Re: [PATCH v2] libbpf: Improve version handling when attaching uprobe

[Andrii Nakryiko]

On Sun, Apr 23, 2023 at 11:55 AM Espen Grindhaug
<espen.grindhaug@xxxxxxxxx> wrote:
>
> This change fixes the handling of versions in elf_find_func_offset.
> In the previous implementation, we incorrectly assumed that the
> version information would be present in the string found in the
> string table.
>
> We now look up the correct version string in the version symbol
> table before constructing the full name and then comparing.
>
> This patch adds support for both name@version and name@@version to
> match output of the various elf parsers.
>
> Signed-off-by: Espen Grindhaug <espen.grindhaug@xxxxxxxxx>
> ---
> Changes since v1:
> - Added test
> ---
>  tools/lib/bpf/libbpf.c                        | 148 +++++++++++++++---
>  .../selftests/bpf/prog_tests/attach_probe.c   |  37 +++++
>  .../selftests/bpf/progs/test_attach_probe.c   |  15 ++
>  3 files changed, 181 insertions(+), 19 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 49cd304ae3bc..ef5e11ce6241 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -10620,31 +10620,94 @@ static int perf_event_uprobe_open_legacy(const char *probe_name, bool retprobe,
>  }
>
>  /* Return next ELF section of sh_type after scn, or first of that type if scn is NULL. */
> -static Elf_Scn *elf_find_next_scn_by_type(Elf *elf, int sh_type, Elf_Scn *scn)
> +static Elf_Scn *elf_find_next_scn_by_type(Elf *elf, int sh_type, Elf_Scn *scn, size_t *idx)
>  {
>         while ((scn = elf_nextscn(elf, scn)) != NULL) {
>                 GElf_Shdr sh;
>
>                 if (!gelf_getshdr(scn, &sh))
>                         continue;
> -               if (sh.sh_type == sh_type)
> +               if (sh.sh_type == sh_type) {
> +                       if (idx)
> +                               *idx = sh.sh_link;
>                         return scn;
> +               }
> +       }
> +       return NULL;
> +}
> +
> +static Elf_Data *elf_find_data_by_type(Elf *elf, int sh_type, size_t *idx)
> +{
> +       Elf_Scn *scn = elf_find_next_scn_by_type(elf, sh_type, NULL, idx);
> +
> +       if (scn)
> +               return elf_getdata(scn, NULL);
> +
> +       return NULL;
> +}
> +
> +static Elf64_Verdef *elf_verdef_by_offset(Elf_Data *data, size_t offset)
> +{
> +       if (offset + sizeof(Elf64_Verdef) > data->d_size)
> +               return NULL;
> +
> +       return (Elf64_Verdef *)((char *) data->d_buf + offset);
> +}
> +
> +static Elf64_Versym *elf_versym_by_idx(Elf_Data *data, size_t idx)
> +{
> +       if (idx >= data->d_size / sizeof(Elf64_Versym))
> +               return NULL;
> +
> +       return (Elf64_Versym *)(data->d_buf + idx * sizeof(Elf64_Versym));
> +}
> +
> +static Elf64_Verdaux *elf_verdaux_by_offset(Elf_Data *data, size_t offset)
> +{
> +       if (offset + sizeof(Elf64_Verdaux) > data->d_size)
> +               return NULL;
> +
> +       return (Elf64_Verdaux *)((char *) data->d_buf + offset);
> +}
> +
> +#define ELF_VERSYM_HIDDEN 0x8000
> +#define ELF_VERSYM_IDX_MASK 0x7fff
> +
> +static Elf64_Verdaux *elf_get_verdaux_by_versym(Elf_Data *verdef_data, Elf64_Versym *versym)
> +{
> +       size_t offset = 0;
> +
> +       while (offset + sizeof(Elf64_Verdef) <= verdef_data->d_size) {
> +               Elf64_Verdef *verdef = elf_verdef_by_offset(verdef_data, offset);
> +
> +               if (!verdef)
> +                       break;
> +
> +               if (verdef->vd_ndx == (*versym & ELF_VERSYM_IDX_MASK))
> +                       return elf_verdaux_by_offset(verdef_data, offset + verdef->vd_aux);
> +
> +               if (verdef->vd_next == 0)
> +                       break;
> +
> +               offset += verdef->vd_next;
>         }
>         return NULL;
>  }

So all of the above is quite cryptic and unclear. I, for example,
don't really know how symbol versioning information is laid out in
ELF, and it's very hard to follow the logic here. Please add comments
explaining the format and how different sections are related to each
other.

