Accessing 'reg.write_index' directly triggers a -Waddress-of-packed-member warning due to potential unaligned pointer access:
perf_test.c:239:38: warning: taking address of packed member 'write_index' of class or structure 'user_reg' may result in an unaligned pointer value [-Waddress-of-packed-member] 239 | ASSERT_NE(-1, write(self->data_fd, ®.write_index, | ^~~~~~~~~~~~~~~
Use memcpy() instead to safely copy the value and avoid unaligned pointer access across architectures.
Signed-off-by: Ankit Khushwaha ankitkhushwaha.linux@gmail.com --- tools/testing/selftests/user_events/perf_test.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/tools/testing/selftests/user_events/perf_test.c b/tools/testing/selftests/user_events/perf_test.c index 201459d8094d..e4385f4aa231 100644 --- a/tools/testing/selftests/user_events/perf_test.c +++ b/tools/testing/selftests/user_events/perf_test.c @@ -201,6 +201,7 @@ TEST_F(user, perf_empty_events) { struct perf_event_mmap_page *perf_page; int page_size = sysconf(_SC_PAGESIZE); int id, fd; + __u32 write_index; __u32 *val;
reg.size = sizeof(reg); @@ -236,7 +237,8 @@ TEST_F(user, perf_empty_events) { ASSERT_EQ(1 << reg.enable_bit, self->check);
/* Ensure write shows up at correct offset */ - ASSERT_NE(-1, write(self->data_fd, ®.write_index, + memcpy(&write_index, ®.write_index, sizeof(reg.write_index)); + ASSERT_NE(-1, write(self->data_fd, &write_index, sizeof(reg.write_index))); val = (void *)(((char *)perf_page) + perf_page->data_offset); ASSERT_EQ(PERF_RECORD_SAMPLE, *val);
On Mon, 27 Oct 2025 17:04:39 +0530 Ankit Khushwaha ankitkhushwaha.linux@gmail.com wrote:
Accessing 'reg.write_index' directly triggers a -Waddress-of-packed-member warning due to potential unaligned pointer access:
perf_test.c:239:38: warning: taking address of packed member 'write_index' of class or structure 'user_reg' may result in an unaligned pointer value [-Waddress-of-packed-member] 239 | ASSERT_NE(-1, write(self->data_fd, ®.write_index, | ^~~~~~~~~~~~~~~
Use memcpy() instead to safely copy the value and avoid unaligned pointer access across architectures.
...
--- a/tools/testing/selftests/user_events/perf_test.c +++ b/tools/testing/selftests/user_events/perf_test.c @@ -201,6 +201,7 @@ TEST_F(user, perf_empty_events) { struct perf_event_mmap_page *perf_page; int page_size = sysconf(_SC_PAGESIZE); int id, fd;
- __u32 write_index; __u32 *val;
reg.size = sizeof(reg); @@ -236,7 +237,8 @@ TEST_F(user, perf_empty_events) { ASSERT_EQ(1 << reg.enable_bit, self->check); /* Ensure write shows up at correct offset */
- ASSERT_NE(-1, write(self->data_fd, ®.write_index,
- memcpy(&write_index, ®.write_index, sizeof(reg.write_index));
- ASSERT_NE(-1, write(self->data_fd, &write_index, sizeof(reg.write_index)));
Simply casting &write_index to void* would fix this?
val = (void *)(((char *)perf_page) + perf_page->data_offset); ASSERT_EQ(PERF_RECORD_SAMPLE, *val);
On Mon, Oct 27, 2025 at 04:25:21PM -0700, Andrew Morton wrote:
On Mon, 27 Oct 2025 17:04:39 +0530 Ankit Khushwaha ankitkhushwaha.linux@gmail.com wrote:
Accessing 'reg.write_index' directly triggers a -Waddress-of-packed-member warning due to potential unaligned pointer access:
perf_test.c:239:38: warning: taking address of packed member 'write_index' of class or structure 'user_reg' may result in an unaligned pointer value [-Waddress-of-packed-member] 239 | ASSERT_NE(-1, write(self->data_fd, ®.write_index, | ^~~~~~~~~~~~~~~
Use memcpy() instead to safely copy the value and avoid unaligned pointer access across architectures.
