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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

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. */
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	
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	
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	
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	
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	}
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