Re: [git pull] uaccess-related bits of vfs.git

Al Viro
2017-05-14T18:57:38Z

On Sun, May 14, 2017 at 08:13:56PM +0200, Ingo Molnar wrote:

> I'd say that the CLAC/STAC addition pretty much killed any argument in favor of
> "optimized" __get_user() code, so I'd be very happy to see these interfaces gone
> altogether.

You and everybody else - these interfaces suck. If anything, we want paired
brackets around a series of accesses instead of a single check in front of it.

> So as far as x86 usage goes:
>
> Acked-by: Ingo Molnar <mingo@kernel.org>

Umm... Could you elaborate the situation with xen/page.h stuff? I don't
see any obvious reasons that would guaratee that addresses passed to
__get_user() and __put_user() there would match the set_fs() state.

It might very well be true, but it's not obvious from that code...

BTW, does anybody have a suggestion regarding a test load that would hit
wait4/waitid as hard as possible? I've turned sys_wait4/sys_waitid into
long kernel_wait4(pid_t upid, int *stat_addr, int options, struct rusage *ru)
and
static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop,
int options, struct rusage *ru)
(with struct waitid_info {
pid_t pid;
uid_t uid;
int status;
int why;
};), so that all copying to userland is done in sys_wait4() and friends.
It seems to survive testing without any noticable slowdowns, but that's
just LTP and xfstests - and a bug in my earlier version of that was _not_
caught by the LTP side; xfstests caught