On Mon, Feb 19, 2024 at 07:10:10PM +0100, Christophe JAILLET wrote: > Le 19/02/2024 à 04:39, Kent Overstreet a écrit : > > 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 > > Hi, > > I think that the patch is correct. You didn't test it, I did. In the future, no more patches that you haven't tested - thanks. > snprintf() returns the number of characters wrote _excluding_ the null. That > is why the test is: len >= size_of_the_buffer ==> more space is needed. > The case you describe is handled by the == part of >=. > > On the contrary, if len is < size_of_the_buffer, then we have at least 1 > place for the terminating NULL. It will then eventually lead to: > len_of_the_string + space_for_'\0' == size_of_the_buffer > So it does not overflow. > > > Anyway, feel free to ignore the patch completely if it sounds too risky, > take only half of it (s/printbuf_remaining/printbuf_remaining_size/) if you > are more confident with it, or the complete patch if the explanation above > convinced you. > > From my PoV, the 3 options lead to the same run-time output, with more or > less memory allocated. > > Best regards. > > CJ > > > > > > > > > > > > > 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 > > > > > > > >