>
>  /* Find offset of function name in the provided ELF object. "binary_path" is
>   * the path to the ELF binary represented by "elf", and only used for error
> - * reporting matters. "name" matches symbol name or name@@LIB for library
> - * functions.
> + * reporting matters. "name" matches symbol name, name@LIB or name@@LIB for
> + * library functions.
>   */
>  static 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;
>         long ret = -ENOENT;
> -       size_t name_len;
>         GElf_Ehdr ehdr;
> +       Elf_Data *versym_data = NULL;
> +       Elf_Data *verdef_data = NULL;
> +       size_t verdef_stridx = 0;
>
>         if (!gelf_getehdr(elf, &ehdr)) {
>                 pr_warn("elf: failed to get ehdr from %s: %s\n", binary_path, elf_errmsg(-1));
> @@ -10654,9 +10717,12 @@ static long elf_find_func_offset(Elf *elf, const char *binary_path, const char *
>         /* 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" or "@LIB"? */
> +       is_name_qualified = strstr(name, "@") != NULL;
> +
> +       /* Extract version definition and version symbol table */
> +       versym_data = elf_find_data_by_type(elf, SHT_GNU_versym, NULL);
> +       verdef_data = elf_find_data_by_type(elf, SHT_GNU_verdef, &verdef_stridx);
>
>         /* 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
> @@ -10671,10 +10737,10 @@ static long elf_find_func_offset(Elf *elf, const char *binary_path, const char *
>                 const char *sname;
>                 GElf_Shdr sh;
>
> -               scn = elf_find_next_scn_by_type(elf, sh_types[i], NULL);
> +               scn = elf_find_next_scn_by_type(elf, sh_types[i], NULL, NULL);
>                 if (!scn) {
>                         pr_debug("elf: failed to find symbol table ELF sections in '%s'\n",
> -                                binary_path);
> +                               binary_path);

please don't change unrelated lines of code

>                         continue;
>                 }
>                 if (!gelf_getshdr(scn, &sh))
> @@ -10705,16 +10771,60 @@ static long elf_find_func_offset(Elf *elf, const char *binary_path, const char *
>                         if (!sname)
>                                 continue;
>
> -                       curr_bind = GELF_ST_BIND(sym.st_info);
> +                       if (is_name_qualified) {
> +                               Elf64_Versym *versym;
> +                               Elf64_Verdaux *verdaux;

why are we assuming 64-bit binaries?

> +                               int res;
> +                               char full_name[256];
>
> -                       /* User can specify func, func@@LIB or func@@LIB_VERSION. */
> -                       if (strncmp(sname, 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 && sname[name_len] != '\0' && sname[name_len] != '@')
> -                               continue;
> +                               /* check that name at least starts with sname before building
> +                                * the full name
> +                                */
> +                               if (strncmp(name, sname, strlen(sname)) != 0)
> +                                       continue;
> +
> +                               if (!versym_data || !verdef_data) {
> +                                       pr_warn("elf: failed to find version definition or version symbol table in '%s'\n",
> +                                               binary_path);
> +                                       break;
> +                               }
> +
> +                               versym = elf_versym_by_idx(versym_data, idx);
> +                               if (!versym) {
> +                                       pr_warn("elf: failed to lookup versym for '%s' in '%s'\n",
> +                                               sname, binary_path);
> +                                       continue;
> +                               }
> +
> +                               verdaux = elf_get_verdaux_by_versym(verdef_data, versym);
> +                               if (!verdaux) {
> +                                       pr_warn("elf: failed to lookup verdaux for '%s' in '%s'\n",
> +                                               sname, binary_path);
> +                                       continue;
> +                               }
> +
> +                               res = snprintf(full_name, sizeof(full_name),
> +                                              (*versym & ELF_VERSYM_HIDDEN) ? "%s@%s" :
> +                                                                   "%s@@%s",
> +                                              sname,
> +                                              elf_strptr(elf, verdef_stridx,
> +                                                         verdaux->vda_name));
> +
> +                               if (res < 0 || res >= sizeof(full_name)) {
> +                                       pr_warn("elf: failed to build full name for '%s' in '%s'\n",
> +                                               sname, binary_path);
> +                                       continue;
> +                               }

maybe this version fetching part can be extracted into a helper to
keep the main logic more straightforward?

> +
> +                               if (strcmp(full_name, name) != 0)
> +                                       continue;
> +                       } else {
> +                               /* If name is not qualified, we want to match the symbol name */
> +                               if (strcmp(sname, name) != 0)
> +                                       continue;
> +                       }
> +
> +                       curr_bind = GELF_ST_BIND(sym.st_info);
>
>                         if (ret >= 0) {
>                                 /* handle multiple matches */
> diff --git a/tools/testing/selftests/bpf/prog_tests/attach_probe.c b/tools/testing/selftests/bpf/prog_tests/attach_probe.c
> index 7175af39134f..c3f33f7e9d12 100644
> --- a/tools/testing/selftests/bpf/prog_tests/attach_probe.c
> +++ b/tools/testing/selftests/bpf/prog_tests/attach_probe.c

please split selftests into a separate patch

Also, it's not great that we rely on specific GLIBC versions for
testing. There is no need to rely on glibc here at all. We have our
own test executable and shared library, urandom_read and
liburandom_read.so, which are used exactly for these sort of tests.