...
--- a/tools/testing/selftests/user_events/perf_test.c +++ b/tools/testing/selftests/user_events/perf_test.c @@ -201,6 +201,7 @@ TEST_F(user, perf_empty_events) { struct perf_event_mmap_page *perf_page; int page_size = sysconf(_SC_PAGESIZE); int id, fd;
- __u32 write_index; __u32 *val;
reg.size = sizeof(reg); @@ -236,7 +237,8 @@ TEST_F(user, perf_empty_events) { ASSERT_EQ(1 << reg.enable_bit, self->check); /* Ensure write shows up at correct offset */
- ASSERT_NE(-1, write(self->data_fd, ®.write_index,
- memcpy(&write_index, ®.write_index, sizeof(reg.write_index));
- ASSERT_NE(-1, write(self->data_fd, &write_index, sizeof(reg.write_index)));
Simply casting &write_index to void* would fix this?
yes, this hides the type mismatch from the compiler. But i think casting to void * will not fix the alignment mismatch for packed struct. It works on x86, but might break on other platform.
val = (void *)(((char *)perf_page) + perf_page->data_offset); ASSERT_EQ(PERF_RECORD_SAMPLE, *val);
Thanks Ankit
On Tue, 28 Oct 2025 22:28:10 +0530 Ankit Khushwaha ankitkhushwaha.linux@gmail.com wrote:
@@ -236,7 +237,8 @@ TEST_F(user, perf_empty_events) { ASSERT_EQ(1 << reg.enable_bit, self->check); /* Ensure write shows up at correct offset */
- ASSERT_NE(-1, write(self->data_fd, ®.write_index,
- memcpy(&write_index, ®.write_index, sizeof(reg.write_index));
- ASSERT_NE(-1, write(self->data_fd, &write_index, sizeof(reg.write_index)));
Simply casting &write_index to void* would fix this?
yes, this hides the type mismatch from the compiler. But i think casting to void * will not fix the alignment mismatch for packed struct. It works on x86, but might break on other platform.
It's the second argument to write(2)! write(2) expects a const char *, but void* will work.
On Tue, Oct 28, 2025 at 01:26:05PM -0700, Andrew Morton wrote:
On Tue, 28 Oct 2025 22:28:10 +0530 Ankit Khushwaha ankitkhushwaha.linux@gmail.com wrote:
@@ -236,7 +237,8 @@ TEST_F(user, perf_empty_events) { ASSERT_EQ(1 << reg.enable_bit, self->check); /* Ensure write shows up at correct offset */
- ASSERT_NE(-1, write(self->data_fd, ®.write_index,
- memcpy(&write_index, ®.write_index, sizeof(reg.write_index));
- ASSERT_NE(-1, write(self->data_fd, &write_index, sizeof(reg.write_index)));
Simply casting &write_index to void* would fix this?
yes, this hides the type mismatch from the compiler. But i think casting to void * will not fix the alignment mismatch for packed struct. It works on x86, but might break on other platform.
It's the second argument to write(2)! write(2) expects a const char *, but void* will work.
Hi Andrew, Indeed `ASSERT_NE(-1, write(self->data_fd, (void *)®.write_index, sizeof(reg.write_index)));`
would work. However since `reg` is packed struct, directly taking the address of its member `®.write_index` may lead to unaligned access on some architectures. as indicated by the compiler warning
perf_test.c:239:38: warning: taking address of packed member 'write_index' of class or structure 'user_reg' may result in an unaligned pointer value [-Waddress-of-packed-member] 239 | ASSERT_NE(-1, write(self->data_fd, ®.write_index, | ^~~~~~~~~~~~~~~
Using `memcpy` avoids this by performing a byte-wise copy, which is safe to use for packed structures.
Thanks -- Ankit
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Andrew Morton -
Ankit Khushwaha