linux-kernel - Re: linux-next 20220125 - build failure in objtool with gcc 12

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Date:   Sat, 12 Feb 2022 09:50:28 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Justin Forbes <jmforbes@...uxtx.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Valdis Klētnieks <valdis.kletnieks@...edu>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: linux-next 20220125 - build failure in objtool with gcc 12

On Thu, Jan 27, 2022 at 09:34:34AM -0600, Justin Forbes wrote:
> On Wed, Jan 26, 2022 at 4:57 PM Valdis Klētnieks
> <valdis.kletnieks@...edu> wrote:
> >
> > Fedora Rawhide shipped gcc12, which apparently includes a new warning that
> > causes a build failure.  Apparently, it's unable to figure out that 'ptr' remains
> > valid on failed realloc(), and we only call realloc() again on failures...
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104069 explains (toward
> the end) why gcc is leaving it to be fixed in the kernel.

Yeah, I agree -- it can trigger a free(). I think the solution should
be:

diff --git a/tools/lib/subcmd/subcmd-util.h b/tools/lib/subcmd/subcmd-util.h
index 794a375dad36..de392fc5fd3a 100644
--- a/tools/lib/subcmd/subcmd-util.h
+++ b/tools/lib/subcmd/subcmd-util.h
@@ -49,15 +49,24 @@ static NORETURN inline void die(const char *err, ...)
 
 static inline void *xrealloc(void *ptr, size_t size)
 {
-	void *ret = realloc(ptr, size);
-	if (!ret && !size)
-		ret = realloc(ptr, 1);
+	void *ret;
+
+	/*
+	 * Convert a zero-sized allocation into 1 byte, since
+	 * realloc(ptr, 0) means free(ptr), but we don't want
+	 * to release the memory. For a new allocation (when
+	 * ptr == NULL), avoid triggering NULL-checking error
+	 * conditions for zero-sized allocations.
+	 */
+	if (!size)
+		size = 1;
+	ret = realloc(ptr, size);
 	if (!ret) {
-		ret = realloc(ptr, size);
-		if (!ret && !size)
-			ret = realloc(ptr, 1);
-		if (!ret)
-			die("Out of memory, realloc failed");
+		/*
+		 * If realloc() fails, the original block is left untouched;
+		 * it is not freed or moved.
+		 */
+		die("Out of memory, realloc failed");
 	}
 	return ret;
 }

> 
> 
> >   CC      /usr/src/linux-next/tools/objtool/exec-cmd.o
> >   CC      /usr/src/linux-next/tools/objtool/help.o
> > In file included from help.c:12:
> > In function 'xrealloc',
> >     inlined from 'add_cmdname' at help.c:24:2:
> > subcmd-util.h:56:23: error: pointer may be used after 'realloc' [-Werror=use-after-free]
> >    56 |                 ret = realloc(ptr, size);
> >       |                       ^~~~~~~~~~~~~~~~~~
> > subcmd-util.h:52:21: note: call to 'realloc' here
> >    52 |         void *ret = realloc(ptr, size);
> >       |                     ^~~~~~~~~~~~~~~~~~
> > subcmd-util.h:58:31: error: pointer may be used after 'realloc' [-Werror=use-after-free]
> >    58 |                         ret = realloc(ptr, 1);
> >       |                               ^~~~~~~~~~~~~~~
> > subcmd-util.h:52:21: note: call to 'realloc' here
> >    52 |         void *ret = realloc(ptr, size);
> >       |                     ^~~~~~~~~~~~~~~~~~
> > cc1: all warnings being treated as errors
> > make[4]: *** [/usr/src/linux-next/tools/build/Makefile.build:97: /usr/src/linux-next/tools/objtool/help.o] Error 1
> > make[3]: *** [Makefile:59: /usr/src/linux-next/tools/objtool/libsubcmd-in.o] Error 2
> > make[2]: *** [Makefile:63: /usr/src/linux-next/tools/objtool/libsubcmd.a] Error 2
> > make[1]: *** [Makefile:69: objtool] Error 2
> > make: *** [Makefile:1405: tools/objtool] Error 2

-- 
Kees Cook