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

[Christophe Leroy]

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@xxxxxxxxxx>:
> 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@xxxxxxxxxx>:
>      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(-)