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
39static 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. */
59static 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
72void 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
162static 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
200void *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
235void 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