On Mon, Apr 18, 2022 at 10:10:42AM -0700, Linus Torvalds wrote:
> Ugh. If you do this, you need to have a big comment about how that
> %rcx value gets fixed up with EX_TYPE_UCOPY_LEN (which basically ends
> up doing "%rcx = %rcx*8+%rax" in ex_handler_ucopy_len() for the
> exception case).
Yap, and I reused your text and expanded it. You made me look at that
crazy DEFINE_EXTABLE_TYPE_REG macro finally so that I know what it does
in detail.
So I have the below now, it boots in the guest so it must be perfect.
---
/*
* Default clear user-space.
* Input:
* rdi destination
* rcx count
*
* Output:
* rcx uncopied bytes or 0 if successful.
*/
SYM_FUNC_START(clear_user_original)
mov %rcx,%rax
shr $3,%rcx # qwords
and $7,%rax # rest bytes
test %rcx,%rcx
jz 1f
# do the qwords first
.p2align 4
0: movq $0,(%rdi)
lea 8(%rdi),%rdi
dec %rcx
jnz 0b
1: test %rax,%rax
jz 3f
# now do the rest bytes
2: movb $0,(%rdi)
inc %rdi
decl %eax
jnz 2b
3:
xorl %eax,%eax
RET
_ASM_EXTABLE_UA(0b, 3b)
/*
* The %rcx value gets fixed up with EX_TYPE_UCOPY_LEN (which basically ends
* up doing "%rcx = %rcx*8 + %rax" in ex_handler_ucopy_len() for the exception
* case). That is, we use %rax above at label 2: for simpler asm but the number
* of uncleared bytes will land in %rcx, as expected by the caller.
*
* %rax at label 3: still needs to be cleared in the exception case because this
* is called from inline asm and the compiler expects %rax to be zero when exiting
* the inline asm, in case it might reuse it somewhere.
*/
_ASM_EXTABLE_TYPE_REG(2b, 3b, EX_TYPE_UCOPY_LEN8, %rax)
SYM_FUNC_END(clear_user_original)
EXPORT_SYMBOL(clear_user_original)
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
