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Re: [PATCH] lib/autoconf/c.m4: fix NULL pointer dereference in _AC_LANG_

From: Nicolas IOOSS
Subject: Re: [PATCH] lib/autoconf/c.m4: fix NULL pointer dereference in _AC_LANG_IO_PROGRAM
Date: Tue, 29 Jun 2021 09:56:44 +0200

On Mon, Jun 28, 2021 at 7:48 PM Zack Weinberg <> wrote:
> On Mon, Jun 28, 2021 at 10:17 AM Nicolas Iooss <> 
> wrote:
> > * lib/autoconf/c.m4: return an error when f is NULL.
> > ---
> >  lib/autoconf/c.m4 | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
> > index 9f881f0bc10c..0d9efa62d450 100644
> > --- a/lib/autoconf/c.m4
> > +++ b/lib/autoconf/c.m4
> > @@ -114,7 +114,7 @@ $2
> >  m4_define([_AC_LANG_IO_PROGRAM(C)],
> >  [AC_LANG_PROGRAM([@%:@include <stdio.h>],
> >  [FILE *f = fopen ("conftest.out", "w");
> > - return ferror (f) || fclose (f) != 0;
> > + return !f || ferror (f) || fclose (f) != 0;
> >  ])])
> Thank you for the bug report and the patch. The change looks correct
> and is simple enough not to require a copyright assignment. I will
> apply it to Autoconf trunk in the next few days.
> Our test suite does not catch this bug, which probably means it never
> tests AC_LANG_IO_PROGRAM inside AC_RUN_IFELSE. Would you be willing to
> write a test case, perhaps based on the that caused you
> to discover the bug? If you don't have time to write a test yourself,
> could you at least tell us about how you found the bug?

Thanks for your quick reply! Actually I found this bug running clang's
static code analyzer on a project named secp256k1. When using
"scan-build -enable-checker alpha.unix.Stream", this tool spot an
issue while running ./configure and I found out it was a real bug. For
more details, I described on
the exact command line I used. So the file that I used is

I don't know how to reproduce the case of failing fopen without using
a static code analyzer, and I don't have time to create a reproducer
which would be simple enough to be integrated in a test. So I do not
plan to write a test myself.

Best regards,


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