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

From: Kees Cook
Date: Sat Feb 12 2022 - 12:50:38 EST


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@xxxxxx> 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