Re: [PATCH v3] perf dso: Fix kallsyms DSO detection with fallback logic

Hello, Thanks for the review.
On 01/07/26 05:21, Namhyung Kim wrote:

On Fri, Jun 26, 2026 at 09:40:53PM +0530, Tanushree Shah wrote:

quoted

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). Use strcmp() for exact matching of
fixed names and strict validation for guest kallsyms with embedded PID
to prevent path traversal attacks.

Fixes: ebf0b332732d ("perf dso: fix dso__is_kallsyms() check")
Signed-off-by: Tanushree Shah <redacted>
---
v2 -> v3: Replace strncmp() prefix matching with strcmp() for fixed
           kallsyms names and add is_guest_kallsyms_pid_name() to
           strictly validate guest kallsyms with PID format, preventing
           path traversal attacks.

v1 -> v2: Rename DSO__NAME_GUEST_KALLSYMS to DSO__PREFIX_GUEST_KALLSYMS
           to reflect that it is a prefix, not a full name.

v1: https://lore.kernel.org/all/20260410071225.708005-2-tshah@linux.ibm.com/ (local)

  tools/perf/util/dso.h | 57 ++++++++++++++++++++++++++++++++++++++++++-
  1 file changed, 56 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
index ede691e9a249..8763e6f65316 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 +21,40 @@ struct perf_env;
  
  #define DSO__NAME_KALLSYMS	"[kernel.kallsyms]"
  #define DSO__NAME_KCORE		"[kernel.kcore]"
+#define DSO__NAME_GUEST_KALLSYMS		"[guest.kernel.kallsyms]"
+#define DSO__NAME_GUEST_KALLSYMS_PID_PREFIX	"[guest.kernel.kallsyms."
+
+/*
+ * Validate names of the form "[guest.kernel.kallsyms.<pid>]", where
+ * <pid> is the PID of the guest VM and varies per guest, so it
+ * cannot be matched with strcmp() against a fixed string.
+ *
+ * Every character after the fixed prefix must be a decimal digit,
+ * with ']' immediately terminating the digit run and nothing
+ * following it. This rules out '/', "..", or any other character
+ * being smuggled into the name.
+ */
+static inline bool is_guest_kallsyms_pid_name(const char *name)
+{
+	const size_t prefix_len = sizeof(DSO__NAME_GUEST_KALLSYMS_PID_PREFIX) - 1;
+	size_t digits;
+
+	if (strncmp(name, DSO__NAME_GUEST_KALLSYMS_PID_PREFIX, prefix_len) != 0)
+		return false;
+
+	digits = strspn(name + prefix_len, "0123456789");
+	if (digits == 0)
+		return false;
+
+	/* ']' must terminate the digit run, with nothing trailing it */
+	if (name[prefix_len + digits] != ']')
+		return false;
+
+	if (name[prefix_len + digits + 1] != '\0')
+		return false;
+
+	return true;
+}
  
  /**
   * enum dso_binary_type - The kind of DSO generally associated with a memory

@@ -914,8 +949,28 @@ static inline bool dso__is_kcore(const struct dso *dso)
  static inline bool dso__is_kallsyms(const struct dso *dso)
  {
  	enum dso_binary_type bt = dso__binary_type(dso);

I have to check its usage carefully but any chance dso__symtab_type(dso)
instead produces better results?

I did some additional logging and found that symtab_type is not getting 
set while running perf record, so it seems dso__symtab_type(dso) would 
likely have the same issue here.

quoted

+	const char *name;
+
+	if (bt == DSO_BINARY_TYPE__KALLSYMS || bt == DSO_BINARY_TYPE__GUEST_KALLSYMS)
+		return true;
+
+	if (bt != DSO_BINARY_TYPE__NOT_FOUND)
+		return false;
+
+	if (!RC_CHK_ACCESS(dso)->kernel)

I think the proper wrapper is dso__kernel().

Thanks for the pointer. Will use this and send v4.

quoted

+		return false;
+
+	name = RC_CHK_ACCESS(dso)->long_name;

And dso__long_name().

Sure, will use this and send v4.

Thanks,
Namhyung

quoted

+	if (!name)
+		return false;
+
+	if (!strcmp(name, DSO__NAME_KALLSYMS))
+		return true;
+
+	if (!strcmp(name, DSO__NAME_GUEST_KALLSYMS))
+		return true;
  
-	return bt == DSO_BINARY_TYPE__KALLSYMS || bt == DSO_BINARY_TYPE__GUEST_KALLSYMS;
+	return is_guest_kallsyms_pid_name(name);
  }
  
  bool dso__is_object_file(const struct dso *dso);
-- 
2.47.3

Thanks
Tanushree Shah