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Linux/Documentation/vDSO/parse_vdso.c

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  1 /*
  2  * parse_vdso.c: Linux reference vDSO parser
  3  * Written by Andrew Lutomirski, 2011.
  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_64.  In principle it should work on any 64-bit
 15  * architecture that has a vDSO.
 16  */
 17 
 18 #include <stdbool.h>
 19 #include <stdint.h>
 20 #include <string.h>
 21 #include <elf.h>
 22 
 23 /*
 24  * To use this vDSO parser, first call one of the vdso_init_* functions.
 25  * If you've already parsed auxv, then pass the value of AT_SYSINFO_EHDR
 26  * to vdso_init_from_sysinfo_ehdr.  Otherwise pass auxv to vdso_init_from_auxv.
 27  * Then call vdso_sym for each symbol you want.  For example, to look up
 28  * gettimeofday on x86_64, use:
 29  *
 30  *     <some pointer> = vdso_sym("LINUX_2.6", "gettimeofday");
 31  * or
 32  *     <some pointer> = vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
 33  *
 34  * vdso_sym will return 0 if the symbol doesn't exist or if the init function
 35  * failed or was not called.  vdso_sym is a little slow, so its return value
 36  * should be cached.
 37  *
 38  * vdso_sym is threadsafe; the init functions are not.
 39  *
 40  * These are the prototypes:
 41  */
 42 extern void vdso_init_from_auxv(void *auxv);
 43 extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
 44 extern void *vdso_sym(const char *version, const char *name);
 45 
 46 
 47 /* And here's the code. */
 48 
 49 #ifndef __x86_64__
 50 # error Not yet ported to non-x86_64 architectures
 51 #endif
 52 
 53 static struct vdso_info
 54 {
 55         bool valid;
 56 
 57         /* Load information */
 58         uintptr_t load_addr;
 59         uintptr_t load_offset;  /* load_addr - recorded vaddr */
 60 
 61         /* Symbol table */
 62         Elf64_Sym *symtab;
 63         const char *symstrings;
 64         Elf64_Word *bucket, *chain;
 65         Elf64_Word nbucket, nchain;
 66 
 67         /* Version table */
 68         Elf64_Versym *versym;
 69         Elf64_Verdef *verdef;
 70 } vdso_info;
 71 
 72 /* Straight from the ELF specification. */
 73 static unsigned long elf_hash(const unsigned char *name)
 74 {
 75         unsigned long h = 0, g;
 76         while (*name)
 77         {
 78                 h = (h << 4) + *name++;
 79                 if (g = h & 0xf0000000)
 80                         h ^= g >> 24;
 81                 h &= ~g;
 82         }
 83         return h;
 84 }
 85 
 86 void vdso_init_from_sysinfo_ehdr(uintptr_t base)
 87 {
 88         size_t i;
 89         bool found_vaddr = false;
 90 
 91         vdso_info.valid = false;
 92 
 93         vdso_info.load_addr = base;
 94 
 95         Elf64_Ehdr *hdr = (Elf64_Ehdr*)base;
 96         Elf64_Phdr *pt = (Elf64_Phdr*)(vdso_info.load_addr + hdr->e_phoff);
 97         Elf64_Dyn *dyn = 0;
 98 
 99         /*
100          * We need two things from the segment table: the load offset
101          * and the dynamic table.
102          */
103         for (i = 0; i < hdr->e_phnum; i++)
104         {
105                 if (pt[i].p_type == PT_LOAD && !found_vaddr) {
106                         found_vaddr = true;
107                         vdso_info.load_offset = base
108                                 + (uintptr_t)pt[i].p_offset
109                                 - (uintptr_t)pt[i].p_vaddr;
110                 } else if (pt[i].p_type == PT_DYNAMIC) {
111                         dyn = (Elf64_Dyn*)(base + pt[i].p_offset);
112                 }
113         }
114 
115         if (!found_vaddr || !dyn)
116                 return;  /* Failed */
117 
118         /*
119          * Fish out the useful bits of the dynamic table.
