Re: [PATCH 22/22] lkdtm: Obfuscate do_nothing() pointer

[Kees Cook]

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@xxxxxxxxxx>
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202503191453.uFfxQy5R-lkp@xxxxxxxxx/
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

Thanks!

Reviewed-by: Kees Cook <kees@xxxxxxxxxx>

-- 
Kees Cook