linux-kernel - Re: [PATCH v4 06/11] perf dso: Add support for reading the e

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Message-ID: <CAP-5=fXdZZX=wa8EkN-dcpnDjA7J-cQ09BERufAQzoBeoNCT1g@mail.gmail.com>
Date: Thu, 6 Mar 2025 21:30:13 -0800
From: Ian Rogers <irogers@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, Kan Liang <kan.liang@...ux.intel.com>, 
	John Garry <john.g.garry@...cle.com>, Will Deacon <will@...nel.org>, 
	James Clark <james.clark@...aro.org>, Mike Leach <mike.leach@...aro.org>, 
	Leo Yan <leo.yan@...ux.dev>, guoren <guoren@...nel.org>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Charlie Jenkins <charlie@...osinc.com>, 
	Bibo Mao <maobibo@...ngson.cn>, Huacai Chen <chenhuacai@...nel.org>, 
	Catalin Marinas <catalin.marinas@....com>, Jiri Slaby <jirislaby@...nel.org>, 
	Björn Töpel <bjorn@...osinc.com>, 
	Howard Chu <howardchu95@...il.com>, linux-kernel@...r.kernel.org, 
	linux-perf-users@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	"linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>, linux-riscv@...ts.infradead.org, 
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v4 06/11] perf dso: Add support for reading the e_machine
 type for a dso

On Thu, Mar 6, 2025 at 5:22 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> On Mon, Mar 03, 2025 at 09:03:00PM -0800, Ian Rogers wrote:
> > For ELF file dsos read the e_machine from the ELF header. For kernel
> > types assume the e_machine matches the perf tool. In other cases
> > return EM_NONE.
> >
> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > ---
> >  tools/perf/util/dso.c | 54 +++++++++++++++++++++++++++++++++++++++++++
> >  tools/perf/util/dso.h |  1 +
> >  2 files changed, 55 insertions(+)
> >
> > diff --git a/tools/perf/util/dso.c b/tools/perf/util/dso.c
> > index 5c6e85fdae0d..7f2f1af4f73b 100644
> > --- a/tools/perf/util/dso.c
> > +++ b/tools/perf/util/dso.c
> > @@ -1170,6 +1170,60 @@ ssize_t dso__data_read_offset(struct dso *dso, struct machine *machine,
> >       return data_read_write_offset(dso, machine, offset, data, size, true);
> >  }
> >
> > +uint16_t dso__e_machine(struct dso *dso, struct machine *machine)
> > +{
> > +     uint16_t e_machine = EM_NONE;
> > +     int fd;
> > +
> > +     switch (dso__binary_type(dso)) {
> > +     case DSO_BINARY_TYPE__KALLSYMS:
> > +     case DSO_BINARY_TYPE__GUEST_KALLSYMS:
> > +     case DSO_BINARY_TYPE__VMLINUX:
> > +     case DSO_BINARY_TYPE__GUEST_VMLINUX:
> > +     case DSO_BINARY_TYPE__GUEST_KMODULE:
> > +     case DSO_BINARY_TYPE__GUEST_KMODULE_COMP:
> > +     case DSO_BINARY_TYPE__SYSTEM_PATH_KMODULE:
> > +     case DSO_BINARY_TYPE__SYSTEM_PATH_KMODULE_COMP:
> > +     case DSO_BINARY_TYPE__KCORE:
> > +     case DSO_BINARY_TYPE__GUEST_KCORE:
> > +     case DSO_BINARY_TYPE__BPF_PROG_INFO:
> > +     case DSO_BINARY_TYPE__BPF_IMAGE:
> > +     case DSO_BINARY_TYPE__OOL:
> > +     case DSO_BINARY_TYPE__JAVA_JIT:
>
> I think some of them can be possible in recorded data.  But let's go
> simple with EM_HOST as we haven't supported cross-arch trace.

I think you are agreeing with the code, which I think is right :-) I
don't know a way to go from say /proc/kallsyms text file to an
e_machine number and inventing a way is getting beyond the scope of
this series.

> > +             return EM_HOST;
> > +     case DSO_BINARY_TYPE__DEBUGLINK:
> > +     case DSO_BINARY_TYPE__BUILD_ID_CACHE:
> > +     case DSO_BINARY_TYPE__BUILD_ID_CACHE_DEBUGINFO:
> > +     case DSO_BINARY_TYPE__SYSTEM_PATH_DSO:
> > +     case DSO_BINARY_TYPE__OPENEMBEDDED_DEBUGINFO:
> > +     case DSO_BINARY_TYPE__FEDORA_DEBUGINFO:
> > +     case DSO_BINARY_TYPE__UBUNTU_DEBUGINFO:
> > +     case DSO_BINARY_TYPE__MIXEDUP_UBUNTU_DEBUGINFO:
> > +     case DSO_BINARY_TYPE__BUILDID_DEBUGINFO:
> > +             break;
> > +     case DSO_BINARY_TYPE__NOT_FOUND:
> > +     default:
> > +             return EM_NONE;
> > +     }
> > +
> > +     pthread_mutex_lock(&dso__data_open_lock);
>
> Hmm.. I'm afraid it'd slow down perf trace a bit more.  It sees
> occasional LOST events.  But it may be ok as it's cached in thread later.

Agreed, I think the caching will handle this. The lock is pre-existing
and is used to guard use of the dso_data->fd. The lock could be made
reader-writer, it could be placed within the dso_data it could also be
sharded. I've not done any of that here as I believe the processing is
all handled on a single thread anyway.

> > +
> > +     /*
> > +      * dso__data(dso)->fd might be closed if other thread opened another
> > +      * file (dso) due to open file limit (RLIMIT_NOFILE).
> > +      */
> > +     try_to_open_dso(dso, machine);
> > +     fd = dso__data(dso)->fd;
> > +     if (fd >= 0) {
> > +             _Static_assert(offsetof(Elf32_Ehdr, e_machine) == 18, "Unexpected offset");
> > +             _Static_assert(offsetof(Elf64_Ehdr, e_machine) == 18, "Unexpected offset");
> > +             if (pread(fd, &e_machine, sizeof(e_machine), 18) != sizeof(e_machine))
>
> I think it needs to check the endianess and swap the data.

Agreed. Will send a fix in v5.

Thanks,
Ian

> Thanks,
> Namhyung
>
>
> > +                     e_machine = EM_NONE;
> > +     }
> > +     pthread_mutex_unlock(&dso__data_open_lock);
> > +     return e_machine;
> > +}
> > +
> >  /**
> >   * dso__data_read_addr - Read data from dso address
> >   * @dso: dso object
> > diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
> > index f3ca2a5e7670..ba9b83db061a 100644
> > --- a/tools/perf/util/dso.h
> > +++ b/tools/perf/util/dso.h
> > @@ -818,6 +818,7 @@ int dso__data_file_size(struct dso *dso, struct machine *machine);
> >  off_t dso__data_size(struct dso *dso, struct machine *machine);
> >  ssize_t dso__data_read_offset(struct dso *dso, struct machine *machine,
> >                             u64 offset, u8 *data, ssize_t size);
> > +uint16_t dso__e_machine(struct dso *dso, struct machine *machine);
> >  ssize_t dso__data_read_addr(struct dso *dso, struct map *map,
> >                           struct machine *machine, u64 addr,
> >                           u8 *data, ssize_t size);
> > --
> > 2.48.1.711.g2feabab25a-goog
> >