Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged

[Andy Lutomirski]

> On Jan 15, 2019, at 9:00 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
>> On Wed, Jan 16, 2019 at 12:42 PM Josh Snyder <joshs@xxxxxxxxxxx> wrote:
>> 
>> For Netflix, losing accurate information from the mincore syscall would
>> lengthen database cluster maintenance operations from days to months.  We
>> rely on cross-process mincore to migrate the contents of a page cache from
>> machine to machine, and across reboots.
> 
> Ok, this is the kind of feedback we need, and means I guess we can't
> just use the mapping existence for mincore.
> 
> The two other ways that we considered were:
> 
> (a) owner of the file gets to know cache information for that file.
> 
> (b) having the fd opened *writably* gets you cache residency information.
> 
> Sadly, taking a look at happycache, you open the file read-only, so
> (b) doesn't work.
> 
> Judging just from the source code, I can't tell how the user ownership
> works. Any input on that?
> 
> And if you're not the owner of the file, do you have another
> suggestion for that "Yes, I have the right to see what's in-core for
> this file". Because the problem is literally that if it's some random
> read-only system file, the kernel shouldn't leak access patterns to
> it..
> 
> 


Something like CAP_DAC_READ_SEARCH might not be crazy.