[PATCH] perf evsel: fix buffer offset when building the EPERM error string

[PATCH] perf evsel: fix buffer offset when building the EPERM error string

[Gu Bin]

When building the EPERM error string, the scnprintf() that emits "No
permission to enable %s event" uses (msg, size) instead of (msg +
printed, size - printed) like every other line in this case, overwriting
the "Access to performance monitoring..." preamble and truncating the
following hint. The message shown to the user for a failed open with
EPERM is therefore garbled: the preamble is lost and stale bytes from
the overwritten content remain in the buffer.

Use the correct offset and remaining size.

Fixes: c1034eb06920 ("perf tool: Make perf tool aware of SELinux access control")
Signed-off-by: Gu Bin <gubin@cmss.chinamobile.com>
---
 tools/perf/util/evsel.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index d060fca8090f..6d69f63b7fb1 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -4590,7 +4590,7 @@ int evsel__open_strerror(struct evsel *evsel, struct target *target,
 		}
 
 		if (err == EPERM)
-			printed += scnprintf(msg, size,
+			printed += scnprintf(msg + printed, size - printed,
 				"No permission to enable %s event.\n\n", evsel__name(evsel));
 
 		return printed + scnprintf(msg + printed, size - printed,
-- 
2.55.0.windows.3

Re: [PATCH] perf evsel: fix buffer offset when building the EPERM error string

[sashiko-bot]

> When building the EPERM error string, the scnprintf() that emits "No
> permission to enable %s event" uses (msg, size) instead of (msg +
> printed, size - printed) like every other line in this case, overwriting
> the "Access to performance monitoring..." preamble and truncating the
> following hint. The message shown to the user for a failed open with
> EPERM is therefore garbled: the preamble is lost and stale bytes from
> the overwritten content remain in the buffer.
> 
> Use the correct offset and remaining size.
> 
> Fixes: c1034eb06920 ("perf tool: Make perf tool aware of SELinux access control")
> Signed-off-by: Gu Bin <gubin@cmss.chinamobile.com>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261104103607.1903-1-gubin@cmss.chinamobile.com?part=1