linux-kernel - [PATCH v3 0/3] Switch get/put unaligned to use memcpy

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Message-ID: <20250626054826.433453-1-irogers@google.com>
Date: Wed, 25 Jun 2025 22:48:23 -0700
From: Ian Rogers <irogers@...gle.com>
To: 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>, Christophe Leroy <christophe.leroy@...roup.eu>, 
	"Jason A. Donenfeld" <Jason@...c4.com>, linux-kernel@...r.kernel.org, 
	linux-perf-users@...r.kernel.org
Cc: Ian Rogers <irogers@...gle.com>
Subject: [PATCH v3 0/3] Switch get/put unaligned to use memcpy

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://lore.kernel.org/lkml/20250625202311.23244-2-ebiggers@kernel.org/

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.

v3: switch to __unqual_scalar_typeof, reducing the code, and use an
    uninitialized variable rather than a cast of 0 to try to avoid a
    sparse warning about not using NULL. The code is trying to
    navigate a minefield of uninitialized and casting warnings,
    hopefully the best balance has been struck, but the code will fail
    for cases like:
    const void *val = get_unaligned((const void * const *)ptr);
    due to __unqual_scalar_typeof leaving the 2nd const of the cast in
    place. Thankfully no code does this - tested with an
    allyesconfig. Support would be achievable by using void* as a
    default case in __unqual_scalar_typeof, it just doesn't seem worth
    it for a fairly unusual const case.

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://lore.kernel.org/lkml/c57de5bf-d55c-48c5-9dfa-e2fb844dafe9@csgroup.eu/

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             | 39 +++++++++++++++++++++++-----
 tools/include/linux/compiler_types.h | 22 ++++++++++++++++
 tools/include/linux/unaligned.h      |  4 ---
 tools/include/vdso/unaligned.h       | 39 +++++++++++++++++++++++-----
 4 files changed, 88 insertions(+), 16 deletions(-)

-- 
2.50.0.727.gbf7dc18ff4-goog