linux-kernel - Re: [PATCH] ALSA: bcd2000: refactor deprecated strncpy

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Date:   Sat, 29 Jul 2023 14:00:55 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
        alsa-devel@...a-project.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ALSA: bcd2000: refactor deprecated strncpy

On Fri, 28 Jul 2023 00:09:29 +0200,
Justin Stitt wrote:
> 
> `strncpy` is deprecated for use on NUL-terminated destination strings [1].
> 
> A suitable replacement is `strscpy` [2] due to the fact that it
> guarantees NUL-termination on its destination buffer argument which is
> _not_ always the case for `strncpy`!
> 
> It should be noted that, in this case, the destination buffer has a
> length strictly greater than the source string. Moreover, the source
> string is NUL-terminated (and so is the destination) which means there
> was no real bug happening here. Nonetheless, this patch would get us one
> step closer to eliminating the `strncpy` API in the kernel, as its use
> is too ambiguous. We need to favor less ambiguous replacements such as:
> strscpy, strscpy_pad, strtomem and strtomem_pad (amongst others).
> 
> Technically, my patch yields subtly different behavior. The original
> implementation with `strncpy` would fill the entire destination buffer
> with null bytes [3] while `strscpy` will leave the junk, uninitialized
> bytes trailing after the _mandatory_ NUL-termination. So, if somehow
> `card->driver` or `card->shortname` require this NUL-padding behavior
> then `strscpy_pad` should be used. My interpretation, though, is that
> the aforementioned fields are just fine as NUL-terminated strings.
> Please correct my assumptions if needed and I'll send in a v2.
> 
> [1]: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
> [2]: manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html
> [3]: https://linux.die.net/man/3/strncpy
> 
> Link: https://github.com/KSPP/linux/issues/90
> Link: https://lore.kernel.org/r/20230727-sound-xen-v1-1-89dd161351f1@google.com (related ALSA patch)
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>

Applied now.  Thanks.


Takashi