Re: [PATCH bpf-next v2 3/5] libbpf: Handle <orig_name>.llvm.<hash> symbol properly

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On Thu, Mar 21, 2024 at 1:01 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
> With CONFIG_LTO_CLANG_THIN enabled, with some of previous
> version of kernel code base ([1]), I hit the following
> error:
>    test_ksyms:PASS:kallsyms_fopen 0 nsec
>    test_ksyms:FAIL:ksym_find symbol 'bpf_link_fops' not found
>    #118     ksyms:FAIL
>
> The reason is that 'bpf_link_fops' is renamed to
>    bpf_link_fops.llvm.8325593422554671469
> Due to cross-file inlining, the static variable 'bpf_link_fops'
> in syscall.c is used by a function in another file. To avoid
> potential duplicated names, the llvm added suffix
> '.llvm.<hash>' ([2]) to 'bpf_link_fops' variable.
> Such renaming caused a problem in libbpf if 'bpf_link_fops'
> is used in bpf prog as a ksym as 'bpf_link_fops' does not
> match any symbol in /proc/kallsyms.
>
> To fix this issue, libbpf needs to understand that suffix '.llvm.<hash>'
> is caused by clang lto kernel and to process such symbols properly.
>
> With latest bpf-next code base built with CONFIG_LTO_CLANG_THIN,
> I cannot reproduce the above failure any more. But such an issue
> could happen with other symbols.
>
> For example, with my current kernel, I got the following from
> /proc/kallsyms:
>   ffffffff84782154 d __func__.net_ratelimit.llvm.6135436931166841955
>   ffffffff85f0a500 d tk_core.llvm.726630847145216431
>   ffffffff85fdb960 d __fs_reclaim_map.llvm.10487989720912350772
>   ffffffff864c7300 d fake_dst_ops.llvm.54750082607048300
>
> I could not easily create a selftest to test newly-added
> libbpf functionality with a static C test since I do not know
> which symbol is cross-file inlined. But based on my particular kernel,
> the following test change can run successfully.
>
>   diff --git a/tools/testing/selftests/bpf/prog_tests/ksyms.c b/tools/testing/selftests/bpf/prog_tests/ksyms.c
>   index 6a86d1f07800..904a103f7b1d 100644
>   --- a/tools/testing/selftests/bpf/prog_tests/ksyms.c
>   +++ b/tools/testing/selftests/bpf/prog_tests/ksyms.c
>   @@ -42,6 +42,7 @@ void test_ksyms(void)
>           ASSERT_EQ(data->out__bpf_link_fops, link_fops_addr, "bpf_link_fops");
>           ASSERT_EQ(data->out__bpf_link_fops1, 0, "bpf_link_fops1");
>           ASSERT_EQ(data->out__btf_size, btf_size, "btf_size");
>   +       ASSERT_NEQ(data->out__fake_dst_ops, 0, "fake_dst_ops");
>           ASSERT_EQ(data->out__per_cpu_start, per_cpu_start_addr, "__per_cpu_start");
>
>    cleanup:
>   diff --git a/tools/testing/selftests/bpf/progs/test_ksyms.c b/tools/testing/selftests/bpf/progs/test_ksyms.c
>   index 6c9cbb5a3bdf..fe91eef54b66 100644
>   --- a/tools/testing/selftests/bpf/progs/test_ksyms.c
>   +++ b/tools/testing/selftests/bpf/progs/test_ksyms.c
>   @@ -9,11 +9,13 @@ __u64 out__bpf_link_fops = -1;
>    __u64 out__bpf_link_fops1 = -1;
>    __u64 out__btf_size = -1;
>    __u64 out__per_cpu_start = -1;
>   +__u64 out__fake_dst_ops = -1;
>
>    extern const void bpf_link_fops __ksym;
>    extern const void __start_BTF __ksym;
>    extern const void __stop_BTF __ksym;
>    extern const void __per_cpu_start __ksym;
>   +extern const void fake_dst_ops __ksym;
>    /* non-existing symbol, weak, default to zero */
>    extern const void bpf_link_fops1 __ksym __weak;
>
>   @@ -23,6 +25,7 @@ int handler(const void *ctx)
>           out__bpf_link_fops = (__u64)&bpf_link_fops;
>           out__btf_size = (__u64)(&__stop_BTF - &__start_BTF);
>           out__per_cpu_start = (__u64)&__per_cpu_start;
>   +       out__fake_dst_ops = (__u64)&fake_dst_ops;
>
>           out__bpf_link_fops1 = (__u64)&bpf_link_fops1;
>
> This patch fixed the issue in libbpf such that if clang lto kernel
> is enabled and the symbol resolution is for ksym's,
> the suffix '.llvm.<hash>' will be ignored during comparison of
