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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1	/*
2	 * parse_vdso.c: Linux reference vDSO parser
3	 * Written by Andrew Lutomirski, 2011.
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_64.  In principle it should work on any 64-bit
15	 * architecture that has a vDSO.
16	 */
17	
18	#include <stdbool.h>
19	#include <stdint.h>
20	#include <string.h>
21	#include <elf.h>
22	
23	/*
24	 * To use this vDSO parser, first call one of the vdso_init_* functions.
25	 * If you've already parsed auxv, then pass the value of AT_SYSINFO_EHDR
26	 * to vdso_init_from_sysinfo_ehdr.  Otherwise pass auxv to vdso_init_from_auxv.
27	 * Then call vdso_sym for each symbol you want.  For example, to look up
28	 * gettimeofday on x86_64, use:
29	 *
30	 *     <some pointer> = vdso_sym("LINUX_2.6", "gettimeofday");
31	 * or
32	 *     <some pointer> = vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
33	 *
34	 * vdso_sym will return 0 if the symbol doesn't exist or if the init function
35	 * failed or was not called.  vdso_sym is a little slow, so its return value
36	 * should be cached.
37	 *
38	 * vdso_sym is threadsafe; the init functions are not.
39	 *
40	 * These are the prototypes:
41	 */
42	extern void vdso_init_from_auxv(void *auxv);
43	extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
44	extern void *vdso_sym(const char *version, const char *name);
45	
46	
47	/* And here's the code. */
48	
49	#ifndef __x86_64__
50	# error Not yet ported to non-x86_64 architectures
51	#endif
52	
53	static struct vdso_info
54	{
55		bool valid;
56	
57		/* Load information */
58		uintptr_t load_addr;
59		uintptr_t load_offset;  /* load_addr - recorded vaddr */
60	
61		/* Symbol table */
62		Elf64_Sym *symtab;
63		const char *symstrings;
64		Elf64_Word *bucket, *chain;
65		Elf64_Word nbucket, nchain;
66	
67		/* Version table */
68		Elf64_Versym *versym;
69		Elf64_Verdef *verdef;
70	} vdso_info;
71	
72	/* Straight from the ELF specification. */
73	static unsigned long elf_hash(const unsigned char *name)
74	{
75		unsigned long h = 0, g;
76		while (*name)
77		{
78			h = (h << 4) + *name++;
79			if (g = h & 0xf0000000)
80				h ^= g >> 24;
81			h &= ~g;
82		}
83		return h;
84	}
85	
86	void vdso_init_from_sysinfo_ehdr(uintptr_t base)
87	{
88		size_t i;
89		bool found_vaddr = false;
90	
91		vdso_info.valid = false;
92	
93		vdso_info.load_addr = base;
94	
95		Elf64_Ehdr *hdr = (Elf64_Ehdr*)base;
96		Elf64_Phdr *pt = (Elf64_Phdr*)(vdso_info.load_addr + hdr->e_phoff);
97		Elf64_Dyn *dyn = 0;
98	
99		/*
100		 * We need two things from the segment table: the load offset
101		 * and the dynamic table.
102		 */
103		for (i = 0; i < hdr->e_phnum; i++)
104		{
105			if (pt[i].p_type == PT_LOAD && !found_vaddr) {
106				found_vaddr = true;
107				vdso_info.load_offset =	base
108					+ (uintptr_t)pt[i].p_offset
109					- (uintptr_t)pt[i].p_vaddr;
110			} else if (pt[i].p_type == PT_DYNAMIC) {
111				dyn = (Elf64_Dyn*)(base + pt[i].p_offset);
112			}
113		}
114	
115		if (!found_vaddr || !dyn)
116			return;  /* Failed */
117	
118		/*
119		 * Fish out the useful bits of the dynamic table.
