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Date: Sun, 16 Sep 2018 14:34:47 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Cc: Felix Janda <felix.janda@...teo.de>
Subject: Re: [PATCH] Fix off-by-one buffer overflow in getdelim
On Sun, Sep 16, 2018 at 02:25:42PM -0400, 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
>
> This patch raised a potential conformance issue, that by a strict
> reading of the spec, getdelim is only permitted to realloc if the
> caller-provided buffer length is insufficient:
>
> "If *lineptr is a null pointer or if the object pointed to by
> *lineptr is of insufficient size, an object shall be allocated as
> if by malloc() or the object shall be reallocated as if by
> realloc(), respectively, ..."
>
> I'm going to change the +1 to +!z and add a comment. The idea is that
> the +1 was only needed in order for the result to fit if the delimiter
> has not already been found; if the memchr found it, an exact-sized
> buffer was being expanded unnecessarily.
>
> I'm replying to this thread and CC'ing in case there are any problems
> I'm missing in my new fix.
This fix actually looks insufficient; it doesn't fix the case where
the getc produces EOF rather than a character.
Rich
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