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
>
