Re: [PATCH v4 2/5] perf dso: Allow reading DSO data from an explicit file

[Ian Rogers]

On Fri, Oct 2, 2026 at 11:46 AM Alireza Haghdoost via B4 Relay
<devnull+haghdoost.uber.com@xxxxxxxxxx> wrote:
>
> From: Alireza Haghdoost <haghdoost@xxxxxxxx>
>
> The DSO data cache derives the file to open from the DSO's binary type,
> which can resolve to the runtime image rather than the file a symbol
> table was read from. The lazy symbol loader added later in this series
> reads symbol names at string-table offsets in the file the symbol table
> came from. With split debuginfo, the data cache would apply those
> debuginfo offsets to the runtime image.
>
> This patch adds dso__data_set_path() so a DSO can be configured to read
> from one exact file while keeping the data cache's descriptor eviction
> and reopening. The path is used only when the data cache opens the file;
> dso__get_filename() and its debuginfo callers are unchanged. Such DSOs
> may be owned privately rather than being part of a dsos collection, so
> the patch also drops the assertion that every opened data DSO is in one.

I think some context is missing here. Why do I want a DSO that isn't
part of a machine's DSOs? I can see a use for testing, do you want
this feature for more than testing?

Thanks,
Ian

> Add a DSO data test that reads through an explicit path, closes the
> descriptor, and reads an uncached offset to exercise reopening.
>
> Signed-off-by: Alireza Haghdoost <haghdoost@xxxxxxxx>
> ---
>  tools/perf/tests/dso-data.c | 42 ++++++++++++++++++++++++++++++++++++++++++
>  tools/perf/util/dso.c       | 30 ++++++++++++++++++++++++++----
>  tools/perf/util/dso.h       |  2 ++
>  3 files changed, 70 insertions(+), 4 deletions(-)
>
> diff --git a/tools/perf/tests/dso-data.c b/tools/perf/tests/dso-data.c
> index 46bc3f597260..fbfb2f08d3ba 100644
> --- a/tools/perf/tests/dso-data.c
> +++ b/tools/perf/tests/dso-data.c
> @@ -393,11 +393,53 @@ static int test__dso_data_reopen(struct test_suite *test __maybe_unused, int sub
>         return 0;
>  }
>
> +static int test__dso_data_path(struct test_suite *test __maybe_unused, int subtest __maybe_unused)
> +{
> +       u8 expect[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
> +       char *file = test_file(TEST_FILE_SIZE);
> +       struct dso *dso;
> +       long nr, nr_end;
> +       u8 buf[10];
> +
> +       TEST_ASSERT_VAL("No test file", file);
> +       nr = open_files_cnt();
> +
> +       /*
> +        * The DSO name does not exist and the DSO is not in a dsos
> +        * collection; reads must come from the configured path.
> +        */
> +       dso = dso__new("/nonexistent/perf-test-dso-data-path");
> +       TEST_ASSERT_VAL("Failed to create dso", dso);
> +       dso__set_binary_type(dso, DSO_BINARY_TYPE__SYSTEM_PATH_DSO);
> +       TEST_ASSERT_VAL("Failed to set path", !dso__data_set_path(dso, file));
> +
> +       TEST_ASSERT_VAL("Wrong size",
> +                       dso__data_read_offset(dso, NULL, 10, buf, 10) == 10);
> +       TEST_ASSERT_VAL("Wrong data", !memcmp(buf, expect, 10));
> +
> +       /* An uncached offset after close must reopen the configured path. */
> +       dso__data_close(dso);
> +       memset(buf, 0, sizeof(buf));
> +       TEST_ASSERT_VAL("Wrong size after reopen",
> +                       dso__data_read_offset(dso, NULL, DSO__DATA_CACHE_SIZE * 2 + 10,
> +                                             buf, 10) == 10);
> +       TEST_ASSERT_VAL("Wrong data after reopen",
> +                       buf[0] == (DSO__DATA_CACHE_SIZE * 2 + 10) % 10);
> +
> +       dso__data_close(dso);
> +       dso__put(dso);
> +       unlink(file);
> +
> +       nr_end = open_files_cnt();
