[PATCH 3/5] perf dso: Use stored fd error instead of stale errno in file_read() and file_size()

[Arnaldo Carvalho de Melo]

From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>

file_read() and file_size() use ret = -errno when
dso__data(dso)->fd is negative after try_to_open_dso() fails.  By this
point errno has been through mutex_lock(), nsinfo__mountns_enter(), and
multiple open() attempts inside try_to_open_dso() — it no longer
reflects the actual open failure.  If errno happens to be 0, ret = 0
looks like EOF rather than an error, and file_size() callers like
dso__data_size() would then report a zero-sized file instead of
failing.

dso__data(dso)->fd is always negative on failure — -errno from
__open_dso() when no filename could be built (e.g. -EINVAL, -ENOENT),
or -1 when do_open() itself failed — and never 0, so use it directly
instead of reading the stale global errno.

Fixes: 33bdedcea2d7 ("perf tools: Protect dso cache fd with a mutex")
Reported-by: sashiko-bot <sashiko-bot@xxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
---
 tools/perf/util/dso.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/dso.c b/tools/perf/util/dso.c
index 124193453675ca18..32ae5c78cdb906e6 100644
--- a/tools/perf/util/dso.c
+++ b/tools/perf/util/dso.c
@@ -1029,7 +1029,8 @@ static ssize_t file_read(struct dso *dso, struct machine *machine,
 
 	if (dso__data(dso)->fd < 0) {
 		dso__data(dso)->status = DSO_DATA_STATUS_ERROR;
-		ret = -errno;
+		/* fd is always negative here: -errno or -1, never 0 */
+		ret = dso__data(dso)->fd;
 		goto out;
 	}
 
@@ -1151,8 +1152,9 @@ static int file_size(struct dso *dso, struct machine *machine)
 	try_to_open_dso(dso, machine);
 
 	if (dso__data(dso)->fd < 0) {
-		ret = -errno;
 		dso__data(dso)->status = DSO_DATA_STATUS_ERROR;
+		/* fd is always negative here: -errno or -1, never 0 */
+		ret = dso__data(dso)->fd;
 		goto out;
 	}
 
-- 
2.55.0