120		 */
121		Elf64_Word *hash = 0;
122		vdso_info.symstrings = 0;
123		vdso_info.symtab = 0;
124		vdso_info.versym = 0;
125		vdso_info.verdef = 0;
126		for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
127			switch (dyn[i].d_tag) {
128			case DT_STRTAB:
129				vdso_info.symstrings = (const char *)
130					((uintptr_t)dyn[i].d_un.d_ptr
131					 + vdso_info.load_offset);
132				break;
133			case DT_SYMTAB:
134				vdso_info.symtab = (Elf64_Sym *)
135					((uintptr_t)dyn[i].d_un.d_ptr
136					 + vdso_info.load_offset);
137				break;
138			case DT_HASH:
139				hash = (Elf64_Word *)
140					((uintptr_t)dyn[i].d_un.d_ptr
141					 + vdso_info.load_offset);
142				break;
143			case DT_VERSYM:
144				vdso_info.versym = (Elf64_Versym *)
145					((uintptr_t)dyn[i].d_un.d_ptr
146					 + vdso_info.load_offset);
147				break;
148			case DT_VERDEF:
149				vdso_info.verdef = (Elf64_Verdef *)
150					((uintptr_t)dyn[i].d_un.d_ptr
151					 + vdso_info.load_offset);
152				break;
153			}
154		}
155		if (!vdso_info.symstrings || !vdso_info.symtab || !hash)
156			return;  /* Failed */
157	
158		if (!vdso_info.verdef)
159			vdso_info.versym = 0;
160	
161		/* Parse the hash table header. */
162		vdso_info.nbucket = hash[0];
163		vdso_info.nchain = hash[1];
164		vdso_info.bucket = &hash[2];
165		vdso_info.chain = &hash[vdso_info.nbucket + 2];
166	
167		/* That's all we need. */
168		vdso_info.valid = true;
169	}
170	
171	static bool vdso_match_version(Elf64_Versym ver,
172				       const char *name, Elf64_Word hash)
173	{
174		/*
175		 * This is a helper function to check if the version indexed by
176		 * ver matches name (which hashes to hash).
177		 *
178		 * The version definition table is a mess, and I don't know how
179		 * to do this in better than linear time without allocating memory
180		 * to build an index.  I also don't know why the table has
181		 * variable size entries in the first place.
182		 *
183		 * For added fun, I can't find a comprehensible specification of how
184		 * to parse all the weird flags in the table.
185		 *
186		 * So I just parse the whole table every time.
187		 */
188	
189		/* First step: find the version definition */
190		ver &= 0x7fff;  /* Apparently bit 15 means "hidden" */
191		Elf64_Verdef *def = vdso_info.verdef;
192		while(true) {
193			if ((def->vd_flags & VER_FLG_BASE) == 0
194			    && (def->vd_ndx & 0x7fff) == ver)
195				break;
196	
197			if (def->vd_next == 0)
198				return false;  /* No definition. */
199	
200			def = (Elf64_Verdef *)((char *)def + def->vd_next);
201		}
202	
203		/* Now figure out whether it matches. */
204		Elf64_Verdaux *aux = (Elf64_Verdaux*)((char *)def + def->vd_aux);
205		return def->vd_hash == hash
206			&& !strcmp(name, vdso_info.symstrings + aux->vda_name);
207	}
208	
209	void *vdso_sym(const char *version, const char *name)
210	{
211		unsigned long ver_hash;
212		if (!vdso_info.valid)
213			return 0;
214	
215		ver_hash = elf_hash(version);
216		Elf64_Word chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
217	
218		for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) {
219			Elf64_Sym *sym = &vdso_info.symtab[chain];
220	
221			/* Check for a defined global or weak function w/ right name. */
222			if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
223				continue;
224			if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
225			    ELF64_ST_BIND(sym->st_info) != STB_WEAK)
226				continue;
227			if (sym->st_shndx == SHN_UNDEF)
228				continue;
229			if (strcmp(name, vdso_info.symstrings + sym->st_name))
230				continue;
231	
232			/* Check symbol version. */
233			if (vdso_info.versym
234			    && !vdso_match_version(vdso_info.versym[chain],
235						   version, ver_hash))
236				continue;
237	
238			return (void *)(vdso_info.load_offset + sym->st_value);
239		}
240	
241		return 0;
242	}
243	
244	void vdso_init_from_auxv(void *auxv)
245	{
246		Elf64_auxv_t *elf_auxv = auxv;
247		for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++)
248		{
249			if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) {
250				vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val);
251				return;
252			}
253		}
254	
255		vdso_info.valid = false;
256	}
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