> bpf prog ksym vs. symbols in /proc/kallsyms, this resolved the issue.
>
> Note that currently kernel does not support gcc build with lto.
>
>   [1] https://lore.kernel.org/bpf/20240302165017.1627295-1-yonghong.song@xxxxxxxxx/
>   [2] https://github.com/llvm/llvm-project/blob/release/18.x/llvm/include/llvm/IR/ModuleSummaryIndex.h#L1714-L1719
>
> Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
> ---
>  tools/lib/bpf/libbpf.c | 64 +++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 63 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index a7a89269148c..8c3861192bc8 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -664,6 +664,7 @@ struct bpf_object {
>         bool loaded;
>         bool has_subcalls;
>         bool has_rodata;
> +       bool need_kallsyms;
>
>         struct bpf_gen *gen_loader;
>
> @@ -8016,14 +8017,73 @@ static int libbpf_kallsyms_parse(kallsyms_cb_t cb, void *ctx)
>         return err;
>  }
>
> +static int check_lto_kernel(void)
> +{
> +       static int check_lto = 2;
> +       char buf[PATH_MAX];
> +       struct utsname uts;
> +       gzFile file;
> +       int len;
> +
> +       if (check_lto != 2)
> +               return check_lto;
> +
> +       uname(&uts);
> +       len = snprintf(buf, PATH_MAX, "/boot/config-%s", uts.release);
> +       if (len < 0) {
> +               check_lto = -EINVAL;
> +               goto out;
> +       } else if (len >= PATH_MAX) {
> +               check_lto = -ENAMETOOLONG;
> +               goto out;
> +       }
> +
> +       /* gzopen also accepts uncompressed files. */
> +       file = gzopen(buf, "re");
> +       if (!file)
> +               file = gzopen("/proc/config.gz", "re");
> +
> +       if (!file) {
> +               check_lto = -ENOENT;
> +               goto out;
> +       }
> +
> +       check_lto = 0;
> +       while (gzgets(file, buf, sizeof(buf))) {
> +               /* buf also contains '\n', skip it during comparison. */
> +               if (!strncmp(buf, "CONFIG_LTO_CLANG=y", 18)) {
> +                       check_lto = 1;
> +                       break;
> +               }
> +       }
> +
> +       gzclose(file);
> +out:
> +       return check_lto;
> +}
> +
>  static int kallsyms_cb(unsigned long long sym_addr, char sym_type,
>                        const char *sym_name, void *ctx)
>  {
> +       int lto_enabled = check_lto_kernel();
> +       char orig_name[PATH_MAX], *res;
>         struct bpf_object *obj = ctx;
>         const struct btf_type *t;
>         struct extern_desc *ext;
>
> -       ext = find_extern_by_name(obj, sym_name);
> +       /* Only check static variables in data sections */
> +       if (sym_type == 'd' && obj->need_kallsyms && lto_enabled == 1) {

why bother grepping config.gz ?
I see no harm in doing below strstr unconditionally.

> +               strcpy(orig_name, sym_name);
> +               res = strstr(orig_name, ".llvm.");
> +               if (res) {
> +                       *res = '\0';
> +                       pr_debug("extern (ksym) '%s': use original name '%s' for comparison\n",
> +                                sym_name, orig_name);
> +               }
> +               ext = find_extern_by_name(obj, orig_name);
> +       } else {
> +               ext = find_extern_by_name(obj, sym_name);
> +       }
>         if (!ext || ext->type != EXT_KSYM)
>                 return 0;
>
> @@ -8322,7 +8382,9 @@ static int bpf_object__resolve_externs(struct bpf_object *obj,
>                         return -EINVAL;
>         }
>         if (need_kallsyms) {
> +               obj->need_kallsyms = true;
>                 err = bpf_object__read_kallsyms_file(obj);
> +               obj->need_kallsyms = false;
>                 if (err)
>                         return -EINVAL;
>         }
> --
> 2.43.0
>





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