Re: [PATCH] selftests/mm: Fix thread state check in uffd-unit-tests

[Mike Rapoport <rppt@kernel.org>]

On Wed, Dec 10, 2025 at 05:14:08PM +0800, Wake Liu wrote:
> In the thread_state_get() function, the logic to find the thread's state
> character was using `sizeof(header) - 1` to calculate the offset from
> the "State:\t" string.
> 
> The `header` variable is a `const char *` pointer. `sizeof()` on a
> pointer returns the size of the pointer itself, not the length of the
> string literal it points to. This makes the code's behavior dependent
> on the architecture's pointer size.
> 
> This bug was identified on a 32-bit ARM build (`gsi_tv_arm`) for
> Android, running on an ARMv8-based device, compiled with Clang 19.0.1.
> 
> On this 32-bit architecture, `sizeof(char *)` is 4. The expression
> `sizeof(header) - 1` resulted in an incorrect offset of 3, causing the
> test to read the wrong character from `/proc/[tid]/status` and fail.
> 
> On 64-bit architectures, `sizeof(char *)` is 8, so the expression
> coincidentally evaluates to 7, which matches the length of "State:\t".
> This is why the bug likely remained hidden on 64-bit builds.
> 
> To fix this and make the code portable and correct across all
> architectures, this patch replaces `sizeof(header) - 1` with
> `strlen(header)`. The `strlen()` function correctly calculates the
> string's length, ensuring the correct offset is always used.
> 
> Signed-off-by: Wake Liu <wakel@google.com>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  tools/testing/selftests/mm/uffd-unit-tests.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
> index f4807242c5b2..6f5e404a446c 100644
> --- a/tools/testing/selftests/mm/uffd-unit-tests.c
> +++ b/tools/testing/selftests/mm/uffd-unit-tests.c
> @@ -1317,7 +1317,7 @@ static thread_state thread_state_get(pid_t tid)
>  		p = strstr(tmp, header);
>  		if (p) {
>  			/* For example, "State:\tD (disk sleep)" */
> -			c = *(p + sizeof(header) - 1);
> +			c = *(p + strlen(header));
>  			return c == 'D' ?
>  			    THR_STATE_UNINTERRUPTIBLE : THR_STATE_UNKNOWN;
>  		}
> -- 
> 2.52.0.223.gf5cc29aaa4-goog
> 

-- 
Sincerely yours,
Mike.