Re: [PATCH] btrfs: Replace memcpy + NUL termination in _btrfs_printk

[david laight <david.laight@runbox.com>]

On Sun, 30 Nov 2025 01:55:17 +0100
Thorsten Blum <thorsten.blum@linux.dev> wrote:

> Use strscpy() to copy the NUL-terminated source string 'fmt' to the
> destination buffer 'lvl' instead of using memcpy() followed by a manual
> NUL termination.  No functional changes.

Why?
The code has just got the length of part of the format string, it wants
a copy of it with a '\0' terminator.
So memcpy() is correct and strscpy() just expensive.
The code is actually strange (and strangely written), but 'size' is always 2.

One might question why btrfs has to invent its own 'printk' scheme...

But PRINTK_MAX_SINGLE_HEADER_LEN is only used here - so take out the 'MAX_'
and use it inside printk_skip_level'.
Then here use the constant PRINTK_SINGLE_HEADER_LEN instead of 'size'.
Since lvl[] is initialised it will all be zero, so after the
mempy(lvl, fmt, 2) (which will be inlined) there is no need to the
lvl[2] = 0 at all.

	David

> 
> Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
> ---
>  fs/btrfs/messages.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/messages.c b/fs/btrfs/messages.c
> index a0cf8effe008..083e228e6d6c 100644
> --- a/fs/btrfs/messages.c
> +++ b/fs/btrfs/messages.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>  
> +#include <linux/string.h>
>  #include "fs.h"
>  #include "messages.h"
>  #include "discard.h"
> @@ -229,8 +230,7 @@ void __cold _btrfs_printk(const struct btrfs_fs_info *fs_info, const char *fmt,
>  		size_t size = printk_skip_level(fmt) - fmt;
>  
>  		if (kern_level >= '0' && kern_level <= '7') {
> -			memcpy(lvl, fmt,  size);
> -			lvl[size] = '\0';
> +			strscpy(lvl, fmt, size + 1);
>  			type = logtypes[kern_level - '0'];
>  			ratelimit = &printk_limits[kern_level - '0'];
>  		}