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

[Oleg Nesterov <oleg@redhat.com>]

On 12/03, Linus Torvalds wrote:
>
> On Tue, Dec 3, 2013 at 10:49 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > See the patch below. For review only
>
> Looks completely broken. Where do you guarantee that it's just a single page?

Yes, it is always a single page on all supported architectures.

This is even documented. I believe that "NOTE:" comment above
uprobe_write_opcode() tries to say this but I guess this comment
should be cleanuped.

And note also

	/* uprobe_write_opcode() assumes we don't cross page boundary */
	BUG_ON((uprobe->offset & ~PAGE_MASK) +
			UPROBE_SWBP_INSN_SIZE > PAGE_SIZE);

in prepare_uprobe().

> Yes, on x86, UPROBE_SWBP_INSN_SIZE is a single byte.

And powerpc checks addr & 3 to ensure it doesn't cross the page.

> 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.

Well, put_user() obviously can't work, mm != current->mm.
So we need get_user_pages() at least.

> No need to
> invalidate caches, even, because if you overwrite the first byte of an
> instruction, it all "just works".

I can't comment this, I do not know how the hardware actually works.

> Either the instruction decoding gets
> the old one, or it gets the new one.

Funny that.

I have asked why access_process_vm() can't work when I saw the initial
version of uprobes patches. I was told this can't work (even on x86).

And I was told that this idiotic games were suggested by someone
named Linus Torvalds ;)

> And on non-x86, UPROBE_SWBP_INSN_SIZE is not necessarily 1, so it
> could cross a page boundary.

Yes. If we support such an architecture we should obviously update
uprobe_write_opcode() accordingly.

Oleg.