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libbpf: Support symbol versioning for uprobe

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In current implementation, we assume that symbol found in .dynsym section
would have a version suffix and use it to compare with symbol user supplied.
According to the spec ([0]), this assumption is incorrect, the version info
of dynamic symbols are stored in .gnu.version and .gnu.version_d sections
of ELF objects. For example:

    $ nm -D /lib/x86_64-linux-gnu/libc.so.6 | grep rwlock_wrlock
    000000000009b1a0 T __pthread_rwlock_wrlock@GLIBC_2.2.5
    000000000009b1a0 T pthread_rwlock_wrlock@@GLIBC_2.34
    000000000009b1a0 T pthread_rwlock_wrlock@GLIBC_2.2.5

    $ readelf -W --dyn-syms /lib/x86_64-linux-gnu/libc.so.6 | grep rwlock_wrlock
      706: 000000000009b1a0   878 FUNC    GLOBAL DEFAULT   15 __pthread_rwlock_wrlock@GLIBC_2.2.5
      2568: 000000000009b1a0   878 FUNC    GLOBAL DEFAULT   15 pthread_rwlock_wrlock@@GLIBC_2.34
      2571: 000000000009b1a0   878 FUNC    GLOBAL DEFAULT   15 pthread_rwlock_wrlock@GLIBC_2.2.5

In this case, specify pthread_rwlock_wrlock@@GLIBC_2.34 or
pthread_rwlock_wrlock@GLIBC_2.2.5 in bpf_uprobe_opts::func_name won't work.
Because the qualified name does NOT match `pthread_rwlock_wrlock` (without
version suffix) in .dynsym sections.

This commit implements the symbol versioning for dynsym and allows user to
specify symbol in the following forms:
  - func
  - func@LIB_VERSION
  - func@@LIB_VERSION

In case of symbol conflicts, error out and users should resolve it by
specifying a qualified name.

  [0]: https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/symversion.html

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20230918024813.237475-3-hengqi.chen@gmail.com
This commit is contained in:
Hengqi Chen 2023-09-18 02:48:12 +00:00 committed by Andrii Nakryiko
parent 7257cee652
commit bb7fa09399
2 changed files with 124 additions and 12 deletions
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134
tools/lib/bpf/elf.c
View File

