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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4601f2f67c6sm23925419f8f.16.2026.06.05.01.23.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 05 Jun 2026 01:23:20 -0700 (PDT) Date: Fri, 5 Jun 2026 09:23:19 +0100 From: David Laight To: Nathan Chancellor Cc: Linus Torvalds , Askar Safin , metze@samba.org, akpm@linux-foundation.org, axboe@kernel.dk, brauner@kernel.org, david@kernel.org, dhowells@redhat.com, hch@infradead.org, jack@suse.cz, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, miklos@szeredi.hu, netdev@vger.kernel.org, patches@lists.linux.dev, pfalcato@suse.de, viro@zeniv.linux.org.uk, willy@infradead.org Subject: Re: [PATCH 2/3] vmsplice: make vmsplice a trivial wrapper for preadv2/pwritev2 Message-ID: <20260605092319.5a1bba74@pumpkin> In-Reply-To: <20260605015724.GA520134@ax162> References: <20260603211736.755139-1-safinaskar@gmail.com> <20260604100609.6b37f500@pumpkin> <20260604183829.63c35fd9@pumpkin> <20260604223216.73468830@pumpkin> <20260605015724.GA520134@ax162> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 4 Jun 2026 18:57:24 -0700 Nathan Chancellor wrote: > On Thu, Jun 04, 2026 at 10:32:16PM +0100, David Laight wrote: > > Talking of broken compilers, had you noticed that: > > struct foo { > > int a; > > char c[32]; > > }; > > > > int b(struct foo *f) > > { > > return __builtin_object_size(f->c, 1); > > } > > returns -1 (size unknown/indefinite). > > You can't use __builtin_object_size() to stop code running off the end > > of anything referenced by address - even when the size is constant. > > That is the entire point of using '-fstrict-flex-arrays=3' in the > kernel: > > df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3") > https://godbolt.org/z/bvfrh7W58 > > Without it, all trailing arrays in structures are treated as flexible > arrays, even those with fixed sizes. > strict-flex-arrays got added in gcc 13.1 and clang 15.0; it isn't supported by the gcc 12.2 on the debian 12 system I'm building kernels on. __buitin_object_size() itself is in gcc 4.1.2 and clang 3.0. Neither are flex arrays mentioned in the gcc docs for __builtin_object_size(). Someone might have used (eg) 'char x[4]' as a flex array to include the padding, but no one would have used anything that extended the structure. And the chance of those hitting __builtin_object_size() is even smaller. -- David