Re: [PATCH 6/7] arm64: start using 'asm goto' for put_user() when available

[Linus Torvalds]

On Tue, 11 Jun 2024 at 14:56, Nathan Chancellor <nathan@xxxxxxxxxx> 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