From 5bf1557e3d6a69113649d831276ea2f97585fc33 Mon Sep 17 00:00:00 2001 From: Eduard Zingerman Date: Thu, 3 Oct 2024 14:03:07 -0700 Subject: [PATCH] selftests/bpf: Fix backtrace printing for selftests crashes 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: 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 Signed-off-by: Andrii Nakryiko Tested-by: Tony Ambardar Reviewed-by: Tony Ambardar Acked-by: Daniel Xu Link: https://lore.kernel.org/bpf/20241003210307.3847907-1-eddyz87@gmail.com --- tools/testing/selftests/bpf/test_progs.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/tools/testing/selftests/bpf/test_progs.c b/tools/testing/selftests/bpf/test_progs.c index 7846f7f989088..7421874380c29 100644 --- a/tools/testing/selftests/bpf/test_progs.c +++ b/tools/testing/selftests/bpf/test_progs.c @@ -20,11 +20,13 @@ #include "network_helpers.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 /* backtrace */ -#endif - -/* Default backtrace funcs if missing at link */ +#else __weak int backtrace(void **buffer, int size) { return 0; @@ -34,6 +36,7 @@ __weak void backtrace_symbols_fd(void *const *buffer, int size, int fd) { dprintf(fd, "\n"); } +#endif /*__GLIBC__ */ int env_verbosity = 0; -- 2.49.0