linux-kernel - Re: [PATCH 6/7] arm64: start using 'asm goto' for put

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Date: Tue, 11 Jun 2024 16:29:17 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Peter Anvin <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>, 
	Thomas Gleixner <tglx@...utronix.de>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, 
	"the arch/x86 maintainers" <x86@...nel.org>, linux-arm-kernel@...ts.infradead.org, 
	linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 6/7] arm64: start using 'asm goto' for put_user() when available

On Tue, 11 Jun 2024 at 14:56, Nathan Chancellor <nathan@...nel.org> wrote:
>
> > +#ifdef CONFIG_CC_HAS_ASM_GOTO
>
> This symbol was eliminated two years ago with commit a0a12c3ed057 ("asm
> goto: eradicate CC_HAS_ASM_GOTO") since all supported compilers have
> support for it.

Hah. And I was trying to be a good boy and keep the old setup working.

Instead - because the HAS_ASM_GOTO config variable no longer exists -
I didn't actually test the new case at all, and it only worked because
the old case did in fact work.

Because fixing the broken #ifdef also showed that the

  +       _ASM_EXTABLE_##type##ACCESS_ZERO(1b, %l2)

line was wrong and was a copy-and-paste error from the get_user case
(that zeroes the result register on error).

It should be just

  +       _ASM_EXTABLE_##type##ACCESS(1b, %l2)

and to make the nasty copy_to_kernel_nofault_loop() build I also need
to do the proper _ASM_EXTABLE_KACCESS macro without the zeroing that
didn't exist.

Oops.

It would be nice to get rid of the CC_HAS_ASM_GOTO_OUTPUT thing too,
but that's probably a decade away ;(

But at least  this made me now go and actually test the _actual_ old
compiler case (no asm goto output). Perhaps ironically, I did get
*that* one right. That's the case where I had actually checked the new
code for get_user().

             Linus