linux-kernel - Re: [PATCH] ASoC: intel: avs: refactor strncpy usage in topology

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Message-ID: <202307252157.90B1933@keescook>
Date:   Tue, 25 Jul 2023 22:04:00 -0700
From:   Kees Cook <keescook@...omium.org>
To:     justinstitt@...gle.com
Cc:     Cezary Rojewski <cezary.rojewski@...el.com>,
        Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        Liam Girdwood <liam.r.girdwood@...ux.intel.com>,
        Peter Ujfalusi <peter.ujfalusi@...ux.intel.com>,
        Bard Liao <yung-chuan.liao@...ux.intel.com>,
        Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        Kai Vehmanen <kai.vehmanen@...ux.intel.com>,
        Mark Brown <broonie@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org, Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH] ASoC: intel: avs: refactor strncpy usage in topology

On Tue, Jul 25, 2023 at 10:08:38PM +0000, justinstitt@...gle.com wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings
> [1].
> 
> A suitable replacement is `strscpy` [2].

For future patches like this, can you include the rationale for _why_
strscpy() is suitable? (i.e. how did you check that it doesn't need
zero-padding, the dest is expected to always be NUL-terminated, etc.)

I had fun looking through this code -- it's a bit of a maze! I can see
in the initializer for "route" (soc_tplg_dapm_graph_elems_load()), that
all the strings pointed at from "elem" are being checked for NUL-termination.

That this code is doing an in-place rewrite of the string is pretty
unusual (i.e. specifically casting around the "const"), but it looks
quite intentional. :)

> There are some hopes that someday the `strncpy` api could be ripped out

This can be rephrased, perhaps, as:

There is a goal to eliminate the `strncpy` API in the kernel, as its
use is too ambiguous, instead moving to the unambiguous replacements
(strscpy, strscpy_pad, strtomem, strtomem_pad, strlcpy) [1].

> due to the vast number of suitable replacements (strscpy, strscpy_pad,
> strtomem, strtomem_pad, strlcpy) [1].
> 
> [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
> 
> ---
^^^^ this triple-dash is going to cause some tooling to lose your S-o-b,
as it indicates the end of the commit log.

> 
> 
> Link: https://github.com/KSPP/linux/issues/90
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>

But otherwise, looks good:

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  sound/soc/intel/avs/topology.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/sound/soc/intel/avs/topology.c b/sound/soc/intel/avs/topology.c
> index cdb4ec500261..45d0eb2a8e71 100644
> --- a/sound/soc/intel/avs/topology.c
> +++ b/sound/soc/intel/avs/topology.c
> @@ -1388,12 +1388,12 @@ static int avs_route_load(struct snd_soc_component *comp, int index,
>  		port = __ffs(mach->mach_params.i2s_link_mask);
>  
>  		snprintf(buf, len, route->source, port);
> -		strncpy((char *)route->source, buf, len);
> +		strscpy((char *)route->source, buf, len);
>  		snprintf(buf, len, route->sink, port);
> -		strncpy((char *)route->sink, buf, len);
> +		strscpy((char *)route->sink, buf, len);
>  		if (route->control) {
>  			snprintf(buf, len, route->control, port);
> -			strncpy((char *)route->control, buf, len);
> +			strscpy((char *)route->control, buf, len);
>  		}
>  	}
>  
> 
> ---
> base-commit: 0b4a9fdc9317440a71d4d4c264a5650bf4a90f3c
> change-id: 20230725-sound-soc-intel-avs-remove-deprecated-strncpy-2bc41a5a5f81
> 
> Best regards,
> -- 
> Justin Stitt <justinstitt@...gle.com>
> 

-- 
Kees Cook