Document the vDSO and add a reference parser (98eedc3a) · Commits · 戴 / test

Documentation/ABI/stable/vdso

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		On some architectures, when the kernel loads any userspace program it
		maps an ELF DSO into that program's address space. This DSO is called
		the vDSO and it often contains useful and highly-optimized alternatives
		to real syscalls.

		These functions are called just like ordinary C function according to
		your platform's ABI. Call them from a sensible context. (For example,
		if you set CS on x86 to something strange, the vDSO functions are
		within their rights to crash.) In addition, if you pass a bad
		pointer to a vDSO function, you might get SIGSEGV instead of -EFAULT.

		To find the DSO, parse the auxiliary vector passed to the program's
		entry point. The AT_SYSINFO_EHDR entry will point to the vDSO.

		The vDSO uses symbol versioning; whenever you request a symbol from the
		vDSO, specify the version you are expecting.

		Programs that dynamically link to glibc will use the vDSO automatically.
		Otherwise, you can use the reference parser in Documentation/vDSO/parse_vdso.c.

		Unless otherwise noted, the set of symbols with any given version and the
		ABI of those symbols is considered stable. It may vary across architectures,
		though.

		(As of this writing, this ABI documentation as been confirmed for x86_64.
		The maintainers of the other vDSO-using architectures should confirm
		that it is correct for their architecture.)
		No newline at end of file

Documentation/vDSO/parse_vdso.c

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		/*
		* parse_vdso.c: Linux reference vDSO parser
		* Written by Andrew Lutomirski, 2011.
		*
		* This code is meant to be linked in to various programs that run on Linux.
		* As such, it is available with as few restrictions as possible. This file
		* is licensed under the Creative Commons Zero License, version 1.0,
		* available at http://creativecommons.org/publicdomain/zero/1.0/legalcode
		*
		* The vDSO is a regular ELF DSO that the kernel maps into user space when
		* it starts a program. It works equally well in statically and dynamically
		* linked binaries.
		*
		* This code is tested on x86_64. In principle it should work on any 64-bit
		* architecture that has a vDSO.
		*/

		#include <stdbool.h>
		#include <stdint.h>
		#include <string.h>
		#include <elf.h>

		/*
		* To use this vDSO parser, first call one of the vdso_init_* functions.
		* If you've already parsed auxv, then pass the value of AT_SYSINFO_EHDR
		* to vdso_init_from_sysinfo_ehdr. Otherwise pass auxv to vdso_init_from_auxv.
		* Then call vdso_sym for each symbol you want. For example, to look up
		* gettimeofday on x86_64, use:
		*
		* <some pointer> = vdso_sym("LINUX_2.6", "gettimeofday");
		* or
		* <some pointer> = vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
		*
		* vdso_sym will return 0 if the symbol doesn't exist or if the init function
		* failed or was not called. vdso_sym is a little slow, so its return value
		* should be cached.
		*
		* vdso_sym is threadsafe; the init functions are not.
		*
		* These are the prototypes:
		*/
		extern void vdso_init_from_auxv(void *auxv);
		extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
		extern void vdso_sym(const char version, const char *name);


		/* And here's the code. */

		#ifndef __x86_64__
		# error Not yet ported to non-x86_64 architectures
		#endif

		static struct vdso_info
		{
		bool valid;

		/* Load information */
		uintptr_t load_addr;
		uintptr_t load_offset; /* load_addr - recorded vaddr */

		/* Symbol table */
		Elf64_Sym *symtab;
		const char *symstrings;
		Elf64_Word bucket, chain;
		Elf64_Word nbucket, nchain;

		/* Version table */
		Elf64_Versym *versym;
		Elf64_Verdef *verdef;
		} vdso_info;

		/* Straight from the ELF specification. */
		static unsigned long elf_hash(const unsigned char *name)
		{
		unsigned long h = 0, g;
		while (*name)
		{
		h = (h << 4) + *name++;
		if (g = h & 0xf0000000)
		h ^= g >> 24;
		h &= ~g;
		}
		return h;
		}

		void vdso_init_from_sysinfo_ehdr(uintptr_t base)
		{
		size_t i;
		bool found_vaddr = false;

		vdso_info.valid = false;

		vdso_info.load_addr = base;

