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 --- 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; }
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
Oops, thanks for spotting it. It was an accident the size of array is 8 here.. What I should have meant was:
const char header[] = "State:\t";
That should also work with sizeof(). But your fix works, so it's all fine.
Acked-by: Peter Xu peterx@redhat.com
Thanks,
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
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Peter Xu -
Wake Liu