1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
|
#include <assert.h>
#include <stdlib.h>
#include <string.h>
#include <libelfu/libelfu.h>
static void* subFindByName(ElfuElf *me, ElfuScn *ms, void *aux1, void *aux2)
{
char *name = (char*)aux1;
(void)aux2;
if (elfu_mScnName(me, ms)) {
if (!strcmp(elfu_mScnName(me, ms), name)) {
return ms;
}
}
/* Continue */
return NULL;
}
/* Hazard a guess where a function may be found in the PLT */
int elfu_mDynLookupPltAddrByName(ElfuElf *me, char *name, GElf_Addr *result)
{
ElfuScn *relplt;
ElfuScn *plt;
ElfuRel *rel;
GElf_Word j;
relplt = elfu_mScnForall(me, subFindByName, ".rel.plt", NULL);
if (!relplt) {
/* x86-64 uses .rela.plt instead */
relplt = elfu_mScnForall(me, subFindByName, ".rela.plt", NULL);
}
if (!relplt) {
ELFU_WARN("elfu_mDynLookupPltAddr: Could not find .rel.plt section in destination ELF.\n");
return -1;
}
plt = elfu_mScnForall(me, subFindByName, ".plt", NULL);
if (!plt) {
ELFU_WARN("elfu_mDynLookupPltAddr: Could not find .plt section in destination ELF.\n");
return -1;
}
/* Look up name. If the j-th entry in .rel.plt has the name we are
* looking for, we assume that the (j+1)-th entry in .plt is machine
* code to jump to the function.
* Your mileage may vary, but it works on my GNU binaries. */
assert(relplt->linkptr);
j = 0;
CIRCLEQ_FOREACH(rel, &relplt->reltab.rels, elem) {
GElf_Word i;
ElfuSym *sym;
char *symname;
j++;
/* We only consider runtime relocations for functions.
* Technically, these relocations write the functions' addresses
* to the GOT, not the PLT, after the dynamic linker has found
* them. */
if ((me->elfclass == ELFCLASS32 && rel->type != R_386_JMP_SLOT)
|| (me->elfclass == ELFCLASS64 && rel->type != R_X86_64_JUMP_SLOT)) {
continue;
}
/* Get the (rel->sym)-th symbol from the symbol table that
* .rel.plt points to. */
sym = CIRCLEQ_FIRST(&relplt->linkptr->symtab.syms);
for (i = 1; i < rel->sym; i++) {
sym = CIRCLEQ_NEXT(sym, elem);
}
symname = ELFU_SYMSTR(relplt->linkptr, sym->name);
if (!strcmp(symname, name)) {
/* If this is the symbol we are looking for, then in an x86 binary
* the jump to the dynamic symbol is probably at offset (j * 16)
* from the start of the PLT, where j is the PLT entry and 16 is
* the number of bytes the machine code in a PLT entry take. */
GElf_Addr addr = plt->shdr.sh_addr + (16 * j);
ELFU_DEBUG("elfu_mDynLookupPltAddr: Guessing symbol '%s' is in destination memory at %x (PLT entry #%u).\n", name, (unsigned)addr, j);
*result = addr;
return 0;
}
}
ELFU_WARN("elfu_mDynLookupPltAddr: Could not find symbol '%s' in destination ELF.\n", name);
return -1;
}
/* Hazard a guess where a global variable may be found in .bss,
* based on dynamic linking information in .rel.dyn */
int elfu_mDynLookupReldynAddrByName(ElfuElf *me, char *name, GElf_Addr *result)
{
ElfuScn *reldyn;
ElfuRel *rel;
GElf_Word j;
reldyn = elfu_mScnForall(me, subFindByName, ".rel.dyn", NULL);
if (!reldyn) {
/* x86-64 uses .rela.dyn instead */
reldyn = elfu_mScnForall(me, subFindByName, ".rela.dyn", NULL);
}
if (!reldyn) {
ELFU_WARN("elfu_mDynLookupReldynAddrByName: Could not find .rel.dyn section in destination ELF.\n");
return -1;
}
assert(reldyn->linkptr);
j = 0;
CIRCLEQ_FOREACH(rel, &reldyn->reltab.rels, elem) {
ElfuSym *sym;
char *symname;
j++;
/* We only consider COPY relocations for global variables here.
* Technically, these relocations write the variables' contents
* to .bss. */
if ((me->elfclass == ELFCLASS32 && rel->type != R_386_COPY)
|| (me->elfclass == ELFCLASS64 && rel->type != R_X86_64_COPY)) {
continue;
}
/* Get the (rel->sym)-th symbol from the symbol table that
* .rel.dyn points to. */
sym = elfu_mSymtabIndexToSym(reldyn->linkptr, rel->sym);
assert(sym);
symname = elfu_mSymtabSymToName(reldyn->linkptr, sym);
if (!strcmp(symname, name)) {
GElf_Addr addr = rel->offset;
ELFU_DEBUG("elfu_mDynLookupReldynAddrByName: Guessing symbol '%s' is in destination memory at %x (PLT entry #%u).\n", name, (unsigned)addr, j);
*result = addr;
return 0;
}
}
ELFU_WARN("elfu_mDynLookupReldynAddrByName: Could not find or use symbol '%s' in destination ELF.\n", name);
ELFU_WARN(" NOTE: Only R_*_COPY relocations are resolved to global variables.\n");
return -1;
}
|