		Elf64_Ehdr hdr = (Elf64_Ehdr)base;
		Elf64_Phdr pt = (Elf64_Phdr)(vdso_info.load_addr + hdr->e_phoff);
		Elf64_Dyn *dyn = 0;

		/*
		* We need two things from the segment table: the load offset
		* and the dynamic table.
		*/
		for (i = 0; i < hdr->e_phnum; i++)
		{
		if (pt[i].p_type == PT_LOAD && !found_vaddr) {
		found_vaddr = true;
		vdso_info.load_offset = base
		+ (uintptr_t)pt[i].p_offset
		- (uintptr_t)pt[i].p_vaddr;
		} else if (pt[i].p_type == PT_DYNAMIC) {
		dyn = (Elf64_Dyn*)(base + pt[i].p_offset);
		}
		}

		if (!found_vaddr \|\| !dyn)
		return; /* Failed */

		/*
		* Fish out the useful bits of the dynamic table.
		*/
		Elf64_Word *hash = 0;
		vdso_info.symstrings = 0;
		vdso_info.symtab = 0;
		vdso_info.versym = 0;
		vdso_info.verdef = 0;
		for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
		switch (dyn[i].d_tag) {
		case DT_STRTAB:
		vdso_info.symstrings = (const char *)
		((uintptr_t)dyn[i].d_un.d_ptr
		+ vdso_info.load_offset);
		break;
		case DT_SYMTAB:
		vdso_info.symtab = (Elf64_Sym *)
		((uintptr_t)dyn[i].d_un.d_ptr
		+ vdso_info.load_offset);
		break;
		case DT_HASH:
		hash = (Elf64_Word *)
		((uintptr_t)dyn[i].d_un.d_ptr
		+ vdso_info.load_offset);
		break;
		case DT_VERSYM:
		vdso_info.versym = (Elf64_Versym *)
		((uintptr_t)dyn[i].d_un.d_ptr
		+ vdso_info.load_offset);
		break;
		case DT_VERDEF:
		vdso_info.verdef = (Elf64_Verdef *)
		((uintptr_t)dyn[i].d_un.d_ptr
		+ vdso_info.load_offset);
		break;
		}
		}
		if (!vdso_info.symstrings \|\| !vdso_info.symtab \|\| !hash)
		return; /* Failed */

		if (!vdso_info.verdef)
		vdso_info.versym = 0;

		/* Parse the hash table header. */
		vdso_info.nbucket = hash[0];
		vdso_info.nchain = hash[1];
		vdso_info.bucket = &hash[2];
		vdso_info.chain = &hash[vdso_info.nbucket + 2];

		/* That's all we need. */
		vdso_info.valid = true;
		}

		static bool vdso_match_version(Elf64_Versym ver,
		const char *name, Elf64_Word hash)
		{
		/*
		* This is a helper function to check if the version indexed by
		* ver matches name (which hashes to hash).
		*
		* The version definition table is a mess, and I don't know how
		* to do this in better than linear time without allocating memory
		* to build an index. I also don't know why the table has
		* variable size entries in the first place.
		*
		* For added fun, I can't find a comprehensible specification of how
		* to parse all the weird flags in the table.
		*
		* So I just parse the whole table every time.
		*/

		/* First step: find the version definition */
		ver &= 0x7fff; /* Apparently bit 15 means "hidden" */
		Elf64_Verdef *def = vdso_info.verdef;
		while(true) {
		if ((def->vd_flags & VER_FLG_BASE) == 0
		&& (def->vd_ndx & 0x7fff) == ver)
		break;

		if (def->vd_next == 0)
		return false; /* No definition. */

		def = (Elf64_Verdef )((char )def + def->vd_next);
		}

		/* Now figure out whether it matches. */
		Elf64_Verdaux aux = (Elf64_Verdaux)((char *)def + def->vd_aux);
		return def->vd_hash == hash
		&& !strcmp(name, vdso_info.symstrings + aux->vda_name);
		}

		void vdso_sym(const char version, const char *name)
		{
		unsigned long ver_hash;
		if (!vdso_info.valid)
		return 0;

		ver_hash = elf_hash(version);
		Elf64_Word chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];

		for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) {
		Elf64_Sym *sym = &vdso_info.symtab[chain];

