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:
  <backtrace not supported>
  Segmentation fault (core dumped)

Resolve this by hiding stub definitions behind __GLIBC__ macro check
instead of using "weak" attribute.

Fixes: c9a83e7 ("selftests/bpf: Fix compile if backtrace support missing in libc")

CC: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Daniel Xu <dxu@dxuuu.xyz>
Reviewed-by: Tony Ambardar <tony.ambardar@gmail.com>
Tested-by: Tony Ambardar <tony.ambardar@gmail.com>

tools/testing/selftests/bpf/test_progs.c

@@ -20,20 +20,23 @@
 
 #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 <execinfo.h> /* backtrace */
-#endif
-
-/* Default backtrace funcs if missing at link */
-__weak int backtrace(void **buffer, int size)
+#else
+int backtrace(void **buffer, int size)
 {
 	return 0;
 }
 
-__weak void backtrace_symbols_fd(void *const *buffer, int size, int fd)
+void backtrace_symbols_fd(void *const *buffer, int size, int fd)
 {
 	dprintf(fd, "<backtrace not supported>\n");
 }
+#endif /*__GLIBC__ */
 
 int env_verbosity = 0;