linux-kernel - Re: [PATCH v3 6/8] crypto: x86/crc32c - Use idiomatic relative jump table

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Message-ID: <CAMj1kXERhCiKMqCr7wCPitQCcPNj7uuEJsgC9jGxSdBWxAwqdg@mail.gmail.com>
Date: Mon, 14 Oct 2024 11:36:10 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Eric Biggers <ebiggers@...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, 
	Josh Poimboeuf <jpoimboe@...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 v3 6/8] crypto: x86/crc32c - Use idiomatic relative jump table

On Mon, 14 Oct 2024 at 06:28, Eric Biggers <ebiggers@...nel.org> wrote:
>
> On Fri, Oct 11, 2024 at 07:08:54PM +0200, Ard Biesheuvel wrote:
> > diff --git a/arch/x86/crypto/crc32c-pcl-intel-asm_64.S b/arch/x86/crypto/crc32c-pcl-intel-asm_64.S
> > index bbcff1fb78cb..45b005935194 100644
> > --- a/arch/x86/crypto/crc32c-pcl-intel-asm_64.S
> > +++ b/arch/x86/crypto/crc32c-pcl-intel-asm_64.S
> > @@ -53,7 +53,7 @@
> >  .endm
> >
> >  .macro JMPTBL_ENTRY i
> > -.quad .Lcrc_\i
> > +.long .Lcrc_\i - jump_table
> >  .endm
> >
> >  .macro JNC_LESS_THAN j
> > @@ -169,7 +169,8 @@ SYM_FUNC_START(crc_pcl)
> >
> >       ## branch into array
> >       leaq    jump_table(%rip), %bufp
> > -     mov     (%bufp,%rax,8), %bufp
> > +     movslq  (%bufp,%rax,4), len
> > +     addq    len, %bufp
> >       JMP_NOSPEC bufp
>
> I think it would be much better to just get rid of the jump table here.
>

Good point.

> I have done this at
> https://lore.kernel.org/linux-crypto/20241014042447.50197-4-ebiggers@kernel.org/
>

I'll go and take a look - thanks.