock(&read_mutex);
                de = cramfs_read(sb, OFFSET(inode) + offset, sizeof(*de)+CRAMFS_MAXPATHLEN);
                name = (char *)(de+1);

                /*
                 * Namelengths on disk are shifted by two
                 * and the name padded out to 4-byte boundaries
                 * with zeroes.
                 */
                namelen = de->namelen << 2;
                memcpy(buf, name, namelen);
                ino = cramino(de, OFFSET(inode) + offset);
                mode = de->mode;
                mutex_unlock(&read_mutex);
                nextoffset = offset + sizeof(*de) + namelen;
                for (;;) {
                        if (!namelen) {
                                kfree(buf);
                                return -EIO;
                        }
                        if (buf[namelen-1])
                                break;
                        namelen--;
                }
                if (!dir_emit(ctx, buf, namelen, ino, mode >> 12))
                        break;

                ctx->pos = offset = nextoffset;
        }
        kfree(buf);
        return 0;
}

/*
 * Lookup and fill in the inode data..
 */
static struct dentry *cramfs_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
{
        unsigned int offset = 0;
        struct inode *inode = NULL;
        int sorted;

        mutex_lock(&read_mutex);
        sorted = CRAMFS_SB(dir->i_sb)->flags & CRAMFS_FLAG_SORTED_DIRS;
        while (offset < dir->i_size) {
                struct cramfs_inode *de;
                char *name;
                int namelen, retval;
                int dir_off = OFFSET(dir) + offset;

                de = cramfs_read(dir->i_sb, dir_off, sizeof(*de)+CRAMFS_MAXPATHLEN);
                name = (char *)(de+1);

                /* Try to take advantage of sorted directories */
                if (sorted && (dentry->d_name.name[0] < name[0]))
                        break;

                namelen = de->namelen << 2;
                offset += sizeof(*de) + namelen;

                /* Quick check that the name is roughly the right length */
                if (((dentry->d_name.len + 3) & ~3) != namelen)
                        continue;

                for (;;) {
                        if (!namelen) {
                                inode = ERR_PTR(-EIO);
                                goto out;
                        }
                        if (name[namelen-1])
                                break;
                        namelen--;
                }
                if (namelen != dentry->d_name.len)
                        continue;
                retval = memcmp(dentry->d_name.name, name, namelen);
                if (retval > 0)
                        continue;
                if (!retval) {
                        inode = get_cramfs_inode(dir->i_sb, de, dir_off);
                        break;
                }
                /* else (retval < 0) */
                if (sorted)
                        break;
        }
out:
        mutex_unlock(&read_mutex);
        return d_splice_alias(inode, dentry);
}

static int cramfs_readpage(struct file *file, struct page *page)
{
        struct inode *inode = page->mapping->host;
        u32 maxblock;
        int bytes_filled;
        void *pgdata;

        maxblock = (inode->i_size + PAGE_SIZE - 1) >> PAGE_SHIFT;
        bytes_filled = 0;
        pgdata = kmap(page);

        if (page->index < maxblock) {
                struct super_block *sb = inode->i_sb;
                u32 blkptr_offset = OFFSET(inode) + page->index * 4;
                u32 block_ptr, block_start, block_len;
                bool uncompressed, direct;

                mutex_lock(&read_mutex);
                block_ptr = *(u32 *) cramfs_read(sb, blkptr_offset, 4);
                uncompressed = (block_ptr & CRAMFS_BLK_FLAG_UNCOMPRESSED);
                direct = (block_ptr & CRAMFS_BLK_FLAG_DIRECT_PTR);
                block_ptr &= ~CRAMFS_BLK_FLAGS;

                if (direct) {
                        /*
                         * The block pointer is an absolute start pointer,
                         * shifted by 2 bits. The size is included in the
                         * first 2 bytes of the data block when compressed,
                         * or PAGE_SIZE otherwise.
                         */
                        block_start = block_ptr << CRAMFS_BLK_DIRECT_PTR_SHIFT;
                        if (uncompressed) {
                                block_len = PAGE_SIZE;
                                /* if last block: cap to file length */
                                if (page->index == maxblock - 1)
                                        block_len =
                                                offset_in_page(inode->i_size);
                        } else {
                                block_len = *(u16 *)
                                        cramfs_read(sb, block_start, 2);
                                block_start += 2;
                        }
                } else {
                        /*
                         * The block pointer indicates one past the end of
                         * the current block (start of next block). If this
                         * is the first block then it starts where the block
                         * pointer table ends, otherwise its start comes
                         * from the previous block's pointer.
                         */
                        block_start = OFFSET(inode) + maxblock * 4;
                        if (page->index)
                                block_start = *(u32 *)
                                        cramfs_read(sb, blkptr_offset - 4, 4);
                        /* Beware... previous ptr might be a direct ptr */
                        if (unlikely(block_start & CRAMFS_BLK_FLAG_DIRECT_PTR)) {
                                /* See comments on earlier code. */
                                u32 prev_start = block_start;
                                block_start = prev_start & ~CRAMFS_BLK_FLAGS;
                                block_start <<= CRAMFS_BLK_DIRECT_PTR_SHIFT;
                                if (prev_start & CRAMFS_BLK_FLAG_UNCOMPRESSED) {
                                        block_start += PAGE_SIZE;
                                } else {
                                        block_len = *(u16 *)
                                                cramfs_read(sb, block_start, 2);
                                        block_start += 2 + block_len;
                                }
                        }
                        block_start &= ~CRAMFS_BLK_FLAGS;
                        block_len = block_ptr - block_start;
                }

                if (block_len == 0)
                        ; /* hole */
                else if (unlikely(block_len > 2*PAGE_SIZE ||
                                  (uncompressed && block_len > PAGE_SIZE))) {
                        mutex_unlock(&read_mutex);
                        pr_err("bad data blocksize %u\n", block_len);
                        goto err;
                } else if (uncompressed) {
                        memcpy(pgdata,
                               cramfs_read(sb, block_start, block_len),
                               block_len);
                        bytes_filled = block_len;
                } else {
                        bytes_filled = cramfs_uncompress_block(pgdata,
                                 PAGE_SIZE,
                                 cramfs_read(sb, block_start, block_len),
                                 block_len);
                }
                mutex_unlock(&read_mutex);
                if (unlikely(bytes_filled < 0))
                        goto err;
        }

        memset(pgdata + bytes_filled, 0, PAGE_SIZE - bytes_filled);
        flush_dcache_page(page);
        kunmap(page);
        SetPageUptodate(page);
        unlock_page(page);
        return 0;

err:
        kunmap(page);
        ClearPageUptodate(page);
        SetPageError(page);
        unlock_page(page);
        return 0;
}

static const struct address_space_operations cramfs_aops = {
        .readpage = cramfs_readpage
};

/*
 * Our operations:
 */

/*
 * A directory can only readdir
 */
static const struct file_operations cramfs_directory_operations = {
        .llseek         = generic_file_llseek,
        .read           = generic_read_dir,
        .iterate_shared = cramfs_readdir,
};

static const struct inode_operations cramfs_dir_inode_operations = {
        .lookup         = cramfs_lookup,
};

static const struct super_operations cramfs_ops = {
        .statfs         = cramfs_statfs,
};

static int cramfs_get_tree(struct fs_context *fc)
{
        int ret = -ENOPROTOOPT;

        if (IS_ENABLED(CONFIG_CRAMFS_MTD)) {
                ret = get_tree_mtd(fc, cramfs_mtd_fill_super);
                if (!ret)
                        return 0;
        }
        if (IS_ENABLED(CONFIG_CRAMFS_BLOCKDEV))
                ret = get_tree_bdev(fc, cramfs_blkdev_fill_super);
        return ret;
}

static const struct fs_context_operations cramfs_context_ops = {
        .get_tree       = cramfs_get_tree,
        .reconfigure    = cramfs_reconfigure,
};

/*
 * Set up the filesystem mount context.
 */
static int cramfs_init_fs_context(struct fs_context *fc)
{
        fc->ops = &cramfs_context_ops;
        return 0;
}

static struct file_system_type cramfs_fs_type = {
        .owner          = THIS_MODULE,
        .name           = "cramfs",
        .init_fs_context = cramfs_init_fs_context,
        .kill_sb        = cramfs_kill_sb,
        .fs_flags       = FS_REQUIRES_DEV,
};
MODULE_ALIAS_FS("cramfs");

static int __init init_cramfs_fs(void)
{
        int rv;

        rv = cramfs_uncompress_init();
        if (rv < 0)
                return rv;
        rv = register_filesystem(&cramfs_fs_type);
        if (rv < 0)

                cramfs_uncompress_exit();
        return rv;
}

static void __exit exit_cramfs_fs(void)
{
        cramfs_uncompress_exit();
        unregister_filesystem(&cramfs_fs_type);
}

module_init(init_cramfs_fs)
module_exit(exit_cramfs_fs)
MODULE_LICENSE("GPL");