[PATCH] perf dso: Fix kallsyms DSO detection with fallback logic

[PATCH] perf dso: Fix kallsyms DSO detection with fallback logic

[Tanushree Shah]

The current kallsyms detection in dso__is_kallsyms() uses the
dso_binary_type enum which fixes the issue of kallsyms being cached in
the build-id cache for out-of-tree modules.

However, during build-id injection in perf record/inject, dso_binary_type
has not been explicitly set yet,so dso__binary_type() returns
DSO_BINARY_TYPE__NOT_FOUND instead of DSO_BINARY_TYPE__KALLSYMS for the
kernel DSO. The current check then fails to identify it as kallsyms,
causing build-id symlinks to not be created in ~/.debug/.build-id/ and
perf archive to fail with "Cannot stat" errors.

Steps to reproduce the issue:
1. rm -rf ~/.debug/.build-id
2. perf record sleep 1
3. perf archive

Fix by falling back to matching long_name against the known kallsyms
strings explicitly when binary_type is not yet set
(== DSO_BINARY_TYPE__NOT_FOUND).

Fixes: ebf0b332732d ("perf dso: fix dso__is_kallsyms() check")
Signed-off-by: Tanushree Shah <tshah@linux.ibm.com>
---
 tools/perf/util/dso.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
index ede691e9a249..e44071998c49 100644
--- a/tools/perf/util/dso.h
+++ b/tools/perf/util/dso.h
@@ -9,6 +9,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <linux/bitops.h>
+#include <string.h>
 #include "build-id.h"
 #include "debuginfo.h"
 #include "mutex.h"
@@ -20,6 +21,7 @@ struct perf_env;
 
 #define DSO__NAME_KALLSYMS	"[kernel.kallsyms]"
 #define DSO__NAME_KCORE		"[kernel.kcore]"
+#define DSO__NAME_GUEST_KALLSYMS	"[guest.kernel.kallsyms"
 
 /**
  * enum dso_binary_type - The kind of DSO generally associated with a memory
@@ -915,6 +917,14 @@ static inline bool dso__is_kallsyms(const struct dso *dso)
 {
 	enum dso_binary_type bt = dso__binary_type(dso);
 
+	if (bt == DSO_BINARY_TYPE__NOT_FOUND) {
+		return RC_CHK_ACCESS(dso)->kernel &&
+			((strncmp(RC_CHK_ACCESS(dso)->long_name, DSO__NAME_KALLSYMS,
+				strlen(DSO__NAME_KALLSYMS)) == 0) ||
+			(strncmp(RC_CHK_ACCESS(dso)->long_name, DSO__NAME_GUEST_KALLSYMS,
+				strlen(DSO__NAME_GUEST_KALLSYMS)) == 0));
+	}
+
 	return bt == DSO_BINARY_TYPE__KALLSYMS || bt == DSO_BINARY_TYPE__GUEST_KALLSYMS;
 }
 
-- 
2.47.3

Re: [PATCH] perf dso: Fix kallsyms DSO detection with fallback logic

[sashiko-bot]

Sashiko AI review found 1 potential issue(s):
- [Low] The macro `DSO__NAME_GUEST_KALLSYMS` is defined with an unbalanced bracket, which is deceptive and prone to misuse.
--

commit bff739892129d9369935f2dfb42b44d83b2f857d
Author: Tanushree Shah <tshah@linux.ibm.com>

perf dso: Fix kallsyms DSO detection with fallback logic

This commit fixes kallsyms DSO detection during build-id injection when
the binary type is not yet explicitly set. It implements a fallback that
matches the long_name against known kallsyms strings to correctly identify
the kernel DSO.

> diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
> index ede691e9a2495..e44071998c499 100644
> --- a/tools/perf/util/dso.h
> +++ b/tools/perf/util/dso.h
[ ... ]
> @@ -20,6 +21,7 @@ struct perf_env;
>  
>  #define DSO__NAME_KALLSYMS	"[kernel.kallsyms]"
>  #define DSO__NAME_KCORE		"[kernel.kcore]"
> +#define DSO__NAME_GUEST_KALLSYMS	"[guest.kernel.kallsyms"

This isn't a bug, but there is a missing closing bracket in this macro
definition.

While it looks like this was done intentionally to allow prefix matching
against dynamic guest kallsyms names (which append a PID) using strncmp
later in the patch, naming it DSO__NAME_GUEST_KALLSYMS implies it represents
a complete, well-formed identifier like DSO__NAME_KALLSYMS.

Could this macro be renamed to something like DSO__PREFIX_GUEST_KALLSYMS
to make its intended use as a prefix clear, preventing it from being
accidentally used in contexts expecting a full name?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260410071225.708005-2-tshah@linux.ibm.com?part=1