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SubjectRe: load_unaligned_zeropad() on x86-64
On Sat, Nov 14, 2020 at 7:53 AM David Laight <David.Laight@aculab.com> wrote:
>
> The change e419b4cc585680940bc42f8ca8a071d6023fb1bb added
> asm code for load_unaligned_zeropad().
>
> However it doesn't look right for 64bit.
> It masks the address with ~3 not ~7 so the second
> access could still cross a page boundary and fault.

Can you explain more what you think is wrong?

It uses

"and %3,%1\n\t"

for the masking, but note how that's a "%3", not a "$3".

And %3 is this asm argument

"i" (-sizeof(unsigned long)),

which is -4 or -8 (which is the same as ~3 or ~7).

The other masking is to get the byte offset within the unsigned long,
to do the shifting. Again, that uses '%4', which is

"i" (sizeof(unsigned long)-1));

so 3 or 7.

So on my build, the code expands to

1: mov (%rsi),%rdx # MEM[(long unsigned int *)ct_58], ret
2:
.section .fixup,"ax"
3: lea (%rsi),%rcx # MEM[(long unsigned int *)ct_58], dummy
and $-8,%rcx #, dummy
mov (%rcx),%rdx # dummy, ret
leal (%rsi),%ecx # MEM[(long unsigned int *)ct_58]
andl $7,%ecx #
shll $3,%ecx
shr %cl,%rdx # ret
jmp 2b
.previous

which looks ok to me.

It's possible that it's buggy (that page crossing basically never
happens - only with PAGEALLOC debugging, and even then only in some
really odd and unlikely situations). So it gets basically zero test
coverage, which is never a good thing. But if it's buggy, it's not
obvious to me, and I don't see any ~3 issue.

Linus

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