parse_vdso.c

  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 <linux/auxvec.h>
 23#include <linux/elf.h>
 24
 25#include "parse_vdso.h"
 26
 27/* And here's the code. */
 28#ifndef ELF_BITS
 29# if __SIZEOF_LONG__ >= 8
 30#  define ELF_BITS 64
 31# else
 32#  define ELF_BITS 32
 33# endif
 34#endif
 35
 36#define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x
 37#define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x)
 38#define ELF(x) ELF_BITS_XFORM(ELF_BITS, x)
 39
 40#ifdef __s390x__
 41#define ELF_HASH_ENTRY ELF(Xword)
 42#else
 43#define ELF_HASH_ENTRY ELF(Word)
 44#endif
 45
 46static struct vdso_info
 47{
 48	bool valid;
 49
 50	/* Load information */
 51	uintptr_t load_addr;
 52	uintptr_t load_offset;  /* load_addr - recorded vaddr */
 53
 54	/* Symbol table */
 55	ELF(Sym) *symtab;
 56	const char *symstrings;
 57	ELF(Word) *gnu_hash, *gnu_bucket;
 58	ELF_HASH_ENTRY *bucket, *chain;
 59	ELF_HASH_ENTRY nbucket, nchain;
 60
 61	/* Version table */
 62	ELF(Versym) *versym;
 63	ELF(Verdef) *verdef;
 64} vdso_info;
 65
 66/*
 67 * Straight from the ELF specification...and then tweaked slightly, in order to
 68 * avoid a few clang warnings.
 69 */
 70static unsigned long elf_hash(const char *name)
 71{
 72	unsigned long h = 0, g;
 73	const unsigned char *uch_name = (const unsigned char *)name;
 74
 75	while (*uch_name)
 76	{
 77		h = (h << 4) + *uch_name++;
 78		g = h & 0xf0000000;
 79		if (g)
 80			h ^= g >> 24;
 81		h &= ~g;
 82	}
 83	return h;
 84}
 85
 86static uint32_t gnu_hash(const char *name)
 87{
 88	const unsigned char *s = (void *)name;
 89	uint32_t h = 5381;
 90
 91	for (; *s; s++)
 92		h += h * 32 + *s;
 93	return h;
 94}
 95
 96void vdso_init_from_sysinfo_ehdr(uintptr_t base)
 97{
 98	size_t i;
 99	bool found_vaddr = false;
100
101	vdso_info.valid = false;
102
103	vdso_info.load_addr = base;
104
105	ELF(Ehdr) *hdr = (ELF(Ehdr)*)base;
106	if (hdr->e_ident[EI_CLASS] !=
107	    (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) {
108		return;  /* Wrong ELF class -- check ELF_BITS */
109	}
110
111	ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff);
112	ELF(Dyn) *dyn = 0;
113
114	/*
115	 * We need two things from the segment table: the load offset
116	 * and the dynamic table.
117	 */
118	for (i = 0; i < hdr->e_phnum; i++)
119	{
120		if (pt[i].p_type == PT_LOAD && !found_vaddr) {
121			found_vaddr = true;
122			vdso_info.load_offset =	base
123				+ (uintptr_t)pt[i].p_offset
124				- (uintptr_t)pt[i].p_vaddr;
125		} else if (pt[i].p_type == PT_DYNAMIC) {
126			dyn = (ELF(Dyn)*)(base + pt[i].p_offset);
127		}
128	}
129
130	if (!found_vaddr || !dyn)
131		return;  /* Failed */
132
133	/*
134	 * Fish out the useful bits of the dynamic table.
135	 */
136	ELF_HASH_ENTRY *hash = 0;
137	vdso_info.symstrings = 0;
138	vdso_info.gnu_hash = 0;
139	vdso_info.symtab = 0;
140	vdso_info.versym = 0;
141	vdso_info.verdef = 0;
142	for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
143		switch (dyn[i].d_tag) {
144		case DT_STRTAB:
145			vdso_info.symstrings = (const char *)
146				((uintptr_t)dyn[i].d_un.d_ptr
147				 + vdso_info.load_offset);
148			break;
149		case DT_SYMTAB:
150			vdso_info.symtab = (ELF(Sym) *)
151				((uintptr_t)dyn[i].d_un.d_ptr
152				 + vdso_info.load_offset);
153			break;
154		case DT_HASH:
155			hash = (ELF_HASH_ENTRY *)
156				((uintptr_t)dyn[i].d_un.d_ptr
157				 + vdso_info.load_offset);
158			break;
159		case DT_GNU_HASH:
160			vdso_info.gnu_hash =
161				(ELF(Word) *)((uintptr_t)dyn[i].d_un.d_ptr +
162					      vdso_info.load_offset);
163			break;
164		case DT_VERSYM:
165			vdso_info.versym = (ELF(Versym) *)
166				((uintptr_t)dyn[i].d_un.d_ptr
167				 + vdso_info.load_offset);
168			break;
169		case DT_VERDEF:
170			vdso_info.verdef = (ELF(Verdef) *)
171				((uintptr_t)dyn[i].d_un.d_ptr
172				 + vdso_info.load_offset);
173			break;
174		}
175	}
176	if (!vdso_info.symstrings || !vdso_info.symtab ||
177	    (!hash && !vdso_info.gnu_hash))
178		return;  /* Failed */
179
180	if (!vdso_info.verdef)
181		vdso_info.versym = 0;
182
183	/* Parse the hash table header. */
184	if (vdso_info.gnu_hash) {
185		vdso_info.nbucket = vdso_info.gnu_hash[0];
186		/* The bucket array is located after the header (4 uint32) and the bloom
187		 * filter (size_t array of gnu_hash[2] elements).