@ -1,5 +1,8 @@
// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif
#include <libelf.h>
#include <gelf.h>
#include <fcntl.h>
@ -10,6 +13,17 @@
#define STRERR_BUFSIZE 128
/* A SHT_GNU_versym section holds 16-bit words. This bit is set if
* the symbol is hidden and can only be seen when referenced using an
* explicit version number. This is a GNU extension.
*/
#define VERSYM_HIDDEN 0x8000
/* This is the mask for the rest of the data in a word read from a
* SHT_GNU_versym section.
*/
#define VERSYM_VERSION 0x7fff
int elf_open(const char *binary_path, struct elf_fd *elf_fd)
{
char errmsg[STRERR_BUFSIZE];
@ -64,13 +78,18 @@ struct elf_sym {
const char *name;
GElf_Sym sym;
GElf_Shdr sh;
int ver;
bool hidden;
};
struct elf_sym_iter {
Elf *elf;
Elf_Data *syms;
Elf_Data *versyms;
Elf_Data *verdefs;
size_t nr_syms;
size_t strtabidx;
size_t verdef_strtabidx;
size_t next_sym_idx;
struct elf_sym sym;
int st_type;
@ -111,6 +130,26 @@ static int elf_sym_iter_new(struct elf_sym_iter *iter,
iter->nr_syms = iter->syms->d_size / sh.sh_entsize;
iter->elf = elf;
iter->st_type = st_type;
/* Version symbol table is meaningful to dynsym only */
if (sh_type != SHT_DYNSYM)
return 0;
scn = elf_find_next_scn_by_type(elf, SHT_GNU_versym, NULL);
if (!scn)
return 0;
iter->versyms = elf_getdata(scn, 0);
scn = elf_find_next_scn_by_type(elf, SHT_GNU_verdef, NULL);
if (!scn) {
pr_debug("elf: failed to find verdef ELF sections in '%s'\n", binary_path);
return -ENOENT;
}
if (!gelf_getshdr(scn, &sh))
return -EINVAL;
iter->verdef_strtabidx = sh.sh_link;
iter->verdefs = elf_getdata(scn, 0);
return 0;
}
@ -119,6 +158,7 @@ static struct elf_sym *elf_sym_iter_next(struct elf_sym_iter *iter)
struct elf_sym *ret = &iter->sym;
GElf_Sym *sym = &ret->sym;
const char *name = NULL;
GElf_Versym versym;
Elf_Scn *sym_scn;
size_t idx;
@ -138,12 +178,80 @@ static struct elf_sym *elf_sym_iter_next(struct elf_sym_iter *iter)
iter->next_sym_idx = idx + 1;
ret->name = name;
ret->ver = 0;
ret->hidden = false;
if (iter->versyms) {
if (!gelf_getversym(iter->versyms, idx, &versym))
continue;
ret->ver = versym & VERSYM_VERSION;
ret->hidden = versym & VERSYM_HIDDEN;
}
return ret;
}
return NULL;
}
static const char *elf_get_vername(struct elf_sym_iter *iter, int ver)
{
GElf_Verdaux verdaux;
GElf_Verdef verdef;
int offset;
offset = 0;
while (gelf_getverdef(iter->verdefs, offset, &verdef)) {
if (verdef.vd_ndx != ver) {
if (!verdef.vd_next)
break;
offset += verdef.vd_next;
continue;
}
if (!gelf_getverdaux(iter->verdefs, offset + verdef.vd_aux, &verdaux))
break;
return elf_strptr(iter->elf, iter->verdef_strtabidx, verdaux.vda_name);
}
return NULL;
}
static bool symbol_match(struct elf_sym_iter *iter, int sh_type, struct elf_sym *sym,
const char *name, size_t name_len, const char *lib_ver)
{
const char *ver_name;
/* Symbols are in forms of func, func@LIB_VER or func@@LIB_VER
* make sure the func part matches the user specified name
*/
if (strncmp(sym->name, name, name_len) != 0)
return false;
/* ...but we don't want a search for "foo" to match 'foo2" also, so any
* additional characters in sname should be of the form "@@LIB".
*/
if (sym->name[name_len] != '\0' && sym->name[name_len] != '@')
return false;
/* If user does not specify symbol version, then we got a match */
if (!lib_ver)
return true;
/* If user specifies symbol version, for dynamic symbols,
* get version name from ELF verdef section for comparison.
*/
if (sh_type == SHT_DYNSYM) {
ver_name = elf_get_vername(iter, sym->ver);
if (!ver_name)
return false;
return strcmp(ver_name, lib_ver) == 0;
}
/* For normal symbols, it is already in form of func@LIB_VER */
return strcmp(sym->name, name) == 0;
}
/* Transform symbol's virtual address (absolute for binaries and relative
* for shared libs) into file offset, which is what kernel is expecting
@ -166,7 +274,8 @@ static unsigned long elf_sym_offset(struct elf_sym *sym)
long elf_find_func_offset(Elf *elf, const char *binary_path, const char *name)
{
int i, sh_types[2] = { SHT_DYNSYM, SHT_SYMTAB };
bool is_shared_lib, is_name_qualified;
const char *at_symbol, *lib_ver;
bool is_shared_lib;
long ret = -ENOENT;
size_t name_len;
GElf_Ehdr ehdr;
@ -179,9 +288,18 @@ long elf_find_func_offset(Elf *elf, const char *binary_path, const char *name)
/* for shared lib case, we do not need to calculate relative offset */
is_shared_lib = ehdr.e_type == ET_DYN;
name_len = strlen(name);
/* Does name specify "@@LIB"? */
is_name_qualified = strstr(name, "@@") != NULL;
/* Does name specify "@@LIB_VER" or "@LIB_VER" ? */
at_symbol = strchr(name, '@');
if (at_symbol) {
name_len = at_symbol - name;
/* skip second @ if it's @@LIB_VER case */
if (at_symbol[1] == '@')
at_symbol++;
lib_ver = at_symbol + 1;
} else {
name_len = strlen(name);
lib_ver = NULL;
}
/* Search SHT_DYNSYM, SHT_SYMTAB for symbol. This search order is used because if
* a binary is stripped, it may only have SHT_DYNSYM, and a fully-statically
@ -201,13 +319,7 @@ long elf_find_func_offset(Elf *elf, const char *binary_path, const char *name)
goto out;
while ((sym = elf_sym_iter_next(&iter))) {
/* User can specify func, func@@LIB or func@@LIB_VERSION. */
if (strncmp(sym->name, name, name_len) != 0)
continue;
/* ...but we don't want a search for "foo" to match 'foo2" also, so any
* additional characters in sname should be of the form "@@LIB".
*/
if (!is_name_qualified && sym->name[name_len] != '\0' && sym->name[name_len] != '@')
if (!symbol_match(&iter, sh_types[i], sym, name, name_len, lib_ver))
continue;
cur_bind = GELF_ST_BIND(sym->sym.st_info);

2
tools/lib/bpf/libbpf.c
View File

@ -11630,7 +11630,7 @@ static int attach_uprobe(const struct bpf_program *prog, long cookie, struct bpf
*link = NULL;
n = sscanf(prog->sec_name, "%m[^/]/%m[^:]:%m[a-zA-Z0-9_.]+%li",
n = sscanf(prog->sec_name, "%m[^/]/%m[^:]:%m[a-zA-Z0-9_.@]+%li",
&probe_type, &binary_path, &func_name, &offset);
switch (n) {
case 1: