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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 /*
25  * To use this vDSO parser, first call one of the vdso_init_* functions.
26  * If you've already parsed auxv, then pass the value of AT_SYSINFO_EHDR
27  * to vdso_init_from_sysinfo_ehdr.  Otherwise pass auxv to vdso_init_from_auxv.
28  * Then call vdso_sym for each symbol you want.  For example, to look up
29  * gettimeofday on x86_64, use:
30  *
31  *     <some pointer> = vdso_sym("LINUX_2.6", "gettimeofday");
32  * or
33  *     <some pointer> = vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
34  *
35  * vdso_sym will return 0 if the symbol doesn't exist or if the init function
36  * failed or was not called.  vdso_sym is a little slow, so its return value
37  * should be cached.
38  *
39  * vdso_sym is threadsafe; the init functions are not.
40  *
41  * These are the prototypes:
42  */
43 extern void vdso_init_from_auxv(void *auxv);
44 extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
45 extern void *vdso_sym(const char *version, const char *name);
46 
47 
48 /* And here's the code. */
49 #ifndef ELF_BITS
50 # if ULONG_MAX > 0xffffffffUL
51 #  define ELF_BITS 64
52 # else
53 #  define ELF_BITS 32
54 # endif
55 #endif
56 
57 #define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x
58 #define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x)
59 #define ELF(x) ELF_BITS_XFORM(ELF_BITS, x)
60 
61 static struct vdso_info
62 {
63 	bool valid;
64 
65 	/* Load information */
66 	uintptr_t load_addr;
67 	uintptr_t load_offset;  /* load_addr - recorded vaddr */
68 
69 	/* Symbol table */
70 	ELF(Sym) *symtab;
71 	const char *symstrings;
72 	ELF(Word) *bucket, *chain;
73 	ELF(Word) nbucket, nchain;
74 
75 	/* Version table */
76 	ELF(Versym) *versym;
77 	ELF(Verdef) *verdef;
78 } vdso_info;
79 
80 /* Straight from the ELF specification. */
elf_hash(const unsigned char * name)81 static unsigned long elf_hash(const unsigned char *name)
82 {
83 	unsigned long h = 0, g;
84 	while (*name)
85 	{
86 		h = (h << 4) + *name++;
87 		if (g = h & 0xf0000000)
88 			h ^= g >> 24;
89 		h &= ~g;
90 	}
91 	return h;
92 }
93 
vdso_init_from_sysinfo_ehdr(uintptr_t base)94 void vdso_init_from_sysinfo_ehdr(uintptr_t base)
95 {
96 	size_t i;
97 	bool found_vaddr = false;
98 
99 	vdso_info.valid = false;
100 
101 	vdso_info.load_addr = base;
102 
103 	ELF(Ehdr) *hdr = (ELF(Ehdr)*)base;
104 	if (hdr->e_ident[EI_CLASS] !=
105 	    (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) {
106 		return;  /* Wrong ELF class -- check ELF_BITS */
107 	}
108 
109 	ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff);
110 	ELF(Dyn) *dyn = 0;
111 
112 	/*
113 	 * We need two things from the segment table: the load offset
114 	 * and the dynamic table.
115 	 */
116 	for (i = 0; i < hdr->e_phnum; i++)
117 	{
118 		if (pt[i].p_type == PT_LOAD && !found_vaddr) {
119 			found_vaddr = true;
120 			vdso_info.load_offset =	base
121 				+ (uintptr_t)pt[i].p_offset
122 				- (uintptr_t)pt[i].p_vaddr;
123 		} else if (pt[i].p_type == PT_DYNAMIC) {
124 			dyn = (ELF(Dyn)*)(base + pt[i].p_offset);
125 		}
126 	}
127 
128 	if (!found_vaddr || !dyn)
129 		return;  /* Failed */
130 
131 	/*
132 	 * Fish out the useful bits of the dynamic table.
133 	 */
134 	ELF(Word) *hash = 0;
135 	vdso_info.symstrings = 0;
136 	vdso_info.symtab = 0;
137 	vdso_info.versym = 0;
138 	vdso_info.verdef = 0;
139 	for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
140 		switch (dyn[i].d_tag) {
141 		case DT_STRTAB:
142 			vdso_info.symstrings = (const char *)
143 				((uintptr_t)dyn[i].d_un.d_ptr
144 				 + vdso_info.load_offset);
145 			break;
146 		case DT_SYMTAB:
147 			vdso_info.symtab = (ELF(Sym) *)
148 				((uintptr_t)dyn[i].d_un.d_ptr
149 				 + vdso_info.load_offset);
150 			break;
151 		case DT_HASH:
152 			hash = (ELF(Word) *)
153 				((uintptr_t)dyn[i].d_un.d_ptr
154 				 + vdso_info.load_offset);
155 			break;
156 		case DT_VERSYM:
157 			vdso_info.versym = (ELF(Versym) *)
158 				((uintptr_t)dyn[i].d_un.d_ptr
159 				 + vdso_info.load_offset);
160 			break;
161 		case DT_VERDEF:
162 			vdso_info.verdef = (ELF(Verdef) *)
163 				((uintptr_t)dyn[i].d_un.d_ptr
164 				 + vdso_info.load_offset);
165 			break;
166 		}
167 	}
168 	if (!vdso_info.symstrings || !vdso_info.symtab || !hash)
169 		return;  /* Failed */
170 
171 	if (!vdso_info.verdef)
172 		vdso_info.versym = 0;
173 
174 	/* Parse the hash table header. */
175 	vdso_info.nbucket = hash[0];
176 	vdso_info.nchain = hash[1];
177 	vdso_info.bucket = &hash[2];
178 	vdso_info.chain = &hash[vdso_info.nbucket + 2];
179 
180 	/* That's all we need. */
181 	vdso_info.valid = true;
182 }
183 
vdso_match_version(ELF (Versym)ver,const char * name,ELF (Word)hash)184 static bool vdso_match_version(ELF(Versym) ver,
185 			       const char *name, ELF(Word) hash)
186 {
187 	/*
188 	 * This is a helper function to check if the version indexed by
189 	 * ver matches name (which hashes to hash).
190 	 *
191 	 * The version definition table is a mess, and I don't know how
192 	 * to do this in better than linear time without allocating memory
193 	 * to build an index.  I also don't know why the table has
194 	 * variable size entries in the first place.
195 	 *
196 	 * For added fun, I can't find a comprehensible specification of how
197 	 * to parse all the weird flags in the table.
198 	 *
199 	 * So I just parse the whole table every time.
200 	 */
201 
202 	/* First step: find the version definition */
203 	ver &= 0x7fff;  /* Apparently bit 15 means "hidden" */
204 	ELF(Verdef) *def = vdso_info.verdef;
205 	while(true) {
206 		if ((def->vd_flags & VER_FLG_BASE) == 0
207 		    && (def->vd_ndx & 0x7fff) == ver)
208 			break;
209 
210 		if (def->vd_next == 0)
211 			return false;  /* No definition. */
212 
213 		def = (ELF(Verdef) *)((char *)def + def->vd_next);
214 	}
215 
216 	/* Now figure out whether it matches. */
217 	ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux);
218 	return def->vd_hash == hash
219 		&& !strcmp(name, vdso_info.symstrings + aux->vda_name);
220 }
221 
vdso_sym(const char * version,const char * name)222 void *vdso_sym(const char *version, const char *name)
223 {
224 	unsigned long ver_hash;
225 	if (!vdso_info.valid)
226 		return 0;
227 
228 	ver_hash = elf_hash(version);
229 	ELF(Word) chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
230 
231 	for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) {
232 		ELF(Sym) *sym = &vdso_info.symtab[chain];
233 
234 		/* Check for a defined global or weak function w/ right name. */
235 		if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
236 			continue;
237 		if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
238 		    ELF64_ST_BIND(sym->st_info) != STB_WEAK)
239 			continue;
240 		if (sym->st_shndx == SHN_UNDEF)
241 			continue;
242 		if (strcmp(name, vdso_info.symstrings + sym->st_name))
243 			continue;
244 
245 		/* Check symbol version. */
246 		if (vdso_info.versym
247 		    && !vdso_match_version(vdso_info.versym[chain],
248 					   version, ver_hash))
249 			continue;
250 
251 		return (void *)(vdso_info.load_offset + sym->st_value);
252 	}
253 
254 	return 0;
255 }
256 
vdso_init_from_auxv(void * auxv)257 void vdso_init_from_auxv(void *auxv)
258 {
259 	ELF(auxv_t) *elf_auxv = auxv;
260 	for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++)
261 	{
262 		if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) {
263 			vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val);
264 			return;
265 		}
266 	}
267 
268 	vdso_info.valid = false;
269 }
270