linux-kernel - Re: [PATCH 1/2] perf jitdump: Add load

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Message-ID: <aRd7iJJxgT-0G8Ky@google.com>
Date: Fri, 14 Nov 2025 10:57:12 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: 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 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.

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
> >