Re: [PATCH v4 06/11] perf dso: Add support for reading the e_machine type for a dso

Ian Rogers
2025-03-07T05:30:25Z

On Thu, Mar 6, 2025 at 5:22 PM Namhyung Kim <namhyung@kernel.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@google.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
> >