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

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  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 
 14 struct 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 
 29 u64 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 
 83 static 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 
 97 static 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 
107 static 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 
149 int 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 

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