Re: [PATCH] selftests/seccomp: build on aarch64, document ABI

[AKASHI Takahiro <takahiro.akashi@linaro.org>]

On 09/10/2015 04:30 AM, Kees Cook wrote:
> The syscall ABI is inconsistent on aarch64 compat, so at least we should
> document it in the seccomp_bpf tests.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> Can someone with access to native aarch64 double-check this for me? I
> think we need to change these tests to pass if it's expected, but the
> compat behavior seems bad. It means compat code will break under an
> aarch64 kernel, when dealing with syscalls, like through seccomp.
> ---
>   tools/testing/selftests/seccomp/seccomp_bpf.c | 15 +++++++++++++--
>   1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> index 770f47adf295..866ff42e000d 100644
> --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> @@ -33,6 +33,10 @@
>   #include <unistd.h>
>   #include <sys/syscall.h>
>
> +#if defined(__aarch64__) && !defined(__NR_poll)
> +# define __NR_poll 0x49
> +#endif
> +
>   #include "test_harness.h"
>
>   #ifndef PR_SET_PTRACER
> @@ -2124,10 +2128,17 @@ TEST(syscall_restart)
>   	ASSERT_EQ(SIGTRAP, WSTOPSIG(status));
>   	ASSERT_EQ(PTRACE_EVENT_SECCOMP, (status >> 16));
>   	ASSERT_EQ(0, ptrace(PTRACE_GETEVENTMSG, child_pid, NULL, &msg));
> -	ASSERT_EQ(0x200, msg);
> +
> +	/*
> +	 * FIXME:
> +	 * - native ARM does not expose true syscall.
> +	 * - compat ARM on ARM64 does expose true syscall.
> +	 * - native ARM64 hides true syscall even from seccomp.

Are you sure about the last line?
The kernel pushes __NR_compat_restart_syscall to w7 in compat mode, while
__NR_restart_syscall to x8 in native mode. But it is the only difference,
as far as I understand, in terms of restarting a system call.
So the behavior should be basically the same.

-Takahiro AKASHI

> +	 */
> +	ASSERT_EQ(0x200, msg);	/* This will fail on native arm64. */
>   	ret = get_syscall(_metadata, child_pid);
>   #if defined(__arm__)
> -	/* FIXME: ARM does not expose true syscall in registers. */
> +	/* This will fail on arm64 in compat mode. */
>   	EXPECT_EQ(__NR_poll, ret);
>   #else
>   	EXPECT_EQ(__NR_restart_syscall, ret);
>