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Tue, 28 Oct 2025 09:58:20 -0700 (PDT) Received: from fedora ([103.120.31.122]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-29498cf4b0csm121255865ad.10.2025.10.28.09.58.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 28 Oct 2025 09:58:19 -0700 (PDT) Date: Tue, 28 Oct 2025 22:28:10 +0530 From: Ankit Khushwaha To: Andrew Morton Cc: linux-kselftest@vger.kernel.org, Shuah Khan , Bala-Vignesh-Reddy , Wei Yang , linux-kernel@vger.kernel.org, linux-kernel-mentees@lists.linuxfoundation.org, Steven Rostedt Subject: Re: [PATCH] selftests/user_events: Avoid taking address of packed member in perf_test Message-ID: References: <20251027113439.36059-1-ankitkhushwaha.linux@gmail.com> <20251027162521.c56c7f89f6ad4e3d639c408c@linux-foundation.org> Precedence: bulk X-Mailing-List: linux-kernel-mentees@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251027162521.c56c7f89f6ad4e3d639c408c@linux-foundation.org> On Mon, Oct 27, 2025 at 04:25:21PM -0700, Andrew Morton wrote: > On Mon, 27 Oct 2025 17:04:39 +0530 Ankit Khushwaha 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