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

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Message-ID: <202511241347.AF6790AFB@keescook>
Date: Mon, 24 Nov 2025 13:50:00 -0800
From: Kees Cook <kees@...nel.org>
To: Matthew Wilcox <willy@...radead.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 09:44:16PM +0000, Matthew Wilcox wrote:
> On Mon, Nov 24, 2025 at 01:20:21PM -0800, Kees Cook wrote:
> > Yes, -Wconversion (W=3) is mind-blowingly noisy, unfortunately.
> 
> This third one is interesting.
> 
> include/linux/jump_label.h:126:44: error: conversion to ‘long unsigned int’ from ‘s32’ {aka ‘int’} may change the sign of the result [-Werror=sign-conversion]
>   126 |         return (unsigned long)&entry->code + entry->code;
> 
> static inline unsigned long jump_entry_code(const struct jump_entry *entry)
> {
>         return (unsigned long)&entry->code + entry->code;
> }
> 
> The warning is ... not the best phrased, but in terms of divining the
> programmer's intent, I genuinely don't know if this code is supposed
> to zero-extend or sign-extend the s32 to unsigned long.  I know what it
> *does*, but I don't know if it was *supposed to do that*.

This is my core frustration with C: we have SO many things where we have
ambiguous intent. Yes, C may do exactly 1 thing with a given construct,
but it isn't clear that the author's intent matches what actually
happens.

> So I wuold be
> in favour of enabling this warning ... if we have a small army of people
> on tap to get the kernel to build.  There's 374 lines of errors to fix
> from the header files included by scripts/mod/devicetable-offsets.s alone.

I'm for it, but that is a LONG road. I have so many other hills to die
on first. ;)

-- 
Kees Cook