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Linux/arch/arm64/kernel/module-plts.c

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  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 }
198 

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