Messages in this thread | | | Date | Thu, 10 Apr 2014 16:18:06 +0200 | From | Oleg Nesterov <> | Subject | Re: [RFC PATCH 4/6] uprobes/x86: Emulate rip-relative call's |
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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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