linux-kernel - Re: [PATCH v2 5/5] crypto: x86/crc32c - Tweak jump table to validate objtool logic

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Message-ID: <CAMj1kXF_-wSM-rmxS5C_cG-srdUgzKQsSGxp54pZ4pMhXeNsfg@mail.gmail.com>
Date: Fri, 11 Oct 2024 08:32:33 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, 
	keescook@...omium.org, linux-hardening@...r.kernel.org, nathan@...nel.org, 
	Peter Zijlstra <peterz@...radead.org>, Jan Beulich <jbeulich@...e.com>, 
	"Jose E. Marchesi" <jemarch@....org>, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v2 5/5] crypto: x86/crc32c - Tweak jump table to validate
 objtool logic

On Thu, 10 Oct 2024 at 22:34, Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> On Thu, Oct 10, 2024 at 02:28:07PM +0200, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@...nel.org>
> >
> > Tweak the jump table so
> > - the address is taken far way from its use
> > - its offset from the start of .rodata is != 0x0
> > - its type is STT_OBJECT and its size is set to the size of the actual
> >   table
> > - the indirect jump is annotated with a R_X86_64_NONE relocation
> >   pointing to the jump table
> >
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
>
> This needs more "why", I assume the goals are to add the annotations +
> confuse objtool if it doesn't read them properly?
>

As presented, this is just a vehicle to test the other changes in the
series. That is why I split it off from the previous one.

Whether or not we want this code in the tree is up for debate, but I
guess it could be useful as a canary for objtool, given that most
configs now disable jump tables entirely.