LXR linux/arch/arm64/kernel/module-plts.c

   1/*
   2 * Copyright (C) 2014-2016 Linaro Ltd. <ard.biesheuvel@linaro.org>
   3 *
   4 * This program is free software; you can redistribute it and/or modify
   5 * it under the terms of the GNU General Public License version 2 as
   6 * published by the Free Software Foundation.
   7 */
   8
   9#include <linux/elf.h>
  10#include <linux/kernel.h>
  11#include <linux/module.h>
  12#include <linux/sort.h>
  13
  14struct plt_entry {
  15        /*
  16         * A program that conforms to the AArch64 Procedure Call Standard
  17         * (AAPCS64) must assume that a veneer that alters IP0 (x16) and/or
  18         * IP1 (x17) may be inserted at any branch instruction that is
  19         * exposed to a relocation that supports long branches. Since that
  20         * is exactly what we are dealing with here, we are free to use x16
  21         * as a scratch register in the PLT veneers.
  22         */
  23        __le32  mov0;   /* movn x16, #0x....                    */
  24        __le32  mov1;   /* movk x16, #0x...., lsl #16           */
  25        __le32  mov2;   /* movk x16, #0x...., lsl #32           */
  26        __le32  br;     /* br   x16                             */
  27};
  28
  29u64 module_emit_plt_entry(struct module *mod, const Elf64_Rela *rela,
  30                          Elf64_Sym *sym)
  31{
  32        struct plt_entry *plt = (struct plt_entry *)mod->arch.plt->sh_addr;
  33        int i = mod->arch.plt_num_entries;
  34        u64 val = sym->st_value + rela->r_addend;
  35
  36        /*
  37         * We only emit PLT entries against undefined (SHN_UNDEF) symbols,
  38         * which are listed in the ELF symtab section, but without a type
  39         * or a size.
  40         * So, similar to how the module loader uses the Elf64_Sym::st_value
  41         * field to store the resolved addresses of undefined symbols, let's
  42         * borrow the Elf64_Sym::st_size field (whose value is never used by
  43         * the module loader, even for symbols that are defined) to record
  44         * the address of a symbol's associated PLT entry as we emit it for a
  45         * zero addend relocation (which is the only kind we have to deal with
  46         * in practice). This allows us to find duplicates without having to
  47         * go through the table every time.
  48         */
  49        if (rela->r_addend == 0 && sym->st_size != 0) {
  50                BUG_ON(sym->st_size < (u64)plt || sym->st_size >= (u64)&plt[i]);
  51                return sym->st_size;
  52        }
  53
  54        mod->arch.plt_num_entries++;
  55        BUG_ON(mod->arch.plt_num_entries > mod->arch.plt_max_entries);
  56
  57        /*
  58         * MOVK/MOVN/MOVZ opcode:
  59         * +--------+------------+--------+-----------+-------------+---------+
  60         * | sf[31] | opc[30:29] | 100101 | hw[22:21] | imm16[20:5] | Rd[4:0] |
  61         * +--------+------------+--------+-----------+-------------+---------+
  62         *
  63         * Rd     := 0x10 (x16)
  64         * hw     := 0b00 (no shift), 0b01 (lsl #16), 0b10 (lsl #32)
  65         * opc    := 0b11 (MOVK), 0b00 (MOVN), 0b10 (MOVZ)
  66         * sf     := 1 (64-bit variant)
  67         */
  68        plt[i] = (struct plt_entry){
  69                cpu_to_le32(0x92800010 | (((~val      ) & 0xffff)) << 5),
  70                cpu_to_le32(0xf2a00010 | ((( val >> 16) & 0xffff)) << 5),
  71                cpu_to_le32(0xf2c00010 | ((( val >> 32) & 0xffff)) << 5),
  72                cpu_to_le32(0xd61f0200)
  73        };
  74
  75        if (rela->r_addend == 0)
  76                sym->st_size = (u64)&plt[i];
  77
  78        return (u64)&plt[i];
  79}
  80
  81#define cmp_3way(a,b)   ((a) < (b) ? -1 : (a) > (b))
  82
  83static int cmp_rela(const void *a, const void *b)
  84{
  85        const Elf64_Rela *x = a, *y = b;
  86        int i;
  87
  88        /* sort by type, symbol index and addend */
  89        i = cmp_3way(ELF64_R_TYPE(x->r_info), ELF64_R_TYPE(y->r_info));
  90        if (i == 0)
  91                i = cmp_3way(ELF64_R_SYM(x->r_info), ELF64_R_SYM(y->r_info));
  92        if (i == 0)
  93                i = cmp_3way(x->r_addend, y->r_addend);
  94        return i;
  95}
  96
  97static bool duplicate_rel(const Elf64_Rela *rela, int num)
  98{
  99        /*
 100         * Entries are sorted by type, symbol index and addend. That means
 101         * that, if a duplicate entry exists, it must be in the preceding
 102         * slot.
