1 /* 2 * parse_vdso.c: Linux reference vDSO parser 3 * Written by Andrew Lutomirski, 2011-2014. 4 * 5 * This code is meant to be linked in to various programs that run on Linux. 6 * As such, it is available with as few restrictions as possible. This file 7 * is licensed under the Creative Commons Zero License, version 1.0, 8 * available at http://creativecommons.org/publicdomain/zero/1.0/legalcode 9 * 10 * The vDSO is a regular ELF DSO that the kernel maps into user space when 11 * it starts a program. It works equally well in statically and dynamically 12 * linked binaries. 13 * 14 * This code is tested on x86. In principle it should work on any 15 * architecture that has a vDSO. 16 */ 17 18 #include <stdbool.h> 19 #include <stdint.h> 20 #include <string.h> 21 #include <limits.h> 22 #include <elf.h> 23 24 #include "parse_vdso.h" 25 26 /* And here's the code. */ 27 #ifndef ELF_BITS 28 # if ULONG_MAX > 0xffffffffUL 29 # define ELF_BITS 64 30 # else 31 # define ELF_BITS 32 32 # endif 33 #endif 34 35 #define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x 36 #define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x) 37 #define ELF(x) ELF_BITS_XFORM(ELF_BITS, x) 38 39 static struct vdso_info 40 { 41 bool valid; 42 43 /* Load information */ 44 uintptr_t load_addr; 45 uintptr_t load_offset; /* load_addr - recorded vaddr */ 46 47 /* Symbol table */ 48 ELF(Sym) *symtab; 49 const char *symstrings; 50 ELF(Word) *bucket, *chain; 51 ELF(Word) nbucket, nchain; 52 53 /* Version table */ 54 ELF(Versym) *versym; 55 ELF(Verdef) *verdef; 56 } vdso_info; 57 58 /* 59 * Straight from the ELF specification...and then tweaked slightly, in order to 60 * avoid a few clang warnings. 61 */ 62 static unsigned long elf_hash(const char *name) 63 { 64 unsigned long h = 0, g; 65 const unsigned char *uch_name = (const unsigned char *)name; 66 67 while (*uch_name) 68 { 69 h = (h << 4) + *uch_name++; 70 g = h & 0xf0000000; 71 if (g) 72 h ^= g >> 24; 73 h &= ~g; 74 } 75 return h; 76 } 77 78 void vdso_init_from_sysinfo_ehdr(uintptr_t base) 79 { 80 size_t i; 81 bool found_vaddr = false; 82 83 vdso_info.valid = false; 84 85 vdso_info.load_addr = base; 86 87 ELF(Ehdr) *hdr = (ELF(Ehdr)*)base; 88 if (hdr->e_ident[EI_CLASS] != 89 (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) { 90 return; /* Wrong ELF class -- check ELF_BITS */ 91 } 92 93 ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff); 94 ELF(Dyn) *dyn = 0; 95 96 /* 97 * We need two things from the segment table: the load offset 98 * and the dynamic table. 99 */ 100 for (i = 0; i < hdr->e_phnum; i++) 101 { 102 if (pt[i].p_type == PT_LOAD && !found_vaddr) { 103 found_vaddr = true; 104 vdso_info.load_offset = base 105 + (uintptr_t)pt[i].p_offset 106 - (uintptr_t)pt[i].p_vaddr; 107 } else if (pt[i].p_type == PT_DYNAMIC) { 108 dyn = (ELF(Dyn)*)(base + pt[i].p_offset); 109 } 110 } 111 112 if (!found_vaddr || !dyn) 113 return; /* Failed */ 114 115 /* 116 * Fish out the useful bits of the dynamic table. 