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

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Date:   Thu, 27 Jul 2023 22:09:29 +0000
From:   Justin Stitt <justinstitt@...gle.com>
To:     Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>
Cc:     alsa-devel@...a-project.org, linux-kernel@...r.kernel.org,
        Justin Stitt <justinstitt@...gle.com>
Subject: [PATCH] ALSA: bcd2000: 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
`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>
---
 sound/usb/bcd2000/bcd2000.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/sound/usb/bcd2000/bcd2000.c b/sound/usb/bcd2000/bcd2000.c
index 7aec0a95c609..392b4d8e9e76 100644
--- a/sound/usb/bcd2000/bcd2000.c
+++ b/sound/usb/bcd2000/bcd2000.c
@@ -395,8 +395,8 @@ static int bcd2000_probe(struct usb_interface *interface,

 	snd_card_set_dev(card, &interface->dev);

-	strncpy(card->driver, "snd-bcd2000", sizeof(card->driver));
-	strncpy(card->shortname, "BCD2000", sizeof(card->shortname));
+	strscpy(card->driver, "snd-bcd2000", sizeof(card->driver));
+	strscpy(card->shortname, "BCD2000", sizeof(card->shortname));
 	usb_make_path(bcd2k->dev, usb_path, sizeof(usb_path));
 	snprintf(bcd2k->card->longname, sizeof(bcd2k->card->longname),
 		    "Behringer BCD2000 at %s",

---
base-commit: 57012c57536f8814dec92e74197ee96c3498d24e
change-id: 20230727-sound-usb-bcd2000-400b3060a9f9

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