Re: [RFC PATCH 4/6] uprobes/x86: Emulate rip-relative call's

[Oleg Nesterov]

On 04/10, Denys Vlasenko wrote:
>
> There is this monstrosity, "16-bit override for branches" in 64-mode:
>
> 66 e8 nn nn       callw   <offset16>
>
> Nobody sane uses it because it truncates instruction pointer.
>
> Or rather, *I think* it should truncate it (i.e. zero-extend to full width),
> but conceivably some CPUs can be buggy wrt that:
> they can decide to modify only lower 16 bits of IP,
> or even they can decided to use signed <offset16> but apply it
> to full-width RIP.
>
> AMD manuals are not clear on what exactly should happen.
>
> I am sure no one sane uses this form of branch instructions
> in 32-bit and 64-bit code.
>
> I don't think we should be trying to support it "correctly"
> (we can just let program crash with SIGILL or something),
> we only need to make sure we don't overlook its existence
> and thus are not tricked into touching or modifying unrelated data.

And after the quick check it seems that lib/insn.c doesn't parse
"66 e8 nn nn" correctly. It treats the next 2 bytes as the part
of 32bit offset.

> Imagine that   66 e8 nn nn    bytes are exactly at the end of
> a page,

this doesn't matter. We always read MAX_UINSN_BYTES bytes, so

> and we wrongly assume that offset is 32-bit, not 16-bit.
> 66 e8 nn nn.

see above this will always happen.

So I think branch_setup_xol_ops() should simply return -ENOSYS in
this case, and let the default_ code execute it out of line. Given
that nobody should use this insn, this is probably not too bad.

We can teach branch_xol_ops to handle this insn correctly later.

Now the question is, how I can detect this insn correctly? I mean,
using the "right" helpers from lib/insn.c ?

Oleg.

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