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| 1 | +// SPDX-License-Identifier: GPL-2.0 |
| 2 | + |
| 3 | +/* |
| 4 | + * Helper functions for finding the symbol in an ELF which is "nearest" |
| 5 | + * to a given address. |
| 6 | + */ |
| 7 | + |
| 8 | +#include "modpost.h" |
| 9 | + |
| 10 | +struct syminfo { |
| 11 | + unsigned int symbol_index; |
| 12 | + unsigned int section_index; |
| 13 | + Elf_Addr addr; |
| 14 | +}; |
| 15 | + |
| 16 | +/* |
| 17 | + * Container used to hold an entire binary search table. |
| 18 | + * Entries in table are ascending, sorted first by section_index, |
| 19 | + * then by addr, and last by symbol_index. The sorting by |
| 20 | + * symbol_index is used to ensure predictable behavior when |
| 21 | + * multiple symbols are present with the same address; all |
| 22 | + * symbols past the first are effectively ignored, by eliding |
| 23 | + * them in symsearch_fixup(). |
| 24 | + */ |
| 25 | +struct symsearch { |
| 26 | + unsigned int table_size; |
| 27 | + struct syminfo table[]; |
| 28 | +}; |
| 29 | + |
| 30 | +static int syminfo_compare(const void *s1, const void *s2) |
| 31 | +{ |
| 32 | + const struct syminfo *sym1 = s1; |
| 33 | + const struct syminfo *sym2 = s2; |
| 34 | + |
| 35 | + if (sym1->section_index > sym2->section_index) |
| 36 | + return 1; |
| 37 | + if (sym1->section_index < sym2->section_index) |
| 38 | + return -1; |
| 39 | + if (sym1->addr > sym2->addr) |
| 40 | + return 1; |
| 41 | + if (sym1->addr < sym2->addr) |
| 42 | + return -1; |
| 43 | + if (sym1->symbol_index > sym2->symbol_index) |
| 44 | + return 1; |
| 45 | + if (sym1->symbol_index < sym2->symbol_index) |
| 46 | + return -1; |
| 47 | + return 0; |
| 48 | +} |
| 49 | + |
| 50 | +static unsigned int symbol_count(struct elf_info *elf) |
| 51 | +{ |
| 52 | + unsigned int result = 0; |
| 53 | + |
| 54 | + for (Elf_Sym *sym = elf->symtab_start; sym < elf->symtab_stop; sym++) { |
| 55 | + if (is_valid_name(elf, sym)) |
| 56 | + result++; |
| 57 | + } |
| 58 | + return result; |
| 59 | +} |
| 60 | + |
| 61 | +/* |
| 62 | + * Populate the search array that we just allocated. |
| 63 | + * Be slightly paranoid here. The ELF file is mmap'd and could |
| 64 | + * conceivably change between symbol_count() and symsearch_populate(). |
| 65 | + * If we notice any difference, bail out rather than potentially |
| 66 | + * propagating errors or crashing. |
| 67 | + */ |
| 68 | +static void symsearch_populate(struct elf_info *elf, |
| 69 | + struct syminfo *table, |
| 70 | + unsigned int table_size) |
| 71 | +{ |
| 72 | + bool is_arm = (elf->hdr->e_machine == EM_ARM); |
| 73 | + |
| 74 | + for (Elf_Sym *sym = elf->symtab_start; sym < elf->symtab_stop; sym++) { |
| 75 | + if (is_valid_name(elf, sym)) { |
| 76 | + if (table_size-- == 0) |
| 77 | + fatal("%s: size mismatch\n", __func__); |
| 78 | + table->symbol_index = sym - elf->symtab_start; |
| 79 | + table->section_index = get_secindex(elf, sym); |
| 80 | + table->addr = sym->st_value; |
| 81 | + |
| 82 | + /* |
| 83 | + * For ARM Thumb instruction, the bit 0 of st_value is |
| 84 | + * set if the symbol is STT_FUNC type. Mask it to get |
| 85 | + * the address. |
| 86 | + */ |
| 87 | + if (is_arm && ELF_ST_TYPE(sym->st_info) == STT_FUNC) |
| 88 | + table->addr &= ~1; |
| 89 | + |
| 90 | + table++; |
| 91 | + } |
| 92 | + } |
| 93 | + |
| 94 | + if (table_size != 0) |
| 95 | + fatal("%s: size mismatch\n", __func__); |
| 96 | +} |
| 97 | + |
| 98 | +/* |
| 99 | + * Do any fixups on the table after sorting. |
| 100 | + * For now, this just finds adjacent entries which have |
| 101 | + * the same section_index and addr, and it propagates |
| 102 | + * the first symbol_index over the subsequent entries, |
| 103 | + * so that only one symbol_index is seen for any given |
| 104 | + * section_index and addr. This ensures that whether |
| 105 | + * we're looking at an address from "above" or "below" |
| 106 | + * that we see the same symbol_index. |