117 */ 118 ELF(Word) *hash = 0; 119 vdso_info.symstrings = 0; 120 vdso_info.symtab = 0; 121 vdso_info.versym = 0; 122 vdso_info.verdef = 0; 123 for (i = 0; dyn[i].d_tag != DT_NULL; i++) { 124 switch (dyn[i].d_tag) { 125 case DT_STRTAB: 126 vdso_info.symstrings = (const char *) 127 ((uintptr_t)dyn[i].d_un.d_ptr 128 + vdso_info.load_offset); 129 break; 130 case DT_SYMTAB: 131 vdso_info.symtab = (ELF(Sym) *) 132 ((uintptr_t)dyn[i].d_un.d_ptr 133 + vdso_info.load_offset); 134 break; 135 case DT_HASH: 136 hash = (ELF(Word) *) 137 ((uintptr_t)dyn[i].d_un.d_ptr 138 + vdso_info.load_offset); 139 break; 140 case DT_VERSYM: 141 vdso_info.versym = (ELF(Versym) *) 142 ((uintptr_t)dyn[i].d_un.d_ptr 143 + vdso_info.load_offset); 144 break; 145 case DT_VERDEF: 146 vdso_info.verdef = (ELF(Verdef) *) 147 ((uintptr_t)dyn[i].d_un.d_ptr 148 + vdso_info.load_offset); 149 break; 150 } 151 } 152 if (!vdso_info.symstrings || !vdso_info.symtab || !hash) 153 return; /* Failed */ 154 155 if (!vdso_info.verdef) 156 vdso_info.versym = 0; 157 158 /* Parse the hash table header. */ 159 vdso_info.nbucket = hash[0]; 160 vdso_info.nchain = hash[1]; 161 vdso_info.bucket = &hash[2]; 162 vdso_info.chain = &hash[vdso_info.nbucket + 2]; 163 164 /* That's all we need. */ 165 vdso_info.valid = true; 166 } 167 168 static bool vdso_match_version(ELF(Versym) ver, 169 const char *name, ELF(Word) hash) 170 { 171 /* 172 * This is a helper function to check if the version indexed by 173 * ver matches name (which hashes to hash). 174 * 175 * The version definition table is a mess, and I don't know how 176 * to do this in better than linear time without allocating memory 177 * to build an index. I also don't know why the table has 178 * variable size entries in the first place. 179 * 180 * For added fun, I can't find a comprehensible specification of how 181 * to parse all the weird flags in the table. 182 * 183 * So I just parse the whole table every time. 184 */ 185 186 /* First step: find the version definition */ 187 ver &= 0x7fff; /* Apparently bit 15 means "hidden" */ 188 ELF(Verdef) *def = vdso_info.verdef; 189 while(true) { 190 if ((def->vd_flags & VER_FLG_BASE) == 0 191 && (def->vd_ndx & 0x7fff) == ver) 192 break; 193 194 if (def->vd_next == 0) 195 return false; /* No definition. */ 196 197 def = (ELF(Verdef) *)((char *)def + def->vd_next); 198 } 199 200 /* Now figure out whether it matches. */ 201 ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux); 202 return def->vd_hash == hash 203 && !strcmp(name, vdso_info.symstrings + aux->vda_name); 204 } 205 206 void *vdso_sym(const char *version, const char *name) 207 { 208 unsigned long ver_hash; 209 if (!vdso_info.valid) 210 return 0; 211 212 ver_hash = elf_hash(version); 213 ELF(Word) chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket]; 214 215 for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) { 216 ELF(Sym) *sym = &vdso_info.symtab[chain]; 217 218 /* Check for a defined global or weak function w/ right name. */ 219 if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC) 220 continue; 221 if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL && 222 ELF64_ST_BIND(sym->st_info) != STB_WEAK) 223 continue; 224 if (sym->st_shndx == SHN_UNDEF) 225 continue; 226 if (strcmp(name, vdso_info.symstrings + sym->st_name)) 227 continue; 228 229 /* Check symbol version. */ 230 if (vdso_info.versym 231 && !vdso_match_version(vdso_info.versym[chain], 232 version, ver_hash)) 233 continue; 234 235 return (void *)(vdso_info.load_offset + sym->st_value); 236 } 237 238 return 0; 239 } 240 241 void vdso_init_from_auxv(void *auxv) 242 { 243 ELF(auxv_t) *elf_auxv = auxv; 244 for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++) 245 { 246 if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) { 247 vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val); 248 return; 249 } 250 } 251 252 vdso_info.valid = false; 253 } 254