Re: [PATCH] lib/string: replace BUG_ON with WARN_ON_ONCE in strlcat

[Greg KH]

On Mon, Mar 16, 2026 at 04:17:28PM +0000, Josh Law wrote:
> BUG_ON() in a library function is too harsh -- it panics the kernel
> when a caller passes a dest string whose length already meets or
> exceeds count. This is a caller bug, not a reason to bring down the
> entire system.
> 
> Replace with WARN_ON_ONCE() and a safe early return. The return value
> of count signals truncation to the caller, consistent with strlcat
> semantics (return >= count means the output was truncated).
> 
> This follows the guidance in include/asm-generic/bug.h which
> explicitly discourages BUG_ON: "Don't use BUG() or BUG_ON() unless
> there's really no way out."
> 
> Signed-off-by: Josh Law <objecting@xxxxxxxxxxxxx>
> ---
>  lib/string.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/string.c b/lib/string.c
> index b632c71df1a5..ae3eb192534d 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -255,8 +255,9 @@ size_t strlcat(char *dest, const char *src, size_t count)
>  	size_t len = strlen(src);
>  	size_t res = dsize + len;
>  
> -	/* This would be a bug */
> -	BUG_ON(dsize >= count);
> +	/* This would be a caller bug */
> +	if (WARN_ON_ONCE(dsize >= count))
> +		return count;

You still just crashed the system for the few billion or so Linux
systems out there with panic-on-warn enabled :(

And is returning 'count' really the correct thing to do here when you
didn't actually copy any data?

thanks,

greg k-h