Messages in this thread | | | Date | Tue, 19 Sep 2023 20:18:48 +0200 | Subject | Re: [PATCH] bcachefs: Avoid a potential useless over memory allocation in bch2_prt_vprintf() | From | Christophe JAILLET <> |
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Le 19/09/2023 à 15:17, Brian Foster a écrit : > 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?
An off-by-two in fact, IIUC. But yes, that's my point.
We consider that the string is truncated when it may not be (len+1 vs len) and we under-estimate the available space (printbuf_remaining() vs printbuf_remaining()+1 or printbuf_remaining_size())
> >> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr> >> --- >> 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?
Sure, it would be much better. I had not seen this function.
CJ
> > 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 >> > >
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