Re: [PATCH] bcachefs: Avoid a potential useless over memory allocation in bch2_prt_vprintf()

[Brian Foster]

On Sat, Sep 16, 2023 at 08:20:24AM +0200, Christophe JAILLET wrote:
> printbuf_remaining() returns the number of characters we can print to the
> output buffer - i.e. excluding the terminating null.
> 
> vsnprintf() takes the size of the buffer, including the trailing null
> space.
> It is truncated if the returned value is greater than or equal to the size
> of the buffer.
> 
> Knowing all that, buffer sizes and overflow checks can be fixed in order
> to potentially avoid a useless memory over-allocation.
> 

For whatever reason I had a hard time parsing this last sentence.  Do
you mean to say there's an off by one here that leads to an unnecessary
overallocation?

> Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
> ---
> Un-tested
> ---
>  fs/bcachefs/printbuf.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/bcachefs/printbuf.c b/fs/bcachefs/printbuf.c
> index de41f9a14492..77bee9060bfe 100644
> --- a/fs/bcachefs/printbuf.c
> +++ b/fs/bcachefs/printbuf.c
> @@ -54,8 +54,9 @@ void bch2_prt_vprintf(struct printbuf *out, const char *fmt, va_list args)
>  		va_list args2;
>  
>  		va_copy(args2, args);
> -		len = vsnprintf(out->buf + out->pos, printbuf_remaining(out), fmt, args2);
> -	} while (len + 1 >= printbuf_remaining(out) &&
> +		len = vsnprintf(out->buf + out->pos, printbuf_remaining(out) + 1,
> +				fmt, args2);
> +	} while (len >= printbuf_remaining(out) + 1 &&
>  		 !bch2_printbuf_make_room(out, len + 1));

It's amazing how simple arithmetic can make my eyes cross at times. :) I
think I follow the fix after reading the commit log a couple times, but
could we use printbuf_remaining_size() appropriately in these places
that want to check actual buffer size (i.e. including terminating null)
instead of doing the manual size fixup?

Brian

>  
>  	len = min_t(size_t, len,
> @@ -70,9 +71,10 @@ void bch2_prt_printf(struct printbuf *out, const char *fmt, ...)
>  
>  	do {
>  		va_start(args, fmt);
> -		len = vsnprintf(out->buf + out->pos, printbuf_remaining(out), fmt, args);
> +		len = vsnprintf(out->buf + out->pos, printbuf_remaining(out) + 1,
> +				fmt, args);
>  		va_end(args);
> -	} while (len + 1 >= printbuf_remaining(out) &&
> +	} while (len >= printbuf_remaining(out) + 1 &&
>  		 !bch2_printbuf_make_room(out, len + 1));
>  
>  	len = min_t(size_t, len,
> -- 
> 2.34.1
>