linux-kernel - Re: [PATCH] ASoC: Intel: avs: Replace snprintf() with scnprintf()

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Message-ID: <20251112135028.18921229@pumpkin>
Date: Wed, 12 Nov 2025 13:50:28 +0000
From: David Laight <david.laight.linux@...il.com>
To: hariconscious@...il.com
Cc: cezary.rojewski@...el.com, liam.r.girdwood@...ux.intel.com,
 peter.ujfalusi@...ux.intel.com, yung-chuan.liao@...ux.intel.com,
 ranjani.sridharan@...ux.intel.com, kai.vehmanen@...ux.intel.com,
 pierre-louis.bossart@...ux.dev, broonie@...nel.org, perex@...ex.cz,
 tiwai@...e.com, amadeuszx.slawinski@...ux.intel.com,
 sakari.ailus@...ux.intel.com, khalid@...nel.org, shuah@...nel.org,
 david.hunter.linux@...il.com, linux-sound@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ASoC: Intel: avs: Replace snprintf() with scnprintf()

On Wed, 12 Nov 2025 17:32:35 +0530
hariconscious@...il.com wrote:

> From: HariKrishna Sagala <hariconscious@...il.com>
> 
> As per the C99 standard snprintf() returns the length of the data
> that *would have been* written if there were enough space for it.
> It's generally considered safer to use the scnprintf() variant.

Did you actually read the code?
In this case the code is actually rather buggy and can read beyond
the end of 'buf[]'.

Neither snprintf() nor scnprintf() ever return -1 on error.
So the existing code will attempt to write past the end of buf[]
if there are a lot of entries.
Fortunately there is a test for negative lengths - so nothing is
written past the end - but the read can return data off the end.

Changing to scnprintf() stops this happening, but the user will get
truncated data.

	David


> 
> Link: https://github.com/KSPP/linux/issues/105
> Signed-off-by: HariKrishna Sagala <hariconscious@...il.com>
> ---
> This patch replaces snprintf() varaint with scnprintf() in
> scenario to know the actual length of the data rather than *would
> have been* written data of snprintf().
> No functional changes intended.
> Reference Links:
> https://lwn.net/Articles/69419/
> https://www.kernel.org/doc/html/latest/core-api/kernel-api.html#c.snprintf
> 
> Note:
> Compile & boot tested with necessary config parameters.
> Other areas of AVS uses scnprintf() variant.
> 
>  sound/soc/intel/avs/debugfs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/sound/soc/intel/avs/debugfs.c b/sound/soc/intel/avs/debugfs.c
> index 3534de46f9e4..100b95bfcd78 100644
> --- a/sound/soc/intel/avs/debugfs.c
> +++ b/sound/soc/intel/avs/debugfs.c
> @@ -119,7 +119,7 @@ static ssize_t probe_points_read(struct file *file, char __user *to, size_t coun
>  	}
>  
>  	for (i = 0; i < num_desc; i++) {
> -		ret = snprintf(buf + len, PAGE_SIZE - len,
> +		ret = scnprintf(buf + len, PAGE_SIZE - len,
>  			       "Id: %#010x  Purpose: %d  Node id: %#x\n",
>  			       desc[i].id.value, desc[i].purpose, desc[i].node_id.val);
>  		if (ret < 0)
> 
> base-commit: 24172e0d79900908cf5ebf366600616d29c9b417