linux-kernel - Re: [PATCH 22/22] lkdtm: Obfuscate do

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Message-ID: <202503251238.EE695D3@keescook>
Date: Tue, 25 Mar 2025 12:39:00 -0700
From: Kees Cook <kees@...nel.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Miroslav Benes <mbenes@...e.cz>,
	Brendan Jackman <jackmanb@...gle.com>,
	Nathan Chancellor <nathan@...nel.org>,
	kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 22/22] lkdtm: Obfuscate do_nothing() pointer

On Mon, Mar 24, 2025 at 02:56:12PM -0700, Josh Poimboeuf wrote:
> If execute_location()'s memcpy of do_nothing() gets inlined and unrolled
> by the compiler, it copies one word at a time:
> 
>     mov    0x0(%rip),%rax    R_X86_64_PC32    .text+0x1374
>     mov    %rax,0x38(%rbx)
>     mov    0x0(%rip),%rax    R_X86_64_PC32    .text+0x136c
>     mov    %rax,0x30(%rbx)
>     ...
> 
> Those .text references point to the middle of the function, causing
> objtool to complain about their lack of ENDBR.
> 
> Prevent that by resolving the function pointer at runtime rather than
> build time.  This fixes the following warning:
> 
>   drivers/misc/lkdtm/lkdtm.o: warning: objtool: execute_location+0x23: relocation to !ENDBR: .text+0x1378
> 
> Cc: Kees Cook <kees@...nel.org>
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202503191453.uFfxQy5R-lkp@intel.com/
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>

Thanks!

Reviewed-by: Kees Cook <kees@...nel.org>

-- 
Kees Cook