> +       TEST_ASSERT_VAL("failed leaking files", nr == nr_end);
> +       return 0;
> +}
>
>  static struct test_case tests__dso_data[] = {
>         TEST_CASE("read", dso_data),
>         TEST_CASE("cache", dso_data_cache),
>         TEST_CASE("reopen", dso_data_reopen),
> +       TEST_CASE("explicit path", dso_data_path),
>         {       .name = NULL, }
>  };
>
> diff --git a/tools/perf/util/dso.c b/tools/perf/util/dso.c
> index d3017c82ffb5..5c4872810ada 100644
> --- a/tools/perf/util/dso.c
> +++ b/tools/perf/util/dso.c
> @@ -547,8 +547,6 @@ static void dso__list_add(struct dso *dso) EXCLUSIVE_LOCKS_REQUIRED(_dso__data_o
>  #ifdef REFCNT_CHECKING
>         dso__data(dso)->dso = dso__get(dso);
>  #endif
> -       /* Assume the dso is part of dsos, hence the optional reference count above. */
> -       assert(dso__dsos(dso));
>         dso__data_open_cnt++;
>  }
>
> @@ -678,8 +676,11 @@ static int __open_dso(struct dso *dso, struct machine *machine)
>
>         mutex_lock(dso__lock(dso));
>
> -       name = dso__get_filename(dso, machine ? machine->root_dir : "", &decomp,
> -                                dso__binary_type(dso));
> +       if (dso__data(dso)->path)
> +               name = strdup(dso__data(dso)->path);
> +       else
> +               name = dso__get_filename(dso, machine ? machine->root_dir : "",
> +                                        &decomp, dso__binary_type(dso));
>         if (name) {
>                 fd = do_open(name);
>         } else {
> @@ -833,6 +834,26 @@ void dso__data_close(struct dso *dso)
>         mutex_unlock(dso__data_open_lock());
>  }
>
> +/**
> + * dso__data_set_path - Read @dso's data from an explicit file
> + * @dso: dso object
> + * @path: file to open instead of the path derived from the binary type
> + *
> + * Used when the data must come from one specific file, such as the separate
> + * debuginfo file that a symbol table was read from. Must be called before any
> + * data is read, as already cached data is not invalidated.
> + */
> +int dso__data_set_path(struct dso *dso, const char *path)
> +{
> +       char *new_path = strdup(path);
> +
> +       if (!new_path)
> +               return -ENOMEM;
> +       free(dso__data(dso)->path);
> +       dso__data(dso)->path = new_path;
> +       return 0;
> +}
> +
>  static void try_to_open_dso(struct dso *dso, struct machine *machine)
>         EXCLUSIVE_LOCKS_REQUIRED(_dso__data_open_lock)
>  {
> @@ -1783,6 +1804,7 @@ void dso__delete(struct dso *dso)
>         dso__data_close(dso);
>         auxtrace_cache__free(RC_CHK_ACCESS(dso)->auxtrace_cache);
>         dso_cache__free(dso);
> +       zfree(&RC_CHK_ACCESS(dso)->data.path);
>         dso__free_a2l(dso);
>         dso__free_a2l_libbfd(dso);
>         dso__free_libdw(dso);
> diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
> index 3f08d45e7f53..7bcd5ec0c312 100644
> --- a/tools/perf/util/dso.h
> +++ b/tools/perf/util/dso.h
> @@ -264,6 +264,7 @@ struct dso_data {
>  #ifdef REFCNT_CHECKING
>         struct dso       *dso;
>  #endif
> +       char             *path;
>         int              fd;
>         int              status;
>         u32              status_seen;
> @@ -921,6 +922,7 @@ bool dso__data_get_fd(struct dso *dso, struct machine *machine, int *fd)
>         EXCLUSIVE_TRYLOCK_FUNCTION(true, _dso__data_open_lock);
>  void dso__data_put_fd(struct dso *dso) UNLOCK_FUNCTION(_dso__data_open_lock);
>  void dso__data_close(struct dso *dso) LOCKS_EXCLUDED(_dso__data_open_lock);
> +int dso__data_set_path(struct dso *dso, const char *path);
>
>  int dso__data_file_size(struct dso *dso, struct machine *machine);
>  off_t dso__data_size(struct dso *dso, struct machine *machine);
>
> --
> Git-157)
>
>