Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap causes application-level breakage

From: Mark Williamson
Date: Thu Apr 30 2015 - 07:50:13 EST

On Wed, Apr 29, 2015 at 10:02 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Apr 29, 2015 at 1:44 PM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
>> This's no longer true. After recent fixes for "anon_vma endless growing" new vma
>> might reuse old anon_vma from grandparent vma.
> Oh well. I guess that was too simple.
> If Mark is ok with the rule that "it's not reliably if you have two
> nested forks" (ie it only works if you exec for every fork you do), it
> should still work, right? It sounds like Mark doesn't necessarily need
> to handle the *generic* case.

Yes, it sounds like that should be OK for us. Our usecase is pretty
restricted, so we're a long way off requiring a generic solution.

Our code will always fork() a fresh child in which to monitor memory
changes. We run the operations we're interested in, use pagemap to
figure out "what changed" (by comparing whether the pagemap_entry_t
values are different from their parent) and then throw away the child

Currently our code does an entry-by-entry compare of pagemap, so
anything that exposes writes as a change to values in there would
allow us to run unmodified. That would be really nice. That said, I
think we'd still be OK to modify our own code too if we can find a
solution that would continue to function on older kernel releases,
-stable trees, etc.

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