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R . Silva" , Bill Wendling , Justin Stitt , Jann Horn , Przemek Kitszel , Marco Elver , Greg Kroah-Hartman , Sasha Levin , linux-mm@kvack.org, Randy Dunlap , Miguel Ojeda , Vegard Nossum , Harry Yoo , Nathan Chancellor , Peter Zijlstra , Nick Desaulniers , Jonathan Corbet , Jakub Kicinski , Yafang Shao , Tony Ambardar , Alexander Lobakin , Jan Hendrik Farr , Alexander Potapenko , linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org, linux-doc@vger.kernel.org, llvm@lists.linux.dev Subject: Re: [PATCH v5 2/4] slab: Introduce kmalloc_obj() and family Message-ID: References: <20251122014258.do.018-kees@kernel.org> <20251122014304.3417954-2-kees@kernel.org> <202511241119.C547DEF80@keescook> <202511241317.516BDE7B@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <202511241317.516BDE7B@keescook> X-Rspamd-Server: rspam12 X-Rspam-User: X-Rspamd-Queue-Id: B7BB640013 X-Stat-Signature: dwhxibkfo547wzm9a46mbo5yjwgbrg3y X-HE-Tag: 1764020036-318700 X-HE-Meta: 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 95IFhlMZ Yk9lpCLyquWSj60Eh2sP7YxMJsLXSTAbOWkFt18GtIyNz3jB9VXfrFNakBNeRLR2TERfciiDJCXET4Oo0Ol6m/mXpPrwh5q59CNj2rQUL4qR1yNblvj509iZgavFq6qeEkmAG5hzWP9rTTsgx+ncOlYvcbYCwCpqWVHKm9plUX7Q+ubnhtMpvY3GMwI8Qw8Cj4zFr4zacLrie7PC5jy9rXOvzlpxfScBJYiBPkWQy4AQL534D6B2hQZOI5J88HCZxOAxkZ65cMBXHltazTkhcvoBVXBweNCmmOJlJBbp7zpLPVoiEFqAnHi7RSBmAyR3G1DJC4UgnFFIAOqnstQRsBQZCkU1p3QJv5lrjcYPbtvmIjSPKNIUkPb+M9ycOw31Txo29IUcc5vY5gfiCErdqFxsi+wUIPdVeJQV198qw5Xh6xAkdMVI33dTWjw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Nov 24, 2025 at 01:20:21PM -0800, Kees Cook wrote: > > Maybe such a warning already exists and it's just too noisy to even > > start thinking about turning it on? > > Yes, -Wconversion (W=3) is mind-blowingly noisy, unfortunately. It looks like GCC isn't smart enough. The first warning I saw was legit and easy to fix. The second one is bogus: include/linux/err.h: In function ‘PTR_ERR_OR_ZERO’: include/linux/err.h:120:24: error: conversion from ‘long int’ to ‘int’ may change value [-Werror=conversion] 120 | return PTR_ERR(ptr); But GCC can prove that this isn't true; it just chooses not to: #define IS_ERR_VALUE(x) unlikely((unsigned long)(void *)(x) >= (unsigned long)-MAX_ERRNO) static inline bool __must_check IS_ERR(__force const void *ptr) { return IS_ERR_VALUE((unsigned long)ptr); } static inline int __must_check PTR_ERR_OR_ZERO(__force const void *ptr) { if (IS_ERR(ptr)) return PTR_ERR(ptr); So GCC knows in this path that 'ptr' is in the range [-4095..-1] and the conversion from long to int will not change the value. I imagine that fixing this is not high on the GCC developer priority list, but if we filed a bug that might change?