This is a note to let you know that I've just added the patch titled
selftests/bpf: Fix backtrace printing for selftests crashes
to the 6.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
selftests-bpf-fix-backtrace-printing-for-selftests-c.patch
and it can be found in the queue-6.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit a9b9832b126b15a85f9160887f4027a8c5e2b6a5
Author: Eduard Zingerman <eddyz87@xxxxxxxxx>
Date: Thu Oct 3 14:03:07 2024 -0700
selftests/bpf: Fix backtrace printing for selftests crashes
[ Upstream commit 5bf1557e3d6a69113649d831276ea2f97585fc33 ]
test_progs uses glibc specific functions backtrace() and
backtrace_symbols_fd() to print backtrace in case of SIGSEGV.
Recent commit (see fixes) updated test_progs.c to define stub versions
of the same functions with attriubte "weak" in order to allow linking
test_progs against musl libc. Unfortunately this broke the backtrace
handling for glibc builds.
As it turns out, glibc defines backtrace() and backtrace_symbols_fd()
as weak:
$ llvm-readelf --symbols /lib64/libc.so.6 \
| grep -P '( backtrace_symbols_fd| backtrace)$'
4910: 0000000000126b40 161 FUNC WEAK DEFAULT 16 backtrace
6843: 0000000000126f90 852 FUNC WEAK DEFAULT 16 backtrace_symbols_fd
So does test_progs:
$ llvm-readelf --symbols test_progs \
| grep -P '( backtrace_symbols_fd| backtrace)$'
2891: 00000000006ad190 15 FUNC WEAK DEFAULT 13 backtrace
11215: 00000000006ad1a0 41 FUNC WEAK DEFAULT 13 backtrace_symbols_fd
In such situation dynamic linker is not obliged to favour glibc
implementation over the one defined in test_progs.
Compiling with the following simple modification to test_progs.c
demonstrates the issue:
$ git diff
...
\--- a/tools/testing/selftests/bpf/test_progs.c
\+++ b/tools/testing/selftests/bpf/test_progs.c
\@@ -1817,6 +1817,7 @@ int main(int argc, char **argv)
if (err)
return err;
+ *(int *)0xdeadbeef = 42;
err = cd_flavor_subdir(argv[0]);
if (err)
return err;
$ ./test_progs
[0]: Caught signal #11!
Stack trace:
<backtrace not supported>
Segmentation fault (core dumped)
Resolve this by hiding stub definitions behind __GLIBC__ macro check
instead of using "weak" attribute.
Fixes: c9a83e76b5a9 ("selftests/bpf: Fix compile if backtrace support missing in libc")
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Tested-by: Tony Ambardar <tony.ambardar@xxxxxxxxx>
Reviewed-by: Tony Ambardar <tony.ambardar@xxxxxxxxx>
Acked-by: Daniel Xu <dxu@xxxxxxxxx>
Link: https://lore.kernel.org/bpf/20241003210307.3847907-1-eddyz87@xxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/tools/testing/selftests/bpf/test_progs.c b/tools/testing/selftests/bpf/test_progs.c
index 11d4c51c7d211..6068f5bde8fd5 100644
--- a/tools/testing/selftests/bpf/test_progs.c
+++ b/tools/testing/selftests/bpf/test_progs.c
@@ -16,11 +16,13 @@
#include <sys/socket.h>
#include <sys/un.h>
+/* backtrace() and backtrace_symbols_fd() are glibc specific,
+ * use header file when glibc is available and provide stub
+ * implementations when another libc implementation is used.
+ */
#ifdef __GLIBC__
#include <execinfo.h> /* backtrace */
-#endif
-
-/* Default backtrace funcs if missing at link */
+#else
__weak int backtrace(void **buffer, int size)
{
return 0;
@@ -30,6 +32,7 @@ __weak void backtrace_symbols_fd(void *const *buffer, int size, int fd)
{
dprintf(fd, "<backtrace not supported>\n");
}
+#endif /*__GLIBC__ */
static bool verbose(void)
{