188		 */
189		vdso_info.gnu_bucket = vdso_info.gnu_hash + 4 +
190				       sizeof(size_t) / 4 * vdso_info.gnu_hash[2];
191	} else {
192		vdso_info.nbucket = hash[0];
193		vdso_info.nchain = hash[1];
194		vdso_info.bucket = &hash[2];
195		vdso_info.chain = &hash[vdso_info.nbucket + 2];
196	}
197
198	/* That's all we need. */
199	vdso_info.valid = true;
200}
201
202static bool vdso_match_version(ELF(Versym) ver,
203			       const char *name, ELF(Word) hash)
204{
205	/*
206	 * This is a helper function to check if the version indexed by
207	 * ver matches name (which hashes to hash).
208	 *
209	 * The version definition table is a mess, and I don't know how
210	 * to do this in better than linear time without allocating memory
211	 * to build an index.  I also don't know why the table has
212	 * variable size entries in the first place.
213	 *
214	 * For added fun, I can't find a comprehensible specification of how
215	 * to parse all the weird flags in the table.
216	 *
217	 * So I just parse the whole table every time.
218	 */
219
220	/* First step: find the version definition */
221	ver &= 0x7fff;  /* Apparently bit 15 means "hidden" */
222	ELF(Verdef) *def = vdso_info.verdef;
223	while(true) {
224		if ((def->vd_flags & VER_FLG_BASE) == 0
225		    && (def->vd_ndx & 0x7fff) == ver)
226			break;
227
228		if (def->vd_next == 0)
229			return false;  /* No definition. */
230
231		def = (ELF(Verdef) *)((char *)def + def->vd_next);
232	}
233
234	/* Now figure out whether it matches. */
235	ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux);
236	return def->vd_hash == hash
237		&& !strcmp(name, vdso_info.symstrings + aux->vda_name);
238}
239
240static bool check_sym(ELF(Sym) *sym, ELF(Word) i, const char *name,
241		      const char *version, unsigned long ver_hash)
242{
243	/* Check for a defined global or weak function w/ right name. */
244	if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
245		return false;
246	if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
247	    ELF64_ST_BIND(sym->st_info) != STB_WEAK)
248		return false;
249	if (strcmp(name, vdso_info.symstrings + sym->st_name))
250		return false;
251
252	/* Check symbol version. */
253	if (vdso_info.versym &&
254	    !vdso_match_version(vdso_info.versym[i], version, ver_hash))
255		return false;
256
257	return true;
258}
259
260void *vdso_sym(const char *version, const char *name)
261{
262	unsigned long ver_hash;
263	if (!vdso_info.valid)
264		return 0;
265
266	ver_hash = elf_hash(version);
267	ELF(Word) i;
268
269	if (vdso_info.gnu_hash) {
270		uint32_t h1 = gnu_hash(name), h2, *hashval;
271
272		i = vdso_info.gnu_bucket[h1 % vdso_info.nbucket];
273		if (i == 0)
274			return 0;
275		h1 |= 1;
276		hashval = vdso_info.gnu_bucket + vdso_info.nbucket +
277			  (i - vdso_info.gnu_hash[1]);
278		for (;; i++) {
279			ELF(Sym) *sym = &vdso_info.symtab[i];
280			h2 = *hashval++;
281			if (h1 == (h2 | 1) &&
282			    check_sym(sym, i, name, version, ver_hash))
283				return (void *)(vdso_info.load_offset +
284						sym->st_value);
285			if (h2 & 1)
286				break;
287		}
288	} else {
289		i = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
290		for (; i; i = vdso_info.chain[i]) {
291			ELF(Sym) *sym = &vdso_info.symtab[i];
292			if (sym->st_shndx != SHN_UNDEF &&
293			    check_sym(sym, i, name, version, ver_hash))
294				return (void *)(vdso_info.load_offset +
295						sym->st_value);
296		}
297	}
298
299	return 0;
300}