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Date:   Thu, 20 Oct 2022 11:57:12 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        mm-commits@vger.kernel.org, masahiroy@kernel.org,
        gregkh@linuxfoundation.org, andriy.shevchenko@linux.intel.com,
        Stephen Rothwell <sfr@canb.auug.org.au>
Subject: Re: [PATCH -mm] -funsigned-char, x86: make struct
 p4_event_bind::cntr signed array
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On Thu, Oct 20, 2022 at 10:42:25AM -0700, Linus Torvalds wrote:
> On Thu, Oct 20, 2022 at 10:33 AM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> >
> > Or sometimes with objdump, I've had more success by keeping debug
> > symbols, and then trimming offsets from jmps.
> 
> objdump is what I'm using, and it actually seems ok on individual object files.
> 
> Now I just need to script the "do all the object files" and see how
> massive the end result is.

For the a/b build, I start with all*config, then:

# Stop painful noise
CONFIG_KCOV=n
CONFIG_GCOV_KERNEL=n
CONFIG_GCC_PLUGINS=n
CONFIG_IKHEADERS=n
CONFIG_KASAN=n
CONFIG_UBSAN=n
CONFIG_KCSAN=n
CONFIG_KMSAN=n
# Get us source/line details
CONFIG_DEBUG_KERNEL=y
CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y
CONFIG_DEBUG_INFO_REDUCED=n CONFIG_DEBUG_INFO_COMPRESSED=n
CONFIG_DEBUG_INFO_SPLIT=n

And to keep other build-time junk stabilized[1], I build with these make
options:

KBUILD_BUILD_TIMESTAMP=1970-01-01
KBUILD_BUILD_USER=user
KBUILD_BUILD_HOST=host
KBUILD_BUILD_VERSION=1

For the code diff, I use:

objdump --disassemble --demangle --no-show-raw-insn --no-addresses

and when doing the manual examination:

objdump --disassemble --demangle --reloc --source -l --no-show-raw-insn


My not-great way to filter out the movsbl/movzbl, I added this to diff:
	-I '\bmov[sz]bl\b'


-- 
Kees Cook