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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38a8e4b82ebsm7462898f8f.82.2025.01.11.08.58.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 11 Jan 2025 08:58:46 -0800 (PST) Date: Sat, 11 Jan 2025 16:58:44 +0000 From: David Laight To: Vincent Mailhol Cc: Kees Cook , Nathan Chancellor , Nick Desaulniers , Bill Wendling , Justin Stitt , David Laight , linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev Subject: Re: [PATCH] fortify: turn strlen() into an inline function using __builtin_constant_p() Message-ID: <20250111165844.6d95600f@pumpkin> In-Reply-To: References: <20250108-strlen_use_builtin_constant_p-v1-1-611b52e80a9f@wanadoo.fr> <202501081331.E4B6DF747@keescook> 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 Sat, 11 Jan 2025 23:40:41 +0900 Vincent Mailhol wrote: > On Thu. 9 Jan 2025 at 16:52, Vincent Mailhol wrote: ... > Actually, I did more investigation and it is working for some strange > reasons. Whenever the argument of a function named strlen() is a > compile time constant, the compiler (both GCC and clang) will replace > it with the string length on the argument, even if strlen() is > programmed to return something else: > > https://godbolt.org/z/nK4b3fnM7 > > So it is only working because the compiler uses its builtin strlen() > instead of the function we provided. It depends on whether -ffreestanding is set. If not set gcc/clang assume a lot of the basic libc functions have their expected behaviour. David