linux-kernel - Re: [PATCH 0/1] uprobes: Kill __replace_page(), change uprobe_write

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Date:	Tue, 10 Dec 2013 11:38:00 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	Borislav Petkov <bp@...en8.de>,
	Hugh Dickins <hughd@...gle.com>,
	Ingo Molnar <mingo@...nel.org>, Jiri Kosina <jkosina@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/1] uprobes: Kill __replace_page(), change
 uprobe_write_opcode() to rely on gup(WRITE)

On Tue, Dec 10, 2013 at 11:18 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>
> Well, if we use get_user_pages() we have vma for free.

No we don't. get_user_pages() is expensive as hell.

We'd be *much* better off using get_user_pages_fast() if possible -
and I bet _is_ possible in 99% of all cases.

So saying that we get vma "for free" is complete BS. We get it "for
very expensive."

> But I am not sure about "and verify it for now" above. Do you mean
> that uprobe_write_opcode() should do another valid_vma() ?

No. I think we should just do something like

    if (vma->vm_flags & (VM_SHARED | VM_HUGETLB))
        return -EINVAL;

and actually make the logic obvious. We should *also* make sure we do
the right thing for THP, because the VM_HUGETLB check isn't obviously
sufficient. Does the code really do the right thing for a writable
hugepage? I'd like that to be *obvious*.

Quite frankly, I think that adding a few of these kinds of *obvious*
checks and then just looking up the page table _once_ - instead of
three times - would make the code safer and faster.

As it is, it is neither safe nor fast. It *may* be safe if callers do
all the right black magic, but it's by no means obvious.

                   Linus
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