From: Linus Torvalds
> Sent: 23 October 2024 21:08
>
> On Wed, 23 Oct 2024 at 12:17, Linus Torvalds
> <torva...@linux-foundation.org> wrote:
> >
> > NOTE! This is obviously untested and I didn't check that it does the
> > cmp/sbb/or the right way around.
>
> Well, it boots. The code generation (from strncpy_from_user()) seems ok:
>
> movabs $0x123456789abcdef,%rcx
> cmp %rsi,%rcx
> sbb %ecx,%ecx
> or %rsi,%rcx
>
> where obviously that constant is the bogus pre-initialized value, not
> the actual runtime value.
Would it be better to make the 'bogus' constant one that makes
all accesses fail?
So you soon find out it any code doesn't get patched.
I also wonder how big the table of addresses to patch is.
If that gets into inlined functions it could be big.
OTOH having a real function that does access_ok(), clac and address
masking may not problem.
Especially if there is always a (PAGE sized) gap between the highest
user address and the lowest kernel address so the 'size' argument
to access_ok() can be ignored on the assumption that the accesses
are (reasonably) linear.
David
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