On Sun, Feb 18, 2024 at 05:12:28PM +0100, Christophe JAILLET wrote: > 2 issues related to incorrect available space in the output buffer > computation may lead to some extra memory allocation. > > > First: vsnprintf() takes the size of the buffer, *including* the space for > the trailing null. > > But printbuf_remaining() returns the number of characters we can print > to the output buffer, *excluding* the terminating null. > > So, use printbuf_remaining_size(), which includes the space for the > terminating null. nope, buggy. passing printbuf_remaining_size() to snprintf() is correct, but snprintf() returns the number of charecters wrote _excluding_ the null > > > Second: if the return value of vsnprintf() is greater than or equal to the > passed size, the resulting string is truncated. > So, in order to see if some extra space is needed, the check needs to be > changed. > > Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx> > --- > Un-tested > > v2: - Use printbuf_remaining_size() instead of hand-writing it. [Brian Foster] > - Reword the commit log, hoping it is clearer. > - Synch with -next-20240215 > > v1: https://lore.kernel.org/all/0f40108bed3e084057223bdbe32c4b37f8500ff3.1694845203.git.christophe.jaillet@xxxxxxxxxx/ > --- > 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 b27d22925929..8cee9c2f88c4 100644 > --- a/fs/bcachefs/printbuf.c > +++ b/fs/bcachefs/printbuf.c > @@ -55,9 +55,10 @@ 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); > + len = vsnprintf(out->buf + out->pos, printbuf_remaining_size(out), > + fmt, args2); > va_end(args2); > - } while (len + 1 >= printbuf_remaining(out) && > + } while (len >= printbuf_remaining_size(out) && > !bch2_printbuf_make_room(out, len + 1)); > > len = min_t(size_t, len, > @@ -72,9 +73,10 @@ 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_size(out), > + fmt, args); > va_end(args); > - } while (len + 1 >= printbuf_remaining(out) && > + } while (len >= printbuf_remaining_size(out) && > !bch2_printbuf_make_room(out, len + 1)); > > len = min_t(size_t, len, > -- > 2.43.2 >
