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

[Christophe JAILLET]

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@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?

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