Re: [PATCH] perf symbol: Do not use debug file as the binary type

[Ian Rogers]

On Mon, Aug 24, 2026 at 11:23 PM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>
> dso__load() sets the binary type of a DSO to the type of the first symbol
> source found. For a DSO with a separate debug file linked via
> .gnu-debuglink, that is DSO_BINARY_TYPE__DEBUGLINK, which makes
> dso__get_filename() return the name of the debug file instead of the file
> that was actually executed.
>
> Consumers that need to read instruction bytes, such as Intel PT decoding
> in 'perf script', then read from the debug file and produce wrong
> instructions.
>
> Prefer DSO_BINARY_TYPE__BUILD_ID_CACHE, and otherwise
> DSO_BINARY_TYPE__SYSTEM_PATH_DSO, over debug-only types, which restores
> the behaviour of using a file that contains the executed instructions.
>
> This is a workaround. Properly separating the binary file used for
> instructions from the file used for debug symbols is left for later.
>
> Example:
>
>  Create a shared object with a separate .gnu_debuglink debug file. Note
>  that 'objcopy --only-keep-debug' leaves .text as NOBITS, so instructions
>  read from the debug file are zeros:
>
>   # cat > foo.c << EOF
>   unsigned long foo_work(unsigned long n)
>   {
>         unsigned long s = 0;
>
>         for (unsigned long i = 0; i < n; i++)
>                 s = s * 31 + i;
>         return s;
>   }
>   EOF
>   # cat > main.c << EOF
>   #include <stdio.h>
>   unsigned long foo_work(unsigned long n);
>   int main(void)
>   {
>         printf("%lu\n", foo_work(1000));
>         return 0;
>   }
>   EOF
>   # gcc -g -O2 -shared -fPIC -o libfoo.so foo.c
>   # gcc -g -O2 -o main main.c -L. -lfoo -Wl,-rpath,'$ORIGIN'
>   # objcopy --only-keep-debug libfoo.so libfoo.so.debug
>   # objcopy --strip-debug libfoo.so
>   # objcopy --add-gnu-debuglink=libfoo.so.debug libfoo.so
>   # perf record -e intel_pt//u ./main
>
>  Note that branch samples must be requested, because it is the resolving
>  of the branch target symbol that causes dso__load() to be called, and
>  hence the binary type to be set, before the decoder walks the code.
>  With '--itrace=e' alone, nothing loads symbols for libfoo.so, the binary
>  type is left as DSO_BINARY_TYPE__NOT_FOUND, the correct file is read
>  anyway, and no errors are reported either way.
>
>  Before:
>
>   # perf.before script --itrace=be 2>&1 | grep "instruction trace error"
>    instruction trace error type 1 time 2350.467489498 cpu 9 pid 75634 tid 75634 ip 0x77d48480718f code 6: Trace doesn't match instruction
>    instruction trace error type 1 time 2350.467489832 cpu 9 pid 75634 tid 75634 ip 0x77d484807341 code 6: Trace doesn't match instruction
>    instruction trace error type 1 time 2350.467496412 cpu 9 pid 75634 tid 75634 ip 0x5b4de37a8074 code 6: Trace doesn't match instruction
>    instruction trace error type 1 time 2350.467593393 cpu 9 pid 75634 tid 75634 ip 0x77d4848070d0 code 6: Trace doesn't match instruction
>    instruction trace error type 1 time 2350.467593954 cpu 9 pid 75634 tid 75634 ip 0x77d4848075a8 code 6: Trace doesn't match instruction
>    instruction trace error type 1 time 2350.467595728 cpu 9 pid 75634 tid 75634 ip 0x77d4848324de code 6: Trace doesn't match instruction
>   6 instruction trace errors
>
>  After:
>
>   # perf script --itrace=be 2>&1 | grep "instruction trace error"
>   #
>
> Fixes: 5363c306787c8 ("perf symbol: Set binary_type of dso when loading")
> Reported-by: Todd Lipcon <tlipcon@xxxxxxxxxx>
> Closes: https://lore.kernel.org/all/CAGH6UiG=RJLqBU3kLu9XJciPyPO1HZkbAPERguVUMRuWQgqf=A@xxxxxxxxxxxxxx/
> Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>

Just a heads up of  a different using the ELF rather than the debug
problem in addr2line caught in:
https://lore.kernel.org/linux-perf-users/20260824062841.1529489-2-irogers@xxxxxxxxxx/
As you mention, there's probably a wider clean up needed. I dislike
that we currently have dso__name and dso__long_name where the long
name is generally the path to the binary. There should probably be a
helper to get the DSO debuginfo for split cases.

Thanks,
Ian

> ---
>  tools/perf/util/symbol.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
> index cd379ced19e5..1f714b47bbf4 100644
> --- a/tools/perf/util/symbol.c
> +++ b/tools/perf/util/symbol.c
> @@ -1947,7 +1947,16 @@ int dso__load(struct dso *dso, struct map *map)
>                 if (next_slot) {
>                         ss_pos++;
>
> -                       if (dso__binary_type(dso) == DSO_BINARY_TYPE__NOT_FOUND)
> +                       /*
> +                        * The binary type is used to find the file containing
> +                        * the executed instructions, so prefer the types that
> +                        * refer to the actual object over debug-only files such
> +                        * as DSO_BINARY_TYPE__DEBUGLINK.
> +                        */
> +                       if (dso__binary_type(dso) == DSO_BINARY_TYPE__NOT_FOUND ||
> +                           symtab_type == DSO_BINARY_TYPE__BUILD_ID_CACHE ||
> +                           (symtab_type == DSO_BINARY_TYPE__SYSTEM_PATH_DSO &&
> +                            dso__binary_type(dso) != DSO_BINARY_TYPE__BUILD_ID_CACHE))
>                                 dso__set_binary_type(dso, symtab_type);
>
>                         if (syms_ss && runtime_ss)
> --
> 2.53.0
>