		/* Check for a defined global or weak function w/ right name. */
		if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
		continue;
		if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
		ELF64_ST_BIND(sym->st_info) != STB_WEAK)
		continue;
		if (sym->st_shndx == SHN_UNDEF)
		continue;
		if (strcmp(name, vdso_info.symstrings + sym->st_name))
		continue;

		/* Check symbol version. */
		if (vdso_info.versym
		&& !vdso_match_version(vdso_info.versym[chain],
		version, ver_hash))
		continue;

		return (void *)(vdso_info.load_offset + sym->st_value);
		}

		return 0;
		}

		void vdso_init_from_auxv(void *auxv)
		{
		Elf64_auxv_t *elf_auxv = auxv;
		for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++)
		{
		if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) {
		vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val);
		return;
		}
		}

		vdso_info.valid = false;
		}

Documentation/vDSO/vdso_test.c

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		/*
		* vdso_test.c: Sample code to test parse_vdso.c on x86_64
		* Copyright (c) 2011 Andy Lutomirski
		* Subject to the GNU General Public License, version 2
		*
		* You can amuse yourself by compiling with:
		* gcc -std=gnu99 -nostdlib
		* -Os -fno-asynchronous-unwind-tables -flto
		* vdso_test.c parse_vdso.c -o vdso_test
		* to generate a small binary with no dependencies at all.
		*/

		#include <sys/syscall.h>
		#include <sys/time.h>
		#include <unistd.h>
		#include <stdint.h>

		extern void vdso_sym(const char version, const char *name);
		extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
		extern void vdso_init_from_auxv(void *auxv);

		/* We need a libc functions... */
		int strcmp(const char a, const char b)
		{
		/* This implementation is buggy: it never returns -1. */
		while (a \|\| b) {
		if (a != b)
		return 1;
		if (a == 0 \|\| b == 0)
		return 1;
		a++;
		b++;
		}

		return 0;
		}

		/* ...and two syscalls. This is x86_64-specific. */
		static inline long linux_write(int fd, const void *data, size_t len)
		{

		long ret;
		asm volatile ("syscall" : "=a" (ret) : "a" (__NR_write),
		"D" (fd), "S" (data), "d" (len) :
		"cc", "memory", "rcx",
		"r8", "r9", "r10", "r11" );
		return ret;
		}

		static inline void linux_exit(int code)
		{
		asm volatile ("syscall" : : "a" (__NR_exit), "D" (code));
		}

		void to_base10(char *lastdig, uint64_t n)
		{
		while (n) {
		*lastdig = (n % 10) + '0';
		n /= 10;
		lastdig--;
		}
		}

		__attribute__((externally_visible)) void c_main(void **stack)
		{
		/* Parse the stack */
		long argc = (long)*stack;
		stack += argc + 2;

		/* Now we're pointing at the environment. Skip it. */
		while(*stack)
		stack++;
		stack++;

		/* Now we're pointing at auxv. Initialize the vDSO parser. */
		vdso_init_from_auxv((void *)stack);

		/* Find gettimeofday. */
		typedef long (gtod_t)(struct timeval tv, struct timezone *tz);
		gtod_t gtod = (gtod_t)vdso_sym("LINUX_2.6", "__vdso_gettimeofday");

		if (!gtod)
		linux_exit(1);

		struct timeval tv;
		long ret = gtod(&tv, 0);

		if (ret == 0) {
		char buf[] = "The time is .000000\n";
		to_base10(buf + 31, tv.tv_sec);
		to_base10(buf + 38, tv.tv_usec);
		linux_write(1, buf, sizeof(buf) - 1);
		} else {
		linux_exit(ret);
		}

		linux_exit(0);
		}

		/*
		* This is the real entry point. It passes the initial stack into
		* the C entry point.
		*/
		asm (
		".text\n"
		".global _start\n"
		".type _start,@function\n"
		"_start:\n\t"
		"mov %rsp,%rdi\n\t"
		"jmp c_main"
		);

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