120          */
121         Elf64_Word *hash = 0;
122         vdso_info.symstrings = 0;
123         vdso_info.symtab = 0;
124         vdso_info.versym = 0;
125         vdso_info.verdef = 0;
126         for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
127                 switch (dyn[i].d_tag) {
128                 case DT_STRTAB:
129                         vdso_info.symstrings = (const char *)
130                                 ((uintptr_t)dyn[i].d_un.d_ptr
131                                  + vdso_info.load_offset);
132                         break;
133                 case DT_SYMTAB:
134                         vdso_info.symtab = (Elf64_Sym *)
135                                 ((uintptr_t)dyn[i].d_un.d_ptr
136                                  + vdso_info.load_offset);
137                         break;
138                 case DT_HASH:
139                         hash = (Elf64_Word *)
140                                 ((uintptr_t)dyn[i].d_un.d_ptr
141                                  + vdso_info.load_offset);
142                         break;
143                 case DT_VERSYM:
144                         vdso_info.versym = (Elf64_Versym *)
145                                 ((uintptr_t)dyn[i].d_un.d_ptr
146                                  + vdso_info.load_offset);
147                         break;
148                 case DT_VERDEF:
149                         vdso_info.verdef = (Elf64_Verdef *)
150                                 ((uintptr_t)dyn[i].d_un.d_ptr
151                                  + vdso_info.load_offset);
152                         break;
153                 }
154         }
155         if (!vdso_info.symstrings || !vdso_info.symtab || !hash)
156                 return;  /* Failed */
157 
158         if (!vdso_info.verdef)
159                 vdso_info.versym = 0;
160 
161         /* Parse the hash table header. */
162         vdso_info.nbucket = hash[0];
163         vdso_info.nchain = hash[1];
164         vdso_info.bucket = &hash[2];
165         vdso_info.chain = &hash[vdso_info.nbucket + 2];
166 
167         /* That's all we need. */
168         vdso_info.valid = true;
169 }
170 
171 static bool vdso_match_version(Elf64_Versym ver,
172                                const char *name, Elf64_Word hash)
173 {
174         /*
175          * This is a helper function to check if the version indexed by
176          * ver matches name (which hashes to hash).
177          *
178          * The version definition table is a mess, and I don't know how
179          * to do this in better than linear time without allocating memory
180          * to build an index.  I also don't know why the table has
181          * variable size entries in the first place.
182          *
183          * For added fun, I can't find a comprehensible specification of how
184          * to parse all the weird flags in the table.
185          *
186          * So I just parse the whole table every time.
187          */
188 
189         /* First step: find the version definition */
190         ver &= 0x7fff;  /* Apparently bit 15 means "hidden" */
191         Elf64_Verdef *def = vdso_info.verdef;
192         while(true) {
193                 if ((def->vd_flags & VER_FLG_BASE) == 0
194                     && (def->vd_ndx & 0x7fff) == ver)
195                         break;
196 
197                 if (def->vd_next == 0)
198                         return false;  /* No definition. */
199 
200                 def = (Elf64_Verdef *)((char *)def + def->vd_next);
201         }
202 
203         /* Now figure out whether it matches. */
204         Elf64_Verdaux *aux = (Elf64_Verdaux*)((char *)def + def->vd_aux);
205         return def->vd_hash == hash
206                 && !strcmp(name, vdso_info.symstrings + aux->vda_name);
207 }
208 
209 void *vdso_sym(const char *version, const char *name)
210 {
211         unsigned long ver_hash;
212         if (!vdso_info.valid)
213                 return 0;
214 
215         ver_hash = elf_hash(version);
216         Elf64_Word chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
217 
218         for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) {
219                 Elf64_Sym *sym = &vdso_info.symtab[chain];
220 
221                 /* Check for a defined global or weak function w/ right name. */
222                 if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
223                         continue;
224                 if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
225                     ELF64_ST_BIND(sym->st_info) != STB_WEAK)
226                         continue;
227                 if (sym->st_shndx == SHN_UNDEF)
228                         continue;
229                 if (strcmp(name, vdso_info.symstrings + sym->st_name))
230                         continue;
231 
232                 /* Check symbol version. */
233                 if (vdso_info.versym
234                     && !vdso_match_version(vdso_info.versym[chain],
235                                            version, ver_hash))
236                         continue;
237 
238                 return (void *)(vdso_info.load_offset + sym->st_value);
239         }
240 
241         return 0;
242 }
243 
244 void vdso_init_from_auxv(void *auxv)
245 {
246         Elf64_auxv_t *elf_auxv = auxv;
247         for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++)
248         {
249                 if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) {
250                         vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val);
251                         return;
252                 }
253         }
254 
255         vdso_info.valid = false;
256 }
257 

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