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 */
 43extern void vdso_init_from_auxv(void *auxv);
 44extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
 45extern 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
 61static 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. */
 81static 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
 94void 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
184static 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
222void *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
257void 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}