Message-ID: <CAN30aBHWef0kKWPkvx9NZ4nJjdz97PJ7s_68h-ddZ3e8VzYW3A@mail.gmail.com>
Date: Sat, 15 Nov 2025 23:22:04 -0800
From: Fangrui Song <maskray@...rceware.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Namhyung Kim <namhyung@...nel.org>, maskray@...rceware.org, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, James Clark <james.clark@...aro.org>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	linux-perf-users@...r.kernel.org, Eric Biggers <ebiggers@...nel.org>, 
	Pablo Galindo <pablogsal@...il.com>
Subject: Re: [PATCH 1/2] perf jitdump: Add load_addr to build-ID generation

On Fri, Nov 14, 2025 at 11:33 AM Ian Rogers <irogers@...gle.com> wrote:
>
> On Fri, Nov 14, 2025 at 10:57 AM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > On Fri, Nov 14, 2025 at 09:33:29AM -0800, Ian Rogers wrote:
> > > On Fri, Nov 14, 2025 at 1:29 AM Namhyung Kim <namhyung@...nel.org> wrote:
> > > >
> > > > It was reported that python backtrace with JIT dump was broken after the
> > > > change to built-in SHA-1 implementation.  It seems python generates the
> > > > same JIT code for each function.  They will become separate DSOs but the
> > > > contents are the same.  Only difference is in the symbol name.
> > > >
> > > > But this caused a problem that every JIT'ed DSOs will have the same
> > > > build-ID which makes perf confused.  And it resulted in no python
> > > > symbols (from JIT) in the output.
> > >
> > > The lookup of a DSO involves the build ID and the filename. I'm
> > > confused as to why things weren't deduplicated and why no symbols
> > > rather than repeatedly the same symbol?
> >
> > I don't know, but that's the symptom in the original bug report in the
> > python github (see Links: below).  I guess the behavior is
> > non-deterministic.
> >
> > >
> > > > Looking back at the original code before the conversion, it used the
> > > > load_addr as well as the code section to distinguish each DSO.  I think
> > > > we should do the same or use symbol table as an additional input for
> > > > SHA-1.
> > >
> > > Hmm.. the build ID for the contents of the code should be a constant.
> > > As the build ID is a note for the entire ELF file then something is
> > > wrong with the filename handling it seems.
> >
> > When it tries to load symbols from a DSO, it prefer reading from the
> > build-ID cache than the file system since it trusts build-IDs more than
> > the path name.  See dso__load() and binary_type_symtab[].
> >
> > So having multiple DSO's with the same build-ID can be a problem if they
> > are in the build-ID cache.  Normally `perf inject -j` won't add the new
> > JIT-ed DSOs to the build-ID cache but it's still possible.
>
> +Fangrui
>
> I'm surprised that build IDs don't include symbol names but:
> ```
> $ cat a.s
> .text
> .global main
> .global foo
> main:
> foo:
>        ret
> $ cat b.s
> .text
> .global main
> .global bar
> main:
> bar:
>        ret
> $ gcc -Wl,--build-id a.s -o a.out
> $ gcc -Wl,--build-id b.s -o b.out
> $ readelf -n a.out
> ...
>    Build ID: 9dd0371b953db5d72929af5d98552e4ee1043616
> ...
> $ readelf -n b.out
> ...
>    Build ID: 9dd0371b953db5d72929af5d98552e4ee1043616
> ...
> ```
> so ugh. Perhaps we need to have jitdump make a single object file (and
> so 1 build ID) but with multiple unique symbols.
>
> Thanks,
> Ian

Looks like a GNU ld issue. Other linkers should give different build IDs.

 .symtab/.strtab/.shstrtab are special sections and BFD seem to skip
their content for the build ID.
Raised  https://sourceware.org/bugzilla/show_bug.cgi?id=33633 ("ld
--build-id does not use symtab/strtab content")

>
>
>
> > Thanks,
> > Namhyung
> >
> > >
> > > > This patch is a quick-and-dirty fix just to add each byte of the
> > > > load_addr to the first 8 bytes of SHA-1 result.  Probably we need to add
> > > > sha1_update() or similar to update the existing hash value and use it
> > > > here.  I'd like something that can be backported to the stable trees
> > > > easily.
> > > >
> > > > Fixes: e3f612c1d8f3945b ("perf genelf: Remove libcrypto dependency and use built-in sha1()")
> > > > Cc: Eric Biggers <ebiggers@...nel.org>
> > > > Cc: Pablo Galindo <pablogsal@...il.com>
> > > > Link: https://github.com/python/cpython/issues/139544
> > > > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > > > ---
> > > >  tools/perf/util/genelf.c | 9 +++++++++
> > > >  1 file changed, 9 insertions(+)
> > > >
> > > > diff --git a/tools/perf/util/genelf.c b/tools/perf/util/genelf.c
> > > > index 591548b10e34ef6a..a412e6faf70e37f3 100644
> > > > --- a/tools/perf/util/genelf.c
> > > > +++ b/tools/perf/util/genelf.c
> > > > @@ -395,6 +395,15 @@ jit_write_elf(int fd, uint64_t load_addr __maybe_unused, const char *sym,
> > > >          * build-id generation
> > > >          */
> > > >         sha1(code, csize, bnote.build_id);
> > > > +       /* FIXME: update the SHA-1 hash using additional contents */
> > > > +       bnote.build_id[0] += (load_addr >> 0) & 0xff;
> > > > +       bnote.build_id[1] += (load_addr >> 8) & 0xff;
> > > > +       bnote.build_id[2] += (load_addr >> 16) & 0xff;
> > > > +       bnote.build_id[3] += (load_addr >> 24) & 0xff;
> > > > +       bnote.build_id[4] += (load_addr >> 32) & 0xff;
> > > > +       bnote.build_id[5] += (load_addr >> 40) & 0xff;
> > > > +       bnote.build_id[6] += (load_addr >> 48) & 0xff;
> > > > +       bnote.build_id[7] += (load_addr >> 56) & 0xff;
> > > >         bnote.desc.namesz = sizeof(bnote.name); /* must include 0 termination */
> > > >         bnote.desc.descsz = sizeof(bnote.build_id);
> > > >         bnote.desc.type   = NT_GNU_BUILD_ID;
> > > > --
> > > > 2.52.0.rc1.455.g30608eb744-goog
> > > >

Powered by blists - more mailing lists