linux-kernel - [PATCH] ALSA: xen-front: refactor deprecated strncpy

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Message-ID: <20230727-sound-xen-v1-1-89dd161351f1@google.com>
Date:   Thu, 27 Jul 2023 21:53:24 +0000
From:   Justin Stitt <justinstitt@...gle.com>
To:     Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>,
        Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>
Cc:     xen-devel@...ts.xenproject.org, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Justin Stitt <justinstitt@...gle.com>
Subject: [PATCH] ALSA: xen-front: refactor deprecated strncpy

`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
`pcm->name` or `card->driver/shortname/longname` 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
Signed-off-by: Justin Stitt <justinstitt@...gle.com>
---
 sound/xen/xen_snd_front_alsa.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/sound/xen/xen_snd_front_alsa.c b/sound/xen/xen_snd_front_alsa.c
index db917453a473..7a3dfce97c15 100644
--- a/sound/xen/xen_snd_front_alsa.c
+++ b/sound/xen/xen_snd_front_alsa.c
@@ -783,7 +783,7 @@ static int new_pcm_instance(struct xen_snd_front_card_info *card_info,
 	pcm->info_flags = 0;
 	/* we want to handle all PCM operations in non-atomic context */
 	pcm->nonatomic = true;
-	strncpy(pcm->name, "Virtual card PCM", sizeof(pcm->name));
+	strscpy(pcm->name, "Virtual card PCM", sizeof(pcm->name));

 	if (instance_cfg->num_streams_pb)
 		snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
@@ -835,9 +835,9 @@ int xen_snd_front_alsa_init(struct xen_snd_front_info *front_info)
 			goto fail;
 	}

-	strncpy(card->driver, XENSND_DRIVER_NAME, sizeof(card->driver));
-	strncpy(card->shortname, cfg->name_short, sizeof(card->shortname));
-	strncpy(card->longname, cfg->name_long, sizeof(card->longname));
+	strscpy(card->driver, XENSND_DRIVER_NAME, sizeof(card->driver));
+	strscpy(card->shortname, cfg->name_short, sizeof(card->shortname));
+	strscpy(card->longname, cfg->name_long, sizeof(card->longname));

 	ret = snd_card_register(card);
 	if (ret < 0)

---
base-commit: 57012c57536f8814dec92e74197ee96c3498d24e
change-id: 20230727-sound-xen-398c9927b3d8

Best regards,
--
Justin Stitt <justinstitt@...gle.com>