arch/arm/kernel/module-plts.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * Copyright (C) 2014-2017 Linaro Ltd. <ard.biesheuvel@linaro.org>
   4  */
   5
   6 #include <linux/elf.h>
   7 #include <linux/kernel.h>
   8 #include <linux/module.h>
   9 #include <linux/sort.h>
  10
  11 #include <asm/cache.h>
  12 #include <asm/opcodes.h>
  13
  14 #define PLT_ENT_STRIDE          L1_CACHE_BYTES
  15 #define PLT_ENT_COUNT           (PLT_ENT_STRIDE / sizeof(u32))
  16 #define PLT_ENT_SIZE            (sizeof(struct plt_entries) / PLT_ENT_COUNT)
  17
  18 #ifdef CONFIG_THUMB2_KERNEL
  19 #define PLT_ENT_LDR             __opcode_to_mem_thumb32(0xf8dff000 | \
  20                                                         (PLT_ENT_STRIDE - 4))
  21 #else
  22 #define PLT_ENT_LDR             __opcode_to_mem_arm(0xe59ff000 | \
  23                                                     (PLT_ENT_STRIDE - 8))
  24 #endif
  25
  26 struct plt_entries {
  27         u32     ldr[PLT_ENT_COUNT];
  28         u32     lit[PLT_ENT_COUNT];
  29 };
  30
  31 static bool in_init(const struct module *mod, unsigned long loc)
  32 {
  33         return loc - (u32)mod->init_layout.base < mod->init_layout.size;
  34 }
  35
  36 u32 get_module_plt(struct module *mod, unsigned long loc, Elf32_Addr val)
  37 {
  38         struct mod_plt_sec *pltsec = !in_init(mod, loc) ? &mod->arch.core :
  39                                                           &mod->arch.init;
  40
  41         struct plt_entries *plt = (struct plt_entries *)pltsec->plt->sh_addr;
  42         int idx = 0;
  43
  44         /*
  45          * Look for an existing entry pointing to 'val'. Given that the
  46          * relocations are sorted, this will be the last entry we allocated.
  47          * (if one exists).
  48          */
  49         if (pltsec->plt_count > 0) {
  50                 plt += (pltsec->plt_count - 1) / PLT_ENT_COUNT;
  51                 idx = (pltsec->plt_count - 1) % PLT_ENT_COUNT;
  52
  53                 if (plt->lit[idx] == val)
  54                         return (u32)&plt->ldr[idx];
  55
  56                 idx = (idx + 1) % PLT_ENT_COUNT;
  57                 if (!idx)
  58                         plt++;
  59         }
  60
  61         pltsec->plt_count++;
  62         BUG_ON(pltsec->plt_count * PLT_ENT_SIZE > pltsec->plt->sh_size);
  63
  64         if (!idx)
  65                 /* Populate a new set of entries */
  66                 *plt = (struct plt_entries){
  67                         { [0 ... PLT_ENT_COUNT - 1] = PLT_ENT_LDR, },
  68                         { val, }
  69                 };
  70         else
  71                 plt->lit[idx] = val;
  72
  73         return (u32)&plt->ldr[idx];
  74 }
  75
  76 #define cmp_3way(a,b)   ((a) < (b) ? -1 : (a) > (b))
  77
  78 static int cmp_rel(const void *a, const void *b)
  79 {
  80         const Elf32_Rel *x = a, *y = b;
  81         int i;
  82
  83         /* sort by type and symbol index */
  84         i = cmp_3way(ELF32_R_TYPE(x->r_info), ELF32_R_TYPE(y->r_info));
  85         if (i == 0)
  86                 i = cmp_3way(ELF32_R_SYM(x->r_info), ELF32_R_SYM(y->r_info));
  87         return i;
  88 }
  89
  90 static bool is_zero_addend_relocation(Elf32_Addr base, const Elf32_Rel *rel)
  91 {
  92         u32 *tval = (u32 *)(base + rel->r_offset);
  93
  94         /*
  95          * Do a bitwise compare on the raw addend rather than fully decoding
  96          * the offset and doing an arithmetic comparison.
  97          * Note that a zero-addend jump/call relocation is encoded taking the
  98          * PC bias into account, i.e., -8 for ARM and -4 for Thumb2.
  99          */
 100         switch (ELF32_R_TYPE(rel->r_info)) {
 101                 u16 upper, lower;
 102
 103         case R_ARM_THM_CALL:
 104         case R_ARM_THM_JUMP24:
 105                 upper = __mem_to_opcode_thumb16(((u16 *)tval)[0]);
 106                 lower = __mem_to_opcode_thumb16(((u16 *)tval)[1]);
 107
 108                 return (upper & 0x7ff) == 0x7ff && (lower & 0x2fff) == 0x2ffe;
 109
 110         case R_ARM_CALL:
 111         case R_ARM_PC24:
 112         case R_ARM_JUMP24:
 113                 return (__mem_to_opcode_arm(*tval) & 0xffffff) == 0xfffffe;
 114         }
 115         BUG();
 116 }
 117
 118 static bool duplicate_rel(Elf32_Addr base, const Elf32_Rel *rel, int num)
 119 {
 120         const Elf32_Rel *prev;
 121
 122         /*
 123          * Entries are sorted by type and symbol index. That means that,
 124          * if a duplicate entry exists, it must be in the preceding
 125          * slot.
