Subject: Re: [PATCH 13/15] selftests/seccomp: powerpc: Set syscall return during ptrace syscall exit

From: Michael Ellerman <[email-masked]>
Date: Mon, 14 Sep 2020 15:47:13 +1000

Kees Cook <[email-masked]> writes:
> Some archs (like ppc) only support changing the return code during
> syscall exit when ptrace is used. As the syscall number might not
> be available anymore during syscall exit, it needs to be saved
> during syscall enter. Adjust the ptrace tests to do this.

I'm not that across all the fixture stuff, but if I'm reading it right
you're now calling change_syscall() on both entry and exit for all
arches.

That should work, but it no longer tests changing the return code on
entry on the arches that support it, which seems like a backward step?

cheers

> Reported-by: Thadeu Lima de Souza Cascardo <[email-masked]>
> Suggested-by: Thadeu Lima de Souza Cascardo <[email-masked]>
> Link: https://lore.kernel.org/linux-kselftest/[email-masked]/
> Fixes: 58d0a862f573 ("seccomp: add tests for ptrace hole")
> Signed-off-by: Kees Cook <[email-masked]>
> ---
>  tools/testing/selftests/seccomp/seccomp_bpf.c | 34 +++++++++++--------
>  1 file changed, 20 insertions(+), 14 deletions(-)
>
> diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> index bbab2420d708..26c712c6a575 100644
> --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> @@ -1949,12 +1949,19 @@ void tracer_seccomp(struct __test_metadata *_metadata, pid_t tracee,
>  
>  }
>  
> +FIXTURE(TRACE_syscall) {
> +	struct sock_fprog prog;
> +	pid_t tracer, mytid, mypid, parent;
> +	long syscall_nr;
> +};
> +
>  void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
>  		   int status, void *args)
>  {
> -	int ret, nr;
> +	int ret;
>  	unsigned long msg;
>  	static bool entry;
> +	FIXTURE_DATA(TRACE_syscall) *self = args;
>  
>  	/*
>  	 * The traditional way to tell PTRACE_SYSCALL entry/exit
> @@ -1968,24 +1975,23 @@ void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
>  	EXPECT_EQ(entry ? PTRACE_EVENTMSG_SYSCALL_ENTRY
>  			: PTRACE_EVENTMSG_SYSCALL_EXIT, msg);
>  
> -	if (!entry)
> -		return;
> -
> -	nr = get_syscall(_metadata, tracee);
> +	/*
> +	 * Some architectures only support setting return values during
> +	 * syscall exit under ptrace, and on exit the syscall number may
> +	 * no longer be available. Therefore, save it here, and call
> +	 * "change syscall and set return values" on both entry and exit.
> +	 */
> +	if (entry)
> +		self->syscall_nr = get_syscall(_metadata, tracee);
>  
> -	if (nr == __NR_getpid)
> +	if (self->syscall_nr == __NR_getpid)
>  		change_syscall(_metadata, tracee, __NR_getppid, 0);
> -	if (nr == __NR_gettid)
> +	if (self->syscall_nr == __NR_gettid)
>  		change_syscall(_metadata, tracee, -1, 45000);
> -	if (nr == __NR_openat)
> +	if (self->syscall_nr == __NR_openat)
>  		change_syscall(_metadata, tracee, -1, -ESRCH);
>  }
>  
> -FIXTURE(TRACE_syscall) {
> -	struct sock_fprog prog;
> -	pid_t tracer, mytid, mypid, parent;
> -};
> -
>  FIXTURE_VARIANT(TRACE_syscall) {
>  	/*
>  	 * All of the SECCOMP_RET_TRACE behaviors can be tested with either
> @@ -2044,7 +2050,7 @@ FIXTURE_SETUP(TRACE_syscall)
>  	self->tracer = setup_trace_fixture(_metadata,
>  					   variant->use_ptrace ? tracer_ptrace
>  							       : tracer_seccomp,
> -					   NULL, variant->use_ptrace);
> +					   self, variant->use_ptrace);
>  
>  	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
>  	ASSERT_EQ(0, ret);
> -- 
> 2.25.1