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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: c9a83e76b5 ("selftests/bpf: Fix compile if backtrace support missing in libc")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Tony Ambardar <tony.ambardar@gmail.com>
Reviewed-by: Tony Ambardar <tony.ambardar@gmail.com>
Acked-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/bpf/20241003210307.3847907-1-eddyz87@gmail.com
Linux kernel
============
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