 126          */
 127         if (!num)
 128                 return false;
 129
 130         prev = rel + num - 1;
 131         return cmp_rel(rel + num, prev) == 0 &&
 132                is_zero_addend_relocation(base, prev);
 133 }
 134
 135 /* Count how many PLT entries we may need */
 136 static unsigned int count_plts(const Elf32_Sym *syms, Elf32_Addr base,
 137                                const Elf32_Rel *rel, int num, Elf32_Word dstidx)
 138 {
 139         unsigned int ret = 0;
 140         const Elf32_Sym *s;
 141         int i;
 142
 143         for (i = 0; i < num; i++) {
 144                 switch (ELF32_R_TYPE(rel[i].r_info)) {
 145                 case R_ARM_CALL:
 146                 case R_ARM_PC24:
 147                 case R_ARM_JUMP24:
 148                 case R_ARM_THM_CALL:
 149                 case R_ARM_THM_JUMP24:
 150                         /*
 151                          * We only have to consider branch targets that resolve
 152                          * to symbols that are defined in a different section.
 153                          * This is not simply a heuristic, it is a fundamental
 154                          * limitation, since there is no guaranteed way to emit
 155                          * PLT entries sufficiently close to the branch if the
 156                          * section size exceeds the range of a branch
 157                          * instruction. So ignore relocations against defined
 158                          * symbols if they live in the same section as the
 159                          * relocation target.
 160                          */
 161                         s = syms + ELF32_R_SYM(rel[i].r_info);
 162                         if (s->st_shndx == dstidx)
 163                                 break;
 164
 165                         /*
 166                          * Jump relocations with non-zero addends against
 167                          * undefined symbols are supported by the ELF spec, but
 168                          * do not occur in practice (e.g., 'jump n bytes past
 169                          * the entry point of undefined function symbol f').
 170                          * So we need to support them, but there is no need to
 171                          * take them into consideration when trying to optimize
 172                          * this code. So let's only check for duplicates when
 173                          * the addend is zero. (Note that calls into the core
 174                          * module via init PLT entries could involve section
 175                          * relative symbol references with non-zero addends, for
 176                          * which we may end up emitting duplicates, but the init
 177                          * PLT is released along with the rest of the .init
 178                          * region as soon as module loading completes.)
 179                          */
 180                         if (!is_zero_addend_relocation(base, rel + i) ||
 181                             !duplicate_rel(base, rel, i))
 182                                 ret++;
 183                 }
 184         }
 185         return ret;
 186 }
 187
 188 int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 189                               char *secstrings, struct module *mod)
 190 {
 191         unsigned long core_plts = 0;
 192         unsigned long init_plts = 0;
 193         Elf32_Shdr *s, *sechdrs_end = sechdrs + ehdr->e_shnum;
 194         Elf32_Sym *syms = NULL;
 195
 196         /*
 197          * To store the PLTs, we expand the .text section for core module code
 198          * and for initialization code.
 199          */
 200         for (s = sechdrs; s < sechdrs_end; ++s) {
 201                 if (strcmp(".plt", secstrings + s->sh_name) == 0)
 202                         mod->arch.core.plt = s;
 203                 else if (strcmp(".init.plt", secstrings + s->sh_name) == 0)
 204                         mod->arch.init.plt = s;
 205                 else if (s->sh_type == SHT_SYMTAB)
 206                         syms = (Elf32_Sym *)s->sh_addr;
 207         }
 208
 209         if (!mod->arch.core.plt || !mod->arch.init.plt) {
 210                 pr_err("%s: module PLT section(s) missing\n", mod->name);
 211                 return -ENOEXEC;
 212         }
 213         if (!syms) {
 214                 pr_err("%s: module symtab section missing\n", mod->name);
 215                 return -ENOEXEC;
 216         }
 217
 218         for (s = sechdrs + 1; s < sechdrs_end; ++s) {
 219                 Elf32_Rel *rels = (void *)ehdr + s->sh_offset;
 220                 int numrels = s->sh_size / sizeof(Elf32_Rel);
 221                 Elf32_Shdr *dstsec = sechdrs + s->sh_info;
 222
 223                 if (s->sh_type != SHT_REL)
 224                         continue;
 225
 226                 /* ignore relocations that operate on non-exec sections */
 227                 if (!(dstsec->sh_flags & SHF_EXECINSTR))
 228                         continue;
 229
 230                 /* sort by type and symbol index */
 231                 sort(rels, numrels, sizeof(Elf32_Rel), cmp_rel, NULL);
 232
 233                 if (strncmp(secstrings + dstsec->sh_name, ".init", 5) != 0)
 234                         core_plts += count_plts(syms, dstsec->sh_addr, rels,
 235                                                 numrels, s->sh_info);
 236                 else
 237                         init_plts += count_plts(syms, dstsec->sh_addr, rels,
 238                                                 numrels, s->sh_info);
 239         }
 240
 241         mod->arch.core.plt->sh_type = SHT_NOBITS;
 242         mod->arch.core.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
 243         mod->arch.core.plt->sh_addralign = L1_CACHE_BYTES;
 244         mod->arch.core.plt->sh_size = round_up(core_plts * PLT_ENT_SIZE,
 245                                                sizeof(struct plt_entries));
 246         mod->arch.core.plt_count = 0;
 247
 248         mod->arch.init.plt->sh_type = SHT_NOBITS;
 249         mod->arch.init.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
 250         mod->arch.init.plt->sh_addralign = L1_CACHE_BYTES;
 251         mod->arch.init.plt->sh_size = round_up(init_plts * PLT_ENT_SIZE,
 252                                                sizeof(struct plt_entries));
 253         mod->arch.init.plt_count = 0;
 254
 255         pr_debug("%s: plt=%x, init.plt=%x\n", __func__,
 256                  mod->arch.core.plt->sh_size, mod->arch.init.plt->sh_size);
 257         return 0;
 258 }