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 #ifdef __s390x__ 40 #define ELF_HASH_ENTRY ELF(Xword) 41 #else 42 #define ELF_HASH_ENTRY ELF(Word) 43 #endif 44 45 static struct vdso_info 46 { 47 bool valid; 48 49 /* Load information */ 50 uintptr_t load_addr; 51 uintptr_t load_offset; /* load_addr - recorded vaddr */ 52 53 /* Symbol table */ 54 ELF(Sym) *symtab; 55 const char *symstrings; 56 ELF_HASH_ENTRY *bucket, *chain; 57 ELF_HASH_ENTRY nbucket, nchain; 58 59 /* Version table */ 60 ELF(Versym) *versym; 61 ELF(Verdef) *verdef; 62 } vdso_info; 63 64 /* 65 * Straight from the ELF specification...and then tweaked slightly, in order to 66 * avoid a few clang warnings. 67 */ 68 static unsigned long elf_hash(const char *name) 69 { 70 unsigned long h = 0, g; 71 const unsigned char *uch_name = (const unsigned char *)name; 72 73 while (*uch_name) 74 { 75 h = (h << 4) + *uch_name++; 76 g = h & 0xf0000000; 77 if (g) 78 h ^= g >> 24; 79 h &= ~g; 80 } 81 return h; 82 } 83 84 void vdso_init_from_sysinfo_ehdr(uintptr_t base) 85 { 86 size_t i; 87 bool found_vaddr = false; 88 89 vdso_info.valid = false; 90 91 vdso_info.load_addr = base; 92 93 ELF(Ehdr) *hdr = (ELF(Ehdr)*)base; 94 if (hdr->e_ident[EI_CLASS] != 95 (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) { 96 return; /* Wrong ELF class -- check ELF_BITS */ 97 } 98 99 ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff); 100 ELF(Dyn) *dyn = 0; 101 102 /* 103 * We need two things from the segment table: the load offset 104 * and the dynamic table. 105 */ 106 for (i = 0; i < hdr->e_phnum; i++) 107 { 108 if (pt[i].p_type == PT_LOAD && !found_vaddr) { 109 found_vaddr = true; 110 vdso_info.load_offset = base 111 + (uintptr_t)pt[i].p_offset 112 - (uintptr_t)pt[i].p_vaddr; 113 } else if (pt[i].p_type == PT_DYNAMIC) { 114 dyn = (ELF(Dyn)*)(base + pt[i].p_offset); 115 } 116 } 117 118 if (!found_vaddr || !dyn) 119 return; /* Failed */ 120 121 /* 122 * Fish out the useful bits of the dynamic table. 123 */ 124 ELF_HASH_ENTRY *hash = 0; 125 vdso_info.symstrings = 0; 126 vdso_info.symtab = 0; 127 vdso_info.versym = 0; 128 vdso_info.verdef = 0; 129 for (i = 0; dyn[i].d_tag != DT_NULL; i++) { 130 switch (dyn[i].d_tag) { 131 case DT_STRTAB: 132 vdso_info.symstrings = (const char *) 133 ((uintptr_t)dyn[i].d_un.d_ptr 134 + vdso_info.load_offset); 135 break; 136 case DT_SYMTAB: 137 vdso_info.symtab = (ELF(Sym) *) 138 ((uintptr_t)dyn[i].d_un.d_ptr 139 + vdso_info.load_offset); 140 break; 141 case DT_HASH: 142 hash = (ELF_HASH_ENTRY *) 143 ((uintptr_t)dyn[i].d_un.d_ptr 144 + vdso_info.load_offset); 145 break; 146 case DT_VERSYM: 147 vdso_info.versym = (ELF(Versym) *) 148 ((uintptr_t)dyn[i].d_un.d_ptr 149 + vdso_info.load_offset); 150 break; 151 case DT_VERDEF: 152 vdso_info.verdef = (ELF(Verdef) *) 153 ((uintptr_t)dyn[i].d_un.d_ptr 154 + vdso_info.load_offset); 155 break; 156 } 157 } 158 if (!vdso_info.symstrings || !vdso_info.symtab || !hash) 159 return; /* Failed */ 160 161 if (!vdso_info.verdef) 162 vdso_info.versym = 0; 163 164 /* Parse the hash table header. */ 165 vdso_info.nbucket = hash[0]; 166 vdso_info.nchain = hash[1]; 167 vdso_info.bucket = &hash[2]; 168 vdso_info.chain = &hash[vdso_info.nbucket + 2]; 169 170 /* That's all we need. */ 171 vdso_info.valid = true; 172 } 173 174 static bool vdso_match_version(ELF(Versym) ver, 175 const char *name, ELF(Word) hash) 176 { 177 /* 178 * This is a helper function to check if the version indexed by 179 * ver matches name (which hashes to hash). 180 * 181 * The version definition table is a mess, and I don't know how 182 * to do this in better than linear time without allocating memory 183 * to build an index. I also don't know why the table has 184 * variable size entries in the first place. 185 * 186 * For added fun, I can't find a comprehensible specification of how 187 * to parse all the weird flags in the table. 188 * 189 * So I just parse the whole table every time. 190 */ 191 192 /* First step: find the version definition */ 193 ver &= 0x7fff; /* Apparently bit 15 means "hidden" */ 194 ELF(Verdef) *def = vdso_info.verdef; 195 while(true) { 196 if ((def->vd_flags & VER_FLG_BASE) == 0 197 && (def->vd_ndx & 0x7fff) == ver) 198 break; 199 200 if (def->vd_next == 0) 201 return false; /* No definition. */ 202 203 def = (ELF(Verdef) *)((char *)def + def->vd_next); 204 } 205 206 /* Now figure out whether it matches. */ 207 ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux); 208 return def->vd_hash == hash 209 && !strcmp(name, vdso_info.symstrings + aux->vda_name); 210 } 211 212 void *vdso_sym(const char *version, const char *name) 213 { 214 unsigned long ver_hash; 215 if (!vdso_info.valid) 216 return 0; 217 218 ver_hash = elf_hash(version); 219 ELF(Word) chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket]; 220 221 for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) { 222 ELF(Sym) *sym = &vdso_info.symtab[chain]; 223 224 /* Check for a defined global or weak function w/ right name. */ 225 if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC) 226 continue; 227 if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL && 228 ELF64_ST_BIND(sym->st_info) != STB_WEAK) 229 continue; 230 if (sym->st_shndx == SHN_UNDEF) 231 continue; 232 if (strcmp(name, vdso_info.symstrings + sym->st_name)) 233 continue; 234 235 /* Check symbol version. */ 236 if (vdso_info.versym 237 && !vdso_match_version(vdso_info.versym[chain], 238 version, ver_hash)) 239 continue; 240 241 return (void *)(vdso_info.load_offset + sym->st_value); 242 } 243 244 return 0; 245 } 246 247 void vdso_init_from_auxv(void *auxv) 248 { 249 ELF(auxv_t) *elf_auxv = auxv; 250 for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++) 251 { 252 if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) { 253 vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val); 254 return; 255 } 256 } 257 258 vdso_info.valid = false; 259 } 260