netdev - Re: [PATCH bpf-next] selftests/bpf: print backtrace on SIGSEGV in test

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Date:   Mon, 24 Feb 2020 21:33:58 +0000
From:   Song Liu <songliubraving@...com>
To:     Andrii Nakryiko <andriin@...com>
CC:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        "Alexei Starovoitov" <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii.nakryiko@...il.com" <andrii.nakryiko@...il.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next] selftests/bpf: print backtrace on SIGSEGV in
 test_progs



> On Feb 22, 2020, at 9:43 PM, Andrii Nakryiko <andriin@...com> wrote:
> 
> Due to various bugs in test 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@...com>
> ---
> tools/testing/selftests/bpf/Makefile     |  2 +-
> tools/testing/selftests/bpf/test_progs.c | 26 ++++++++++++++++++++++++
> 2 files changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> index 2a583196fa51..50c63c21e6fd 100644
> --- a/tools/testing/selftests/bpf/Makefile
> +++ b/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
> diff --git a/tools/testing/selftests/bpf/test_progs.c b/tools/testing/selftests/bpf/test_progs.c
> index bab1e6f1d8f1..531ab3e7e5e5 100644
> --- a/tools/testing/selftests/bpf/test_progs.c
> +++ b/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 +619,22 @@ 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);
> +	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 +642,16 @@ 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;
> 
> +	env.stdout = stdout;
> +	env.stderr = stderr;

We have the same code in stdio_hijack(). Maybe remove those in 
stdio_hijack()? 

> +	sigaction(SIGSEGV, &sigact, NULL);
> +
> 	err = argp_parse(&argp, argc, argv, 0, NULL, &env);
> 	if (err)
> 		return err;
> -- 
> 2.17.1
>