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Commit 98eedc3a9dbf90cecb91093d2a7fa083942b7d13

Authored by Andy Lutomirski 2011-07-13 21:24:16 +0800

Committed by H. Peter Anvin 2011-07-15 08:57:09 +0800

Exists in master and in 6 other branches

Document the vDSO and add a reference parser

It turns out that parsing the vDSO is nontrivial if you don't already
have an ELF dynamic loader around.  So document it in Documentation/ABI
and add a reference CC0-licenced parser.

This code is dedicated to Go issue 1933:
http://code.google.com/p/go/issues/detail?id=1933

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/a315a9514cd71bcf29436cc31e35aada21a5ff21.1310563276.git.luto@mit.edu
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

Showing 3 changed files with 394 additions and 0 deletions Side-by-side Diff

Documentation/ABI/stable/vdso
Documentation/vDSO/parse_vdso.c
Documentation/vDSO/vdso_test.c

Documentation/ABI/stable/vdso

Diff comments View file @ 98eedc3

	1	+On some architectures, when the kernel loads any userspace program it
	2	+maps an ELF DSO into that program's address space. This DSO is called
	3	+the vDSO and it often contains useful and highly-optimized alternatives
	4	+to real syscalls.
	5	+
	6	+These functions are called just like ordinary C function according to
	7	+your platform's ABI. Call them from a sensible context. (For example,
	8	+if you set CS on x86 to something strange, the vDSO functions are
	9	+within their rights to crash.) In addition, if you pass a bad
	10	+pointer to a vDSO function, you might get SIGSEGV instead of -EFAULT.
	11	+
	12	+To find the DSO, parse the auxiliary vector passed to the program's
	13	+entry point. The AT_SYSINFO_EHDR entry will point to the vDSO.
	14	+
	15	+The vDSO uses symbol versioning; whenever you request a symbol from the
	16	+vDSO, specify the version you are expecting.
	17	+
	18	+Programs that dynamically link to glibc will use the vDSO automatically.
	19	+Otherwise, you can use the reference parser in Documentation/vDSO/parse_vdso.c.
	20	+
	21	+Unless otherwise noted, the set of symbols with any given version and the
	22	+ABI of those symbols is considered stable. It may vary across architectures,
	23	+though.
	24	+
	25	+(As of this writing, this ABI documentation as been confirmed for x86_64.
	26	+ The maintainers of the other vDSO-using architectures should confirm
	27	+ that it is correct for their architecture.)

Documentation/vDSO/parse_vdso.c

Diff comments View file @ 98eedc3

	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	+}

Documentation/vDSO/vdso_test.c

Diff comments View file @ 98eedc3

	1	+/*
	2	+ * vdso_test.c: Sample code to test parse_vdso.c on x86_64
	3	+ * Copyright (c) 2011 Andy Lutomirski
	4	+ * Subject to the GNU General Public License, version 2
	5	+ *
	6	+ * You can amuse yourself by compiling with:
	7	+ * gcc -std=gnu99 -nostdlib
	8	+ * -Os -fno-asynchronous-unwind-tables -flto
	9	+ * vdso_test.c parse_vdso.c -o vdso_test
	10	+ * to generate a small binary with no dependencies at all.
	11	+ */
	12	+
	13	+#include <sys/syscall.h>
	14	+#include <sys/time.h>
	15	+#include <unistd.h>
	16	+#include <stdint.h>
	17	+
	18	+extern void vdso_sym(const char version, const char *name);
	19	+extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
	20	+extern void vdso_init_from_auxv(void *auxv);
	21	+
	22	+/* We need a libc functions... */
	23	+int strcmp(const char a, const char b)
	24	+{
	25	+ /* This implementation is buggy: it never returns -1. */
	26	+ while (a \|\| b) {
	27	+ if (a != b)
	28	+ return 1;
	29	+ if (a == 0 \|\| b == 0)
	30	+ return 1;
	31	+ a++;
	32	+ b++;
	33	+ }
	34	+
	35	+ return 0;
	36	+}
	37	+
	38	+/* ...and two syscalls. This is x86_64-specific. */
	39	+static inline long linux_write(int fd, const void *data, size_t len)
	40	+{
	41	+
	42	+ long ret;
	43	+ asm volatile ("syscall" : "=a" (ret) : "a" (__NR_write),
	44	+ "D" (fd), "S" (data), "d" (len) :
	45	+ "cc", "memory", "rcx",
	46	+ "r8", "r9", "r10", "r11" );
	47	+ return ret;
	48	+}
	49	+
	50	+static inline void linux_exit(int code)
	51	+{
	52	+ asm volatile ("syscall" : : "a" (__NR_exit), "D" (code));
	53	+}
	54	+
	55	+void to_base10(char *lastdig, uint64_t n)
	56	+{
	57	+ while (n) {
	58	+ *lastdig = (n % 10) + '0';
	59	+ n /= 10;
	60	+ lastdig--;
	61	+ }
	62	+}
	63	+
	64	+__attribute__((externally_visible)) void c_main(void **stack)
	65	+{
	66	+ /* Parse the stack */
	67	+ long argc = (long)*stack;
	68	+ stack += argc + 2;
	69	+
	70	+ /* Now we're pointing at the environment. Skip it. */
	71	+ while(*stack)
	72	+ stack++;
	73	+ stack++;
	74	+
	75	+ /* Now we're pointing at auxv. Initialize the vDSO parser. */
	76	+ vdso_init_from_auxv((void *)stack);
	77	+
	78	+ /* Find gettimeofday. */
	79	+ typedef long (gtod_t)(struct timeval tv, struct timezone *tz);
	80	+ gtod_t gtod = (gtod_t)vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
	81	+
	82	+ if (!gtod)
	83	+ linux_exit(1);
	84	+
	85	+ struct timeval tv;
	86	+ long ret = gtod(&tv, 0);
	87	+
	88	+ if (ret == 0) {
	89	+ char buf[] = "The time is .000000\n";
	90	+ to_base10(buf + 31, tv.tv_sec);
	91	+ to_base10(buf + 38, tv.tv_usec);
	92	+ linux_write(1, buf, sizeof(buf) - 1);
	93	+ } else {
	94	+ linux_exit(ret);
	95	+ }
	96	+
	97	+ linux_exit(0);
	98	+}
	99	+
	100	+/*
	101	+ * This is the real entry point. It passes the initial stack into
	102	+ * the C entry point.
	103	+ */
	104	+asm (
	105	+ ".text\n"
	106	+ ".global _start\n"
	107	+ ".type _start,@function\n"
	108	+ "_start:\n\t"
	109	+ "mov %rsp,%rdi\n\t"
	110	+ "jmp c_main"
	111	+ );