linux-kernel - Re: [kees:for-next/lkdtm 6/6] drivers/misc/lkdtm/cfi.c:62:37: warning: cast from pointer to integer of different size

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Date:   Tue, 26 Apr 2022 15:03:33 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Dan Li <ashimida@...ux.alibaba.com>, kbuild-all@...ts.01.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kernel test robot <lkp@...el.com>
Subject: Re: [kees:for-next/lkdtm 6/6] drivers/misc/lkdtm/cfi.c:62:37:
 warning: cast from pointer to integer of different size

Hi Kees,

On Tue, Apr 19, 2022 at 9:20 PM Kees Cook <keescook@...omium.org> wrote:
> On Sun, Apr 17, 2022 at 07:26:33AM +0800, kernel test robot wrote:
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git for-next/lkdtm
> > head:   2e53b877dc1258d4ac3de98f496bb88ec3bf5e25
> > commit: 2e53b877dc1258d4ac3de98f496bb88ec3bf5e25 [6/6] lkdtm: Add CFI_BACKWARD to test ROP mitigations
> > config: sparc-randconfig-r026-20220417 (https://download.01.org/0day-ci/archive/20220417/202204170710.LwCV5Ec0-lkp@intel.com/config)

Just received a failure for m68k-allmodconfig on next-20220426:

drivers/misc/lkdtm/cfi.c:62:37: error: cast from pointer to integer of
different size [-Werror=pointer-to-int-cast]
drivers/misc/lkdtm/cfi.c:62:10: error: cast to pointer from integer of
different size [-Werror=int-to-pointer-cast]
drivers/misc/lkdtm/cfi.c:62:37: error: cast from pointer to integer of
different size [-Werror=pointer-to-int-cast]
drivers/misc/lkdtm/cfi.c:62:10: error: cast to pointer from integer of
different size [-Werror=int-to-pointer-cast]

> > compiler: sparc-linux-gcc (GCC) 11.2.0
> > reproduce (this is a W=1 build):
> >         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> >         chmod +x ~/bin/make.cross
> >         # https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?id=2e53b877dc1258d4ac3de98f496bb88ec3bf5e25
> >         git remote add kees https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git
> >         git fetch --no-tags kees for-next/lkdtm
> >         git checkout 2e53b877dc1258d4ac3de98f496bb88ec3bf5e25
> >         # save the config file to linux build tree
> >         mkdir build_dir
> >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-11.2.0 make.cross O=build_dir ARCH=sparc SHELL=/bin/bash drivers/misc/lkdtm/
> >
> > If you fix the issue, kindly add following tag as appropriate
> > Reported-by: kernel test robot <lkp@...el.com>
> >
> > All warnings (new ones prefixed by >>):
> >
> >    drivers/misc/lkdtm/cfi.c: In function 'set_return_addr_unchecked':
> > >> drivers/misc/lkdtm/cfi.c:62:37: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
> >       62 |         ((__force __typeof__(addr))((__force u64)(addr) | PAGE_OFFSET))
> >          |                                     ^
> >    drivers/misc/lkdtm/cfi.c:72:13: note: in expansion of macro 'no_pac_addr'
> >       72 |         if (no_pac_addr(*ret_addr) == expected)
> >          |             ^~~~~~~~~~~
>
> Hmm. I don't see why this is warning. The macro looks like untagged_addr():
>
> #define untagged_addr(addr)     ({ \
>         u64 __addr = (__force u64)(addr); \
>         __addr &= __untagged_addr(__addr); \
>         (__force __typeof__(addr))__addr; \
> })

The offending macro is:

#define no_pac_addr(addr)      \
          ((__force __typeof__(addr))((__force u64)(addr) | PAGE_OFFSET))

On 32-bit, addr is a 32-bit pointer, which is cast to a 64-bit integer first,
then to a 32-bit pointer again.
All of these need intermediate casts to uintptr_t.

oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds