selftests/bpf: Print backtrace on SIGSEGV in test_progs

Due to various bugs in tests clean up code (usually), if host system is
misconfigured, it happens that test_progs will just crash in the middle of
running a test with little to no indication of where and why the crash
happened. For cases where coredump is not readily available (e.g., inside
a CI), it's very helpful to have a stack trace, which lead to crash, to be
printed out. This change adds a signal handler that will capture and print out
symbolized backtrace:

  $ sudo ./test_progs -t mmap
  test_mmap:PASS:skel_open_and_load 0 nsec
  test_mmap:PASS:bss_mmap 0 nsec
  test_mmap:PASS:data_mmap 0 nsec
  Caught signal #11!
  Stack trace:
  ./test_progs(crash_handler+0x18)[0x42a888]
  /lib64/libpthread.so.0(+0xf5d0)[0x7f2aab5175d0]
  ./test_progs(test_mmap+0x3c0)[0x41f0a0]
  ./test_progs(main+0x160)[0x407d10]
  /lib64/libc.so.6(__libc_start_main+0xf5)[0x7f2aab15d3d5]
  ./test_progs[0x407ebc]
  [1]    1988412 segmentation fault (core dumped)  sudo ./test_progs -t mmap

Unfortunately, glibc's symbolization support is unable to symbolize static
functions, only global ones will be present in stack trace. But it's still a
step forward without adding extra libraries to get a better symbolization.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200225000847.3965188-1-andriin@fb.com

tools/testing/selftests/bpf/Makefile

@@ -20,7 +20,7 @@ CLANG		?= clang
 LLC		?= llc
 LLVM_OBJCOPY	?= llvm-objcopy
 BPF_GCC		?= $(shell command -v bpf-gcc;)
-CFLAGS += -g -Wall -O2 $(GENFLAGS) -I$(CURDIR) -I$(APIDIR)		\
+CFLAGS += -g -rdynamic -Wall -O2 $(GENFLAGS) -I$(CURDIR) -I$(APIDIR)	\
 	  -I$(INCLUDE_DIR) -I$(GENDIR) -I$(LIBDIR) -I$(TOOLSINCDIR)	\
 	  -Dbpf_prog_load=bpf_prog_test_load				\
 	  -Dbpf_load_program=bpf_test_load_program

tools/testing/selftests/bpf/test_progs.c

@@ -6,6 +6,8 @@
 #include "bpf_rlimit.h"
 #include <argp.h>
 #include <string.h>
+#include <signal.h>
+#include <execinfo.h> /* backtrace */
 
 /* defined in test_progs.h */
 struct test_env env = {};
@@ -617,6 +619,23 @@ int cd_flavor_subdir(const char *exec_name)
 	return chdir(flavor);
 }
 
+#define MAX_BACKTRACE_SZ 128
+void crash_handler(int signum)
+{
+	void *bt[MAX_BACKTRACE_SZ];
+	size_t sz;
+
+	sz = backtrace(bt, ARRAY_SIZE(bt));
+
+	if (env.test)
+		dump_test_log(env.test, true);
+	if (env.stdout)
+		stdio_restore();
+
+	fprintf(stderr, "Caught signal #%d!\nStack trace:\n", signum);
+	backtrace_symbols_fd(bt, sz, STDERR_FILENO);
+}
+
 int main(int argc, char **argv)
 {
 	static const struct argp argp = {
@@ -624,8 +643,14 @@ int main(int argc, char **argv)
 		.parser = parse_arg,
 		.doc = argp_program_doc,
 	};
+	struct sigaction sigact = {
+		.sa_handler = crash_handler,
+		.sa_flags = SA_RESETHAND,
+	};
 	int err, i;
 
+	sigaction(SIGSEGV, &sigact, NULL);
+
 	err = argp_parse(&argp, argc, argv, 0, NULL, &env);
 	if (err)
 		return err;