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

H. Peter Anvin
2013-12-03T19:22:06Z

On 12/03/2013 11:00 AM, Linus Torvalds wrote:
>
> Yes, on x86, UPROBE_SWBP_INSN_SIZE is a single byte. But quite
> frankly, on x86, exactly *because* it's a single byte, I don't
> understand why we don't just write the damn thing with a single
> "put_user()", and stop with all the idiotic games. No need to
> invalidate caches, even, because if you overwrite the first byte of an
> instruction, it all "just works". Either the instruction decoding gets
> the old one, or it gets the new one. We already rely on that for the
> kernel bp instruction replacement.
>
> And on non-x86, UPROBE_SWBP_INSN_SIZE is not necessarily 1, so it
> could cross a page boundary. Yes, many architectures will have
> alignment constraints, but I don't see this testing it.
>
> Whatever. I think that code is bad, and you should feel bad. But hey,
> I think it was pretty bad before too.
>

I guess it would have to be checked, but I would be *highly* surprised
if UPROBE_SWBP_INSN_SIZE ever[1] could be anything than the fundamental
instruction quantum, which means it should never be able to wrap a page,
but *also* should mean it should be able to just be put_user()'d
followed by whatever synchronization necessary to make it globally visible.

-hpa