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

Linus Torvalds
2013-12-04T17:15:10Z

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.

Linus