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 /* Straight from the ELF specification. */ 59 static unsigned long elf_hash(const unsigned char *name) 60 { 61 unsigned long h = 0, g; 62 while (*name) 63 { 64 h = (h << 4) + *name++; 65 if (g = h & 0xf0000000) 66 h ^= g >> 24; 67 h &= ~g; 68 } 69 return h; 70 } 71 72 void vdso_init_from_sysinfo_ehdr(uintptr_t base) 73 { 74 size_t i; 75 bool found_vaddr = false; 76 77 vdso_info.valid = false; 78 79 vdso_info.load_addr = base; 80 81 ELF(Ehdr) *hdr = (ELF(Ehdr)*)base; 82 if (hdr->e_ident[EI_CLASS] != 83 (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) { 84 return; /* Wrong ELF class -- check ELF_BITS */ 85 } 86 87 ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff); 88 ELF(Dyn) *dyn = 0; 89 90 /* 91 * We need two things from the segment table: the load offset 92 * and the dynamic table. 93 */ 94 for (i = 0; i < hdr->e_phnum; i++) 95 { 96 if (pt[i].p_type == PT_LOAD && !found_vaddr) { 97 found_vaddr = true; 98 vdso_info.load_offset = base 99 + (uintptr_t)pt[i].p_offset 100 - (uintptr_t)pt[i].p_vaddr; 101 } else if (pt[i].p_type == PT_DYNAMIC) { 102 dyn = (ELF(Dyn)*)(base + pt[i].p_offset); 103 } 104 } 105 106 if (!found_vaddr || !dyn) 107 return; /* Failed */ 108 109 /* 110 * Fish out the useful bits of the dynamic table. 111 */ 112 ELF(Word) *hash = 0; 113 vdso_info.symstrings = 0; 114 vdso_info.symtab = 0; 115 vdso_info.versym = 0; 116 vdso_info.verdef = 0; 117 for (i = 0; dyn[i].d_tag != DT_NULL; i++) { 118 switch (dyn[i].d_tag) { 119 case DT_STRTAB: 120 vdso_info.symstrings = (const char *) 121 ((uintptr_t)dyn[i].d_un.d_ptr 122 + vdso_info.load_offset); 123 break; 124 case DT_SYMTAB: 125 vdso_info.symtab = (ELF(Sym) *) 126 ((uintptr_t)dyn[i].d_un.d_ptr 127 + vdso_info.load_offset); 128 break; 129 case DT_HASH: 130 hash = (ELF(Word) *) 131 ((uintptr_t)dyn[i].d_un.d_ptr 132 + vdso_info.load_offset); 133 break; 134 case DT_VERSYM: 135 vdso_info.versym = (ELF(Versym) *) 136 ((uintptr_t)dyn[i].d_un.d_ptr 137 + vdso_info.load_offset); 138 break; 139 case DT_VERDEF: 140 vdso_info.verdef = (ELF(Verdef) *) 141 ((uintptr_t)dyn[i].d_un.d_ptr 142 + vdso_info.load_offset); 143 break; 144 } 145 } 146 if (!vdso_info.symstrings || !vdso_info.symtab || !hash) 147 return; /* Failed */ 148 149 if (!vdso_info.verdef) 150 vdso_info.versym = 0; 151 152 /* Parse the hash table header. */ 153 vdso_info.nbucket = hash[0]; 154 vdso_info.nchain = hash[1]; 155 vdso_info.bucket = &hash[2]; 156 vdso_info.chain = &hash[vdso_info.nbucket + 2]; 157 158 /* That's all we need. */ 159 vdso_info.valid = true; 160 } 161 162 static bool vdso_match_version(ELF(Versym) ver, 163 const char *name, ELF(Word) hash) 164 { 165 /* 166 * This is a helper function to check if the version indexed by 167 * ver matches name (which hashes to hash). 168 * 169 * The version definition table is a mess, and I don't know how 170 * to do this in better than linear time without allocating memory 171 * to build an index. I also don't know why the table has 172 * variable size entries in the first place. 173 * 174 * For added fun, I can't find a comprehensible specification of how 175 * to parse all the weird flags in the table. 176 * 177 * So I just parse the whole table every time. 178 */ 179 180 /* First step: find the version definition */ 181 ver &= 0x7fff; /* Apparently bit 15 means "hidden" */ 182 ELF(Verdef) *def = vdso_info.verdef; 183 while(true) { 184 if ((def->vd_flags & VER_FLG_BASE) == 0 185 && (def->vd_ndx & 0x7fff) == ver) 186 break; 187 188 if (def->vd_next == 0) 189 return false; /* No definition. */ 190 191 def = (ELF(Verdef) *)((char *)def + def->vd_next); 192 } 193 194 /* Now figure out whether it matches. */ 195 ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux); 196 return def->vd_hash == hash 197 && !strcmp(name, vdso_info.symstrings + aux->vda_name); 198 } 199 200 void *vdso_sym(const char *version, const char *name) 201 { 202 unsigned long ver_hash; 203 if (!vdso_info.valid) 204 return 0; 205 206 ver_hash = elf_hash(version); 207 ELF(Word) chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket]; 208 209 for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) { 210 ELF(Sym) *sym = &vdso_info.symtab[chain]; 211 212 /* Check for a defined global or weak function w/ right name. */ 213 if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC) 214 continue; 215 if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL && 216 ELF64_ST_BIND(sym->st_info) != STB_WEAK) 217 continue; 218 if (sym->st_shndx == SHN_UNDEF) 219 continue; 220 if (strcmp(name, vdso_info.symstrings + sym->st_name)) 221 continue; 222 223 /* Check symbol version. */ 224 if (vdso_info.versym 225 && !vdso_match_version(vdso_info.versym[chain], 226 version, ver_hash)) 227 continue; 228 229 return (void *)(vdso_info.load_offset + sym->st_value); 230 } 231 232 return 0; 233 } 234 235 void vdso_init_from_auxv(void *auxv) 236 { 237 ELF(auxv_t) *elf_auxv = auxv; 238 for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++) 239 { 240 if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) { 241 vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val); 242 return; 243 } 244 } 245 246 vdso_info.valid = false; 247 } 248