| 107 | + * This does leave some duplicate entries in the table; |
| 108 | + * in practice, these are a small fraction of the |
| 109 | + * total number of entries, and they are harmless to |
| 110 | + * the binary search algorithm other than a few occasional |
| 111 | + * unnecessary comparisons. |
| 112 | + */ |
| 113 | +static void symsearch_fixup(struct syminfo *table, unsigned int table_size) |
| 114 | +{ |
| 115 | + /* Don't look at index 0, it will never change. */ |
| 116 | + for (unsigned int i = 1; i < table_size; i++) { |
| 117 | + if (table[i].addr == table[i - 1].addr && |
| 118 | + table[i].section_index == table[i - 1].section_index) { |
| 119 | + table[i].symbol_index = table[i - 1].symbol_index; |
| 120 | + } |
| 121 | + } |
| 122 | +} |
| 123 | + |
| 124 | +void symsearch_init(struct elf_info *elf) |
| 125 | +{ |
| 126 | + unsigned int table_size = symbol_count(elf); |
| 127 | + |
| 128 | + elf->symsearch = NOFAIL(malloc(sizeof(struct symsearch) + |
| 129 | + sizeof(struct syminfo) * table_size)); |
| 130 | + elf->symsearch->table_size = table_size; |
| 131 | + |
| 132 | + symsearch_populate(elf, elf->symsearch->table, table_size); |
| 133 | + qsort(elf->symsearch->table, table_size, |
| 134 | + sizeof(struct syminfo), syminfo_compare); |
| 135 | + |
| 136 | + symsearch_fixup(elf->symsearch->table, table_size); |
| 137 | +} |
| 138 | + |
| 139 | +void symsearch_finish(struct elf_info *elf) |
| 140 | +{ |
| 141 | + free(elf->symsearch); |
| 142 | + elf->symsearch = NULL; |
| 143 | +} |
| 144 | + |
| 145 | +/* |
| 146 | + * Find the syminfo which is in secndx and "nearest" to addr. |
| 147 | + * allow_negative: allow returning a symbol whose address is > addr. |
| 148 | + * min_distance: ignore symbols which are further away than this. |
| 149 | + * |
| 150 | + * Returns a pointer into the symbol table for success. |
| 151 | + * Returns NULL if no legal symbol is found within the requested range. |
| 152 | + */ |
| 153 | +Elf_Sym *symsearch_find_nearest(struct elf_info *elf, Elf_Addr addr, |
| 154 | + unsigned int secndx, bool allow_negative, |
| 155 | + Elf_Addr min_distance) |
| 156 | +{ |
| 157 | + unsigned int hi = elf->symsearch->table_size; |
| 158 | + unsigned int lo = 0; |
| 159 | + struct syminfo *table = elf->symsearch->table; |
| 160 | + struct syminfo target; |
| 161 | + |
| 162 | + target.addr = addr; |
| 163 | + target.section_index = secndx; |
| 164 | + target.symbol_index = ~0; /* compares greater than any actual index */ |
| 165 | + while (hi > lo) { |
| 166 | + unsigned int mid = lo + (hi - lo) / 2; /* Avoids overflow */ |
| 167 | + |
| 168 | + if (syminfo_compare(&table[mid], &target) > 0) |
| 169 | + hi = mid; |
| 170 | + else |
| 171 | + lo = mid + 1; |
| 172 | + } |
| 173 | + |
| 174 | + /* |
| 175 | + * table[hi], if it exists, is the first entry in the array which |
| 176 | + * lies beyond target. table[hi - 1], if it exists, is the last |
| 177 | + * entry in the array which comes before target, including the |
| 178 | + * case where it perfectly matches the section and the address. |
| 179 | + * |
| 180 | + * Note -- if the address we're looking up falls perfectly |
| 181 | + * in the middle of two symbols, this is written to always |
| 182 | + * prefer the symbol with the lower address. |
| 183 | + */ |
| 184 | + Elf_Sym *result = NULL; |
| 185 | + |
| 186 | + if (allow_negative && |
| 187 | + hi < elf->symsearch->table_size && |
| 188 | + table[hi].section_index == secndx && |
| 189 | + table[hi].addr - addr <= min_distance) { |
| 190 | + min_distance = table[hi].addr - addr; |
| 191 | + result = &elf->symtab_start[table[hi].symbol_index]; |
| 192 | + } |
| 193 | + if (hi > 0 && |
| 194 | + table[hi - 1].section_index == secndx && |
| 195 | + addr - table[hi - 1].addr <= min_distance) { |
| 196 | + result = &elf->symtab_start[table[hi - 1].symbol_index]; |
| 197 | + } |
| 198 | + return result; |
| 199 | +} |
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