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

Namhyung Kim
2025-03-07T01:22:23Z

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.

> + 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.

> +
> + /*
> + * 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.

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
>