Re: [PATCH?] uprobes: change uprobe_write_opcode() to modify the page directly

Jon Medhurst (Tixy)
2013-12-05T08:29:08Z

On Wed, 2013-12-04 at 09:15 -0800, Linus Torvalds wrote:
> On Wed, Dec 4, 2013 at 8:54 AM, H. Peter Anvin <hpa@zytor.com> wrote:
> >
> > That is why I talk about the atomic instruction word... most (but not
> > *all*) architectures have a fundamental minimum unit of instructions
> > which is aligned and can be atomically written. Typically this is 1, 2,
> > or 4 bytes.
>
> Note that it's not just about the "atomically written", it's also
> about the guarantee that it's atomically *read*.
>
> x86 can certainly atomically write a 4-byte instruction too, it's just
> that there's no guarantee - even if the instruction is aligned etc -
> that the actual instruction decoding always ends up reading it that
> way. It might re-read an instruction after encountering a prefix byte
> etc etc. So even if it's all properly aligned, the reading side might
> do something odd.

The ARM architecture has similar issues. Even though the instruction
size is mostly fixed, the architecture specification itself only
guarantees a very tiny subset of instructions are safe to modify whilst
there may be concurrent execution of that instruction. I'm quoting a
discussion from a while ago: http://lkml.org/lkml/2012/12/10/346

--
Tixy