On 12/03, Linus Torvalds wrote:
>
> On Tue, Dec 3, 2013 at 10:49 AM, Oleg Nesterov <o...@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.
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