While I was trying to understand all this versioning  some more, I've
wrote a bunch of code already. Please use it as a base and switch
tests to these new urandom_api() (v1 and v2) functions:

diff --git a/tools/testing/selftests/bpf/Makefile
b/tools/testing/selftests/bpf/Makefile
index c49e5403ad0e..ff8685aac28f 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -181,11 +181,12 @@ endif

 # Filter out -static for liburandom_read.so and its dependent targets
so that static builds
 # do not fail. Static builds leave urandom_read relying on
system-wide shared libraries.
-$(OUTPUT)/liburandom_read.so: urandom_read_lib1.c urandom_read_lib2.c
+$(OUTPUT)/liburandom_read.so: urandom_read_lib1.c urandom_read_lib2.c
liburandom_read.map
        $(call msg,LIB,,$@)
        $(Q)$(CLANG) $(filter-out -static,$(CFLAGS) $(LDFLAGS))   \
-                    $^ $(filter-out -static,$(LDLIBS))      \
+                    $(filter %.c,$^) $(filter-out -static,$(LDLIBS))        \
                     -fuse-ld=$(LLD) -Wl,-znoseparate-code -Wl,--build-id=sha1 \
+                    -Wl,--version-script=liburandom_read.map
         \
                     -fPIC -shared -o $@

 $(OUTPUT)/urandom_read: urandom_read.c urandom_read_aux.c
$(OUTPUT)/liburandom_read.so
diff --git a/tools/testing/selftests/bpf/urandom_read.c
b/tools/testing/selftests/bpf/urandom_read.c
index e92644d0fa75..36ccfeb63443 100644
--- a/tools/testing/selftests/bpf/urandom_read.c
+++ b/tools/testing/selftests/bpf/urandom_read.c
@@ -21,6 +21,10 @@ void urand_read_without_sema(int iter_num, int
iter_cnt, int read_sz);
 void urandlib_read_with_sema(int iter_num, int iter_cnt, int read_sz);
 void urandlib_read_without_sema(int iter_num, int iter_cnt, int read_sz);

+int urandlib_api(void);
+__asm__(".symver urandlib_api_old,urandlib_api@LIBURANDOM_READ_1.0.0");
+int urandlib_api_old(void);
+
 unsigned short urand_read_with_sema_semaphore SEC(".probes");

 static __attribute__((noinline))
@@ -83,6 +87,9 @@ int main(int argc, char *argv[])

        urandom_read(fd, count);

+       urandlib_api();
+       urandlib_api_old();
+
        close(fd);
        return 0;
 }
diff --git a/tools/testing/selftests/bpf/urandom_read_lib1.c
b/tools/testing/selftests/bpf/urandom_read_lib1.c
index 86186e24b740..eb02b8a9032c 100644
--- a/tools/testing/selftests/bpf/urandom_read_lib1.c
+++ b/tools/testing/selftests/bpf/urandom_read_lib1.c
@@ -11,3 +11,37 @@ void urandlib_read_with_sema(int iter_num, int
iter_cnt, int read_sz)
 {
        STAP_PROBE3(urandlib, read_with_sema, iter_num, iter_cnt, read_sz);
 }
+
+/* Symbol versioning is different between static and shared library.
+ * Properly versioned symbols are needed for shared library, but
+ * only the symbol of the new version is needed for static library.
+ * Starting with GNU C 10, use symver attribute instead of .symver assembler
+ * directive, which works better with GCC LTO builds.
+ */
+#if defined(__GNUC__) && __GNUC__ >= 10
+
+#define DEFAULT_VERSION(internal_name, api_name, version) \
+       __attribute__((symver(#api_name "@@" #version)))
+#define OMPAT_VERSION(internal_name, api_name, version) \
+       __attribute__((symver(#api_name "@" #version)))
+
+#else
+
+#define COMPAT_VERSION(internal_name, api_name, version) \
+       asm(".symver " #internal_name "," #api_name "@" #version);
+#define DEFAULT_VERSION(internal_name, api_name, version) \
+       asm(".symver " #internal_name "," #api_name "@@" #version);
+
+#endif
+
+COMPAT_VERSION(urandlib_api_v1, urandlib_api, LIBURANDOM_READ_1.0.0)
+int urandlib_api_v1(void)
+{
+       return 1;
+}
+
+DEFAULT_VERSION(urandlib_api_v2, urandlib_api, LIBURANDOM_READ_2.0.0)
+int urandlib_api_v2(void)
+{
+       return 2;
+}


