linux-kernel - Re: [PATCH] selftests/proc: Fix proc-pid-vm for vsyscall=xonly.

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Date:   Thu, 16 Jun 2022 17:01:48 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Dylan Hatch <dylanbhatch@...gle.com>, Shuah Khan <shuah@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH] selftests/proc: Fix proc-pid-vm for vsyscall=xonly.

On 6/16/22 3:10 PM, Dylan Hatch wrote:
> This test would erroneously fail the /proc/$PID/maps case if
> vsyscall=xonly since the existing probe of the vsyscall page only
> succeeds if the process has read permissions. Fix this by checking for
> either no vsyscall mapping OR an execute-only vsyscall mapping in the
> case were probing the vsyscall page segfaults.
> 

Does this fix include skipping the test with a clear message that
says why test is skipped?

> Signed-off-by: Dylan Hatch <dylanbhatch@...gle.com>
> ---
>   tools/testing/selftests/proc/proc-pid-vm.c | 20 +++++++++++++++-----
>   1 file changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/testing/selftests/proc/proc-pid-vm.c b/tools/testing/selftests/proc/proc-pid-vm.c
> index 28604c9f805c..5ca85520131f 100644
> --- a/tools/testing/selftests/proc/proc-pid-vm.c
> +++ b/tools/testing/selftests/proc/proc-pid-vm.c
> @@ -213,9 +213,12 @@ static int make_exe(const uint8_t *payload, size_t len)
>   
>   static bool g_vsyscall = false;
>   
> -static const char str_vsyscall[] =
> +static const char str_vsyscall_rx[] =
>   "ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]\n";
>   
> +static const char str_vsyscall_x[] =
> +"ffffffffff600000-ffffffffff601000 --xp 00000000 00:00 0                  [vsyscall]\n";
> +
>   #ifdef __x86_64__
>   static void sigaction_SIGSEGV(int _, siginfo_t *__, void *___)
>   {
> @@ -261,6 +264,7 @@ int main(void)
>   	int exec_fd;
>   
>   	vsyscall();
> +	const char *str_vsyscall = g_vsyscall ? str_vsyscall_rx : str_vsyscall_x;
>   
>   	atexit(ate);
>   
> @@ -314,7 +318,8 @@ int main(void)
>   
>   	/* Test /proc/$PID/maps */
>   	{
> -		const size_t len = strlen(buf0) + (g_vsyscall ? strlen(str_vsyscall) : 0);
> +		const size_t len_buf0 = strlen(buf0);
> +		const size_t len_vsys = strlen(str_vsyscall);
>   		char buf[256];
>   		ssize_t rv;
>   		int fd;
> @@ -325,11 +330,16 @@ int main(void)
>   			return 1;
>   		}
>   		rv = read(fd, buf, sizeof(buf));
> -		assert(rv == len);
> -		assert(memcmp(buf, buf0, strlen(buf0)) == 0);
>   		if (g_vsyscall) {
> -			assert(memcmp(buf + strlen(buf0), str_vsyscall, strlen(str_vsyscall)) == 0);
> +			assert(rv == len_buf0 + len_vsys);
> +		} else {
> +			/* If vsyscall isn't readable, it's either x-only or not mapped at all */
> +			assert(rv == len_buf0 + len_vsys || rv == len_buf0);
>   		}
> +		assert(memcmp(buf, buf0, len_buf0) == 0);
> +		/* Check for vsyscall mapping if buf is long enough */
> +		if (rv == len_buf0 + len_vsys)
> +			assert(memcmp(buf + len_buf0, str_vsyscall, len_vsys) == 0);
>   	}
>   
>   	/* Test /proc/$PID/smaps */
> 

The change looks good to me. Doesn't look like it skips the test though?

thanks,
-- Shuah