linux-kernel - Re: [PATCH v2 0/3] Switch get/put unaligned to use memcpy

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Message-ID: <51bc73ca-3b1f-4f1a-b75b-7bdeffd7a395@csgroup.eu>
Date: Wed, 18 Jun 2025 13:42:53 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Ian Rogers <irogers@...gle.com>, Eric Biggers <ebiggers@...gle.com>,
 Yuzhuo Jing <yuzhuo@...gle.com>, Andy Lutomirski <luto@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>,
 Vincenzo Frascino <vincenzo.frascino@....com>,
 Arnaldo Carvalho de Melo <acme@...hat.com>, Al Viro
 <viro@...iv.linux.org.uk>, "Jason A. Donenfeld" <Jason@...c4.com>,
 linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v2 0/3] Switch get/put unaligned to use memcpy



Le 17/06/2025 à 22:53, Ian Rogers a écrit :
> The existing type punning approach with packed structs requires
>   -fno-strict-aliasing to be passed to the compiler for
> correctness. This is true in the kernel tree but not in the tools
> directory resulting in this suggested patch from Eric Biggers
>   <ebiggers@...gle.com>:
> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2F20250614044133.660848-2-ebiggers%40kernel.org%2F&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7Cf05413010ecc40ad1bdf08ddade1316a%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638857904894967529%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=29QUBLnUowncZiTH4z74Ec1olUlX0OTYnUNGDvWxX1o%3D&reserved=0
> 
> Requiring -fno-strict-aliasing seems unfortunate and so this patch
> makes the unaligned code work via memcpy for type punning rather than
> the packed attribute.
> 
> v2: switch memcpy to __builtin_memcpy to avoid potential/disallowed
>      memcpy calls in vdso caused by -fno-builtin. Reported by
>      Christophe Leroy <christophe.leroy@...roup.eu>:
>      https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2Fc57de5bf-d55c-48c5-9dfa-e2fb844dafe9%40csgroup.eu%2F&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7Cf05413010ecc40ad1bdf08ddade1316a%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638857904894985987%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=l1LJP3XPKVuhDpiHxxkfWBiNPMYaBT9YXGBFzs6wLpY%3D&reserved=0
> 

Does this new version also fixes the failures reported by the robots:
- arm64-randconfig with clang 21
- i386-randconfig with gcc 12

Christophe

> Ian Rogers (3):
>    vdso: Switch get/put unaligned from packed struct to memcpy
>    tools headers: Update the linux/unaligned.h copy with the kernel
>      sources
>    tools headers: Remove unneeded ignoring of warnings in unaligned.h
> 
>   include/vdso/unaligned.h        | 48 ++++++++++++++++++++++++++++-----
>   tools/include/linux/unaligned.h |  4 ---
>   tools/include/vdso/unaligned.h  | 48 ++++++++++++++++++++++++++++-----
>   3 files changed, 84 insertions(+), 16 deletions(-)
>