linux-kernel - Re: [PATCH 3/6] tools/nolibc: properly align dirent buffer

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Message-ID: <20250419091140.GC31874@1wt.eu>
Date: Sat, 19 Apr 2025 11:11:40 +0200
From: Willy Tarreau <w@....eu>
To: Thomas Weißschuh <linux@...ssschuh.net>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, Shuah Khan <shuah@...nel.org>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 3/6] tools/nolibc: properly align dirent buffer

On Wed, Apr 16, 2025 at 08:40:18PM +0200, Thomas Weißschuh wrote:
> As byte buffer is overlaid with a 'struct dirent64'.
> it has to satisfy the structs alignment requirements.
> 
> Fixes: 665fa8dea90d ("tools/nolibc: add support for directory access")
> Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>

Oh good catch! I already faced crashes in other programs due to AVX
memcpy when doing similar casts without thinking about alignment.

> @@ -58,6 +58,7 @@ int closedir(DIR *dirp)
>  static __attribute__((unused))
>  int readdir_r(DIR *dirp, struct dirent *entry, struct dirent **result)
>  {
> +	__attribute__((aligned(__alignof__(struct linux_dirent64))))
>  	char buf[sizeof(struct linux_dirent64) + NAME_MAX + 1];

I tend to find it more readable when the attribute is on the same line as
the type on variables declaration, often at the end of the line, to keep
declarations alignes, even if that makes longer lines. If alignment concerns
come back often, we could maybe have __nolibc_align(<align>) and maybe even
__nolibc_align_as(<type>) to slightly shorten the lines. Just an idea.

In any case: Acked-by: Willy Tarreau <w@....eu>

Willy