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From:   john.hubbard@gmail.com
To:     "H . Peter Anvin" <hpa@zytor.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, LKML <linux-kernel@vger.kernel.org>,
        John Hubbard <jhubbard@nvidia.com>
Subject: [PATCH 0/1] x86/boot: clear some fields explicitly
Date:   Wed, 24 Jul 2019 16:15:27 -0700
Message-Id: <20190724231528.32381-1-jhubbard@nvidia.com>
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From: John Hubbard <jhubbard@nvidia.com>

Hi,

I just ran across this on the latest (well, any -rc1) linux.git, while
working on something else. It generates an ugly multiline warning:

In file included from arch/x86/kernel/head64.c:35:
In function ?sanitize_boot_params?,
    inlined from ?copy_bootdata? at arch/x86/kernel/head64.c:391:2:
./arch/x86/include/asm/bootparam_utils.h:40:3: warning:
?memset? offset [197, 448] from the object at ?boot_params? is out
of the bounds of referenced subobject ?ext_ramdisk_image? with type
?unsigned int? at offset 192 [-Warray-bounds]
   40 |   memset(&boot_params->ext_ramdisk_image, 0,
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   41 |          (char *)&boot_params->efi_info -
      |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   42 |    (char *)&boot_params->ext_ramdisk_image);
      |    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./arch/x86/include/asm/bootparam_utils.h:43:3: warning: ?memset?
offset [493, 497] from the object at ?boot_params? is out of the
bounds of referenced subobject ?kbd_status? with type
?unsigned char? at offset 491 [-Warray-bounds]
   43 |   memset(&boot_params->kbd_status, 0,
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   44 |          (char *)&boot_params->hdr -
      |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   45 |          (char *)&boot_params->kbd_status);
      |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It looks like this hasn't been touched since 2013, so maybe gcc is
just getting more agressive about warnings. Well, we know it is, actually.

Because struct boot_params is __packed__, normal variable
variable assignment will work just as well as a memset here.
Change three u32 fields to be cleared to zero that way, and
just memset the _pad4 field.

This clears up the build warnings for me.

Testing: just did a basic boot test on my x86_64 machine, no problems
seen. Not that that tests much, but it's something.

John Hubbard (1):
  x86/boot: clear some fields explicitly

 arch/x86/include/asm/bootparam_utils.h | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

-- 
2.22.0