arch/arm64/kernel/module-plts.c

   1 /*
   2  * Copyright (C) 2014-2017 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
  14 static bool in_init(const struct module *mod, void *loc)
  15 {
  16         return (u64)loc - (u64)mod->init_layout.base < mod->init_layout.size;
  17 }
  18
  19 u64 module_emit_plt_entry(struct module *mod, void *loc, const Elf64_Rela *rela,
  20                           Elf64_Sym *sym)
  21 {
  22         struct mod_plt_sec *pltsec = !in_init(mod, loc) ? &mod->arch.core :
  23                                                           &mod->arch.init;
  24         struct plt_entry *plt = (struct plt_entry *)pltsec->plt->sh_addr;
  25         int i = pltsec->plt_num_entries;
  26         u64 val = sym->st_value + rela->r_addend;
  27
  28         plt[i] = get_plt_entry(val);
  29
  30         /*
  31          * Check if the entry we just created is a duplicate. Given that the
  32          * relocations are sorted, this will be the last entry we allocated.
  33          * (if one exists).
  34          */
  35         if (i > 0 && plt_entries_equal(plt + i, plt + i - 1))
  36                 return (u64)&plt[i - 1];
  37
  38         pltsec->plt_num_entries++;
  39         BUG_ON(pltsec->plt_num_entries > pltsec->plt_max_entries);
  40
  41         return (u64)&plt[i];
  42 }
  43
  44 #define cmp_3way(a,b)   ((a) < (b) ? -1 : (a) > (b))
  45
  46 static int cmp_rela(const void *a, const void *b)
  47 {
  48         const Elf64_Rela *x = a, *y = b;
  49         int i;
  50
  51         /* sort by type, symbol index and addend */
  52         i = cmp_3way(ELF64_R_TYPE(x->r_info), ELF64_R_TYPE(y->r_info));
  53         if (i == 0)
  54                 i = cmp_3way(ELF64_R_SYM(x->r_info), ELF64_R_SYM(y->r_info));
  55         if (i == 0)
  56                 i = cmp_3way(x->r_addend, y->r_addend);
  57         return i;
  58 }
  59
  60 static bool duplicate_rel(const Elf64_Rela *rela, int num)
  61 {
  62         /*
  63          * Entries are sorted by type, symbol index and addend. That means
  64          * that, if a duplicate entry exists, it must be in the preceding
  65          * slot.
  66          */
  67         return num > 0 && cmp_rela(rela + num, rela + num - 1) == 0;
  68 }
  69
  70 static unsigned int count_plts(Elf64_Sym *syms, Elf64_Rela *rela, int num,
  71                                Elf64_Word dstidx)
  72 {
  73         unsigned int ret = 0;
  74         Elf64_Sym *s;
  75         int i;
  76
  77         for (i = 0; i < num; i++) {
  78                 switch (ELF64_R_TYPE(rela[i].r_info)) {
  79                 case R_AARCH64_JUMP26:
  80                 case R_AARCH64_CALL26:
  81                         /*
  82                          * We only have to consider branch targets that resolve
  83                          * to symbols that are defined in a different section.
  84                          * This is not simply a heuristic, it is a fundamental
  85                          * limitation, since there is no guaranteed way to emit
  86                          * PLT entries sufficiently close to the branch if the
  87                          * section size exceeds the range of a branch
  88                          * instruction. So ignore relocations against defined
  89                          * symbols if they live in the same section as the
  90                          * relocation target.
  91                          */
  92                         s = syms + ELF64_R_SYM(rela[i].r_info);
  93                         if (s->st_shndx == dstidx)
  94                                 break;
  95
  96                         /*
  97                          * Jump relocations with non-zero addends against
  98                          * undefined symbols are supported by the ELF spec, but
  99                          * do not occur in practice (e.g., 'jump n bytes past
 100                          * the entry point of undefined function symbol f').
 101                          * So we need to support them, but there is no need to
 102                          * take them into consideration when trying to optimize
 103                          * this code. So let's only check for duplicates when
 104                          * the addend is zero: this allows us to record the PLT
 105                          * entry address in the symbol table itself, rather than
 106                          * having to search the list for duplicates each time we
 107                          * emit one.