> @@ -192,6 +192,41 @@ static void test_uprobe_lib(struct test_attach_probe *skel)
>         ASSERT_EQ(skel->bss->uretprobe_byname2_res, 8, "check_uretprobe_byname2_res");
>  }
>
> +static void test_uprobe_lib_with_versions(struct test_attach_probe *skel)
> +{
> +       DECLARE_LIBBPF_OPTS(bpf_uprobe_opts, uprobe_opts);
> +       char absolute_path[256];
> +
> +       /* test attach with a versioned name.
> +        * realpath has two implementations in libc, only the default version will be used.
> +        */
> +       uprobe_opts.func_name = "realpath@@GLIBC_2.3";
> +       uprobe_opts.retprobe = false;
> +       skel->links.handle_uprobe_byversionedname_a =
> +                       bpf_program__attach_uprobe_opts(skel->progs.handle_uprobe_byversionedname_a,
> +                                                       0 /* this pid */,
> +                                                       "libc.so.6",
> +                                                       0, &uprobe_opts);
> +       if (!ASSERT_OK_PTR(skel->links.handle_uprobe_byversionedname_a, "attach_handle_uprobe_byversionedname_a"))
> +               return;
> +
> +       uprobe_opts.func_name = "realpath@GLIBC_2.2.5";
> +       uprobe_opts.retprobe = false;
> +       skel->links.handle_uprobe_byversionedname_b =
> +                       bpf_program__attach_uprobe_opts(skel->progs.handle_uprobe_byversionedname_b,
> +                                                       0 /* this pid */,
> +                                                       "libc.so.6",
> +                                                       0, &uprobe_opts);
> +       if (!ASSERT_OK_PTR(skel->links.handle_uprobe_byversionedname_b, "attach_handle_uprobe_byversionedname_b"))
> +               return;
> +
> +       /* trigger & validate probes */
> +       realpath("/", absolute_path);
> +
> +       ASSERT_EQ(skel->bss->uprobe_byversionedname_a_res, 13, "check_uprobe_byversionedname_a_res");
> +       ASSERT_NEQ(skel->bss->uprobe_byversionedname_b_res, 14, "check_uprobe_byversionedname_b_res");
> +}
> +
>  static void test_uprobe_ref_ctr(struct test_attach_probe *skel)
>  {
>         DECLARE_LIBBPF_OPTS(bpf_uprobe_opts, uprobe_opts);
> @@ -316,6 +351,8 @@ void test_attach_probe(void)
>                 test_kprobe_sleepable();
>         if (test__start_subtest("uprobe-lib"))
>                 test_uprobe_lib(skel);
> +       if (test__start_subtest("uprobe-lib-with-versions"))
> +               test_uprobe_lib_with_versions(skel);
>         if (test__start_subtest("uprobe-sleepable"))
>                 test_uprobe_sleepable(skel);
>         if (test__start_subtest("uprobe-ref_ctr"))
> diff --git a/tools/testing/selftests/bpf/progs/test_attach_probe.c b/tools/testing/selftests/bpf/progs/test_attach_probe.c
> index 68466a6ad18c..079b58901ff8 100644
> --- a/tools/testing/selftests/bpf/progs/test_attach_probe.c
> +++ b/tools/testing/selftests/bpf/progs/test_attach_probe.c
> @@ -17,6 +17,8 @@ int uprobe_byname3_sleepable_res = 0;
>  int uprobe_byname3_res = 0;
>  int uretprobe_byname3_sleepable_res = 0;
>  int uretprobe_byname3_res = 0;
> +int uprobe_byversionedname_a_res = 0;
> +int uprobe_byversionedname_b_res = 0;
>  void *user_ptr = 0;
>
>  SEC("ksyscall/nanosleep")
> @@ -121,5 +123,18 @@ int handle_uretprobe_byname3(struct pt_regs *ctx)
>         return 0;
>  }
>
> +SEC("uprobe")
> +int BPF_UPROBE(handle_uprobe_byversionedname_a, const char *a, char *b)
> +{
> +       uprobe_byversionedname_a_res = 13;
> +       return 0;
> +}
> +
> +SEC("uprobe")
> +int BPF_UPROBE(handle_uprobe_byversionedname_b, const char *a, char *b)
> +{
> +       uprobe_byversionedname_b_res = 14;
> +       return 0;
> +}
>
>  char _license[] SEC("license") = "GPL";
> --
> 2.34.1
>