 103         */
 104        return num > 0 && cmp_rela(rela + num, rela + num - 1) == 0;
 105}
 106
 107static unsigned int count_plts(Elf64_Sym *syms, Elf64_Rela *rela, int num)
 108{
 109        unsigned int ret = 0;
 110        Elf64_Sym *s;
 111        int i;
 112
 113        for (i = 0; i < num; i++) {
 114                switch (ELF64_R_TYPE(rela[i].r_info)) {
 115                case R_AARCH64_JUMP26:
 116                case R_AARCH64_CALL26:
 117                        /*
 118                         * We only have to consider branch targets that resolve
 119                         * to undefined symbols. This is not simply a heuristic,
 120                         * it is a fundamental limitation, since the PLT itself
 121                         * is part of the module, and needs to be within 128 MB
 122                         * as well, so modules can never grow beyond that limit.
 123                         */
 124                        s = syms + ELF64_R_SYM(rela[i].r_info);
 125                        if (s->st_shndx != SHN_UNDEF)
 126                                break;
 127
 128                        /*
 129                         * Jump relocations with non-zero addends against
 130                         * undefined symbols are supported by the ELF spec, but
 131                         * do not occur in practice (e.g., 'jump n bytes past
 132                         * the entry point of undefined function symbol f').
 133                         * So we need to support them, but there is no need to
 134                         * take them into consideration when trying to optimize
 135                         * this code. So let's only check for duplicates when
 136                         * the addend is zero: this allows us to record the PLT
 137                         * entry address in the symbol table itself, rather than
 138                         * having to search the list for duplicates each time we
 139                         * emit one.
 140                         */
 141                        if (rela[i].r_addend != 0 || !duplicate_rel(rela, i))
 142                                ret++;
 143                        break;
 144                }
 145        }
 146        return ret;
 147}
 148
 149int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 150                              char *secstrings, struct module *mod)
 151{
 152        unsigned long plt_max_entries = 0;
 153        Elf64_Sym *syms = NULL;
 154        int i;
 155
 156        /*
 157         * Find the empty .plt section so we can expand it to store the PLT
 158         * entries. Record the symtab address as well.
 159         */
 160        for (i = 0; i < ehdr->e_shnum; i++) {
 161                if (strcmp(".plt", secstrings + sechdrs[i].sh_name) == 0)
 162                        mod->arch.plt = sechdrs + i;
 163                else if (sechdrs[i].sh_type == SHT_SYMTAB)
 164                        syms = (Elf64_Sym *)sechdrs[i].sh_addr;
 165        }
 166
 167        if (!mod->arch.plt) {
 168                pr_err("%s: module PLT section missing\n", mod->name);
 169                return -ENOEXEC;
 170        }
 171        if (!syms) {
 172                pr_err("%s: module symtab section missing\n", mod->name);
 173                return -ENOEXEC;
 174        }
 175
 176        for (i = 0; i < ehdr->e_shnum; i++) {
 177                Elf64_Rela *rels = (void *)ehdr + sechdrs[i].sh_offset;
 178                int numrels = sechdrs[i].sh_size / sizeof(Elf64_Rela);
 179                Elf64_Shdr *dstsec = sechdrs + sechdrs[i].sh_info;
 180
 181                if (sechdrs[i].sh_type != SHT_RELA)
 182                        continue;
 183
 184                /* ignore relocations that operate on non-exec sections */
 185                if (!(dstsec->sh_flags & SHF_EXECINSTR))
 186                        continue;
 187
 188                /* sort by type, symbol index and addend */
 189                sort(rels, numrels, sizeof(Elf64_Rela), cmp_rela, NULL);
 190
 191                plt_max_entries += count_plts(syms, rels, numrels);
 192        }
 193
 194        mod->arch.plt->sh_type = SHT_NOBITS;
 195        mod->arch.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
 196        mod->arch.plt->sh_addralign = L1_CACHE_BYTES;
 197        mod->arch.plt->sh_size = plt_max_entries * sizeof(struct plt_entry);
 198        mod->arch.plt_num_entries = 0;
 199        mod->arch.plt_max_entries = plt_max_entries;
 200        return 0;
 201}
 202