Re: [RFC] [PATCH] drop_pagecache syscall

[Andrea Righi]

On Wed, Apr 27, 2011 at 11:25:47AM -0400, Mike Frysinger wrote:
> On Wed, Apr 27, 2011 at 05:57, Andrea Righi wrote:
> > On Wed, Apr 27, 2011 at 05:50:04AM -0400, Mike Frysinger wrote:
> >> On Wed, Apr 27, 2011 at 05:47, Andrea Righi wrote:
> >> > On Wed, Apr 27, 2011 at 05:10:41AM -0400, Mike Frysinger wrote:
> >> >> On Wed, Apr 27, 2011 at 05:01, Andrea Righi wrote:
> >> >> > On Wed, Apr 27, 2011 at 10:14:53AM +1000, Dave Chinner wrote:
> >> >> >> On Tue, Apr 26, 2011 at 11:35:27PM +0200, Andrea Righi wrote:
> >> >> >> > This functionality can be used by all the applications that want to have a
> >> >> >> > better control over the page cache management (for example to immediately drop
> >> >> >> > pages that for sure will not be reused in the near future, without calling
> >> >> >> > posix_fadvise() for all the files they've touched), or to provide a more fine
> >> >> >> > grained debugging feature usable by the filesystem benchmarks.
> >> >> >> >
> >> >> >> > The system call does not require root privileges and it can be called by any
> >> >> >> > unprivileged application. For example, we can write a userspace tool to run
> >> >> >> > something like this:
> >> >> >> >
> >> >> >> >   $ drop-pagecache /path/file_or_dir
> >> >> >>
> >> >> >> That's a potential DOS vector, I think. Drop the pagecache in a hard
> >> >> >> loop on the root fs of a busy server and watch it crawl...
> >> >> >
> >> >> > Yes, probably we could allow only the CAP_SYS_ADMIN tasks to execute
> >> >> > this syscall.
> >> >>
> >> >> if /proc/sys/vm/drop_caches has any checks other than file permission
> >> >> checks (i.e. UID==0), it'd probably be better to copy those rather
> >> >> than picking something different.
> >> >
> >> > ok, what about checking current_euid() == 0?
> >>
> >> that's not what i meant.  if the drop_caches file already has certain
> >> cap checks/whatever in place, let's use those.  if it doesnt, then
> >> picking a cap level as you proposed makes sense.
> >
> > mmh, drop_caches has a file ownership (root:root) and a permission mask
> > (0644), how to apply the same checks to a system call? The most similar
> > thing seems to check the current euid. Am I missing something?
> 
> my (limited) understanding is that you should be using cap checks, not UID
> -mike

Agreed. This was my initial proposal. It's not very good, but I also
think it's the best option for this case.

Thanks,
-Andrea
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