Re: [PATCH] Fix off-by-one buffer overflow in getdelim

[Felix Janda <felix.janda@posteo.de>]

Rich Felker wrote:
> On Sat, Oct 24, 2015 at 10:43:39PM +0200, Felix Janda wrote:
> > when deciding whether to resize the buffer, the terminating null byte
> > was not taken into account
> > ---
> >  src/stdio/getdelim.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/src/stdio/getdelim.c b/src/stdio/getdelim.c
> > index a88c393..3077490 100644
> > --- a/src/stdio/getdelim.c
> > +++ b/src/stdio/getdelim.c
> > @@ -27,7 +27,7 @@ ssize_t getdelim(char **restrict s, size_t *restrict n, int delim, FILE *restric
> >  	for (;;) {
> >  		z = memchr(f->rpos, delim, f->rend - f->rpos);
> >  		k = z ? z - f->rpos + 1 : f->rend - f->rpos;
> > -		if (i+k >= *n) {
> > +		if (i+k+1 >= *n) {
> >  			if (k >= SIZE_MAX/2-i) goto oom;
> >  			*n = i+k+2;
> >  			if (*n < SIZE_MAX/4) *n *= 2;
> > -- 
> > 2.4.9
> 
> I think you're mistaken. i+k is the space needed so far in the buffer
> (not counting the terminating null byte) and *n is the usable space.
> The equality case of the i+k >= *n conditional covers the need to
> expand the buffer when the new content of length k would exactly fit
> but would not leave room for null termination.
> 
> Just to make sure I wrote a quick test program, which I've attached,
> that should crash in free if the overflow occurs. It does not crash
> and the output demonstrates correct resizing.

Thanks for the test program!

I did not see the 'if (z) break;'. The off-by-one should only occur
when memchr returns 0 but the byte from getc_unlocked is the delimiter.
(This makes it not so easy to observe the bug.)

Felix