linux-kernel - Re: [PATCH v5 2/4] slab: Introduce kmalloc

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Message-ID: <aSTPOYLVzgkWDZR-@casper.infradead.org>
Date: Mon, 24 Nov 2025 21:33:45 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Kees Cook <kees@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>, Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Hyeonggon Yoo <42.hyeyoo@...il.com>,
	"Gustavo A . R . Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, Jann Horn <jannh@...gle.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Marco Elver <elver@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sasha Levin <sashal@...nel.org>, linux-mm@...ck.org,
	Randy Dunlap <rdunlap@...radead.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	Harry Yoo <harry.yoo@...cle.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Jonathan Corbet <corbet@....net>, Jakub Kicinski <kuba@...nel.org>,
	Yafang Shao <laoar.shao@...il.com>,
	Tony Ambardar <tony.ambardar@...il.com>,
	Alexander Lobakin <aleksander.lobakin@...el.com>,
	Jan Hendrik Farr <kernel@...rr.cc>,
	Alexander Potapenko <glider@...gle.com>,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
	linux-doc@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v5 2/4] slab: Introduce kmalloc_obj() and family

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?