 108                          */
 109                         if (rela[i].r_addend != 0 || !duplicate_rel(rela, i))
 110                                 ret++;
 111                         break;
 112                 }
 113         }
 114         return ret;
 115 }
 116
 117 int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 118                               char *secstrings, struct module *mod)
 119 {
 120         unsigned long core_plts = 0;
 121         unsigned long init_plts = 0;
 122         Elf64_Sym *syms = NULL;
 123         Elf_Shdr *tramp = NULL;
 124         int i;
 125
 126         /*
 127          * Find the empty .plt section so we can expand it to store the PLT
 128          * entries. Record the symtab address as well.
 129          */
 130         for (i = 0; i < ehdr->e_shnum; i++) {
 131                 if (!strcmp(secstrings + sechdrs[i].sh_name, ".plt"))
 132                         mod->arch.core.plt = sechdrs + i;
 133                 else if (!strcmp(secstrings + sechdrs[i].sh_name, ".init.plt"))
 134                         mod->arch.init.plt = sechdrs + i;
 135                 else if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
 136                          !strcmp(secstrings + sechdrs[i].sh_name,
 137                                  ".text.ftrace_trampoline"))
 138                         tramp = sechdrs + i;
 139                 else if (sechdrs[i].sh_type == SHT_SYMTAB)
 140                         syms = (Elf64_Sym *)sechdrs[i].sh_addr;
 141         }
 142
 143         if (!mod->arch.core.plt || !mod->arch.init.plt) {
 144                 pr_err("%s: module PLT section(s) missing\n", mod->name);
 145                 return -ENOEXEC;
 146         }
 147         if (!syms) {
 148                 pr_err("%s: module symtab section missing\n", mod->name);
 149                 return -ENOEXEC;
 150         }
 151
 152         for (i = 0; i < ehdr->e_shnum; i++) {
 153                 Elf64_Rela *rels = (void *)ehdr + sechdrs[i].sh_offset;
 154                 int numrels = sechdrs[i].sh_size / sizeof(Elf64_Rela);
 155                 Elf64_Shdr *dstsec = sechdrs + sechdrs[i].sh_info;
 156
 157                 if (sechdrs[i].sh_type != SHT_RELA)
 158                         continue;
 159
 160                 /* ignore relocations that operate on non-exec sections */
 161                 if (!(dstsec->sh_flags & SHF_EXECINSTR))
 162                         continue;
 163
 164                 /* sort by type, symbol index and addend */
 165                 sort(rels, numrels, sizeof(Elf64_Rela), cmp_rela, NULL);
 166
 167                 if (strncmp(secstrings + dstsec->sh_name, ".init", 5) != 0)
 168                         core_plts += count_plts(syms, rels, numrels,
 169                                                 sechdrs[i].sh_info);
 170                 else
 171                         init_plts += count_plts(syms, rels, numrels,
 172                                                 sechdrs[i].sh_info);
 173         }
 174
 175         mod->arch.core.plt->sh_type = SHT_NOBITS;
 176         mod->arch.core.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
 177         mod->arch.core.plt->sh_addralign = L1_CACHE_BYTES;
 178         mod->arch.core.plt->sh_size = (core_plts  + 1) * sizeof(struct plt_entry);
 179         mod->arch.core.plt_num_entries = 0;
 180         mod->arch.core.plt_max_entries = core_plts;
 181
 182         mod->arch.init.plt->sh_type = SHT_NOBITS;
 183         mod->arch.init.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
 184         mod->arch.init.plt->sh_addralign = L1_CACHE_BYTES;
 185         mod->arch.init.plt->sh_size = (init_plts + 1) * sizeof(struct plt_entry);
 186         mod->arch.init.plt_num_entries = 0;
 187         mod->arch.init.plt_max_entries = init_plts;
 188
 189         if (tramp) {
 190                 tramp->sh_type = SHT_NOBITS;
 191                 tramp->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
 192                 tramp->sh_addralign = __alignof__(struct plt_entry);
 193                 tramp->sh_size = sizeof(struct plt_entry);
 194         }
 195
 196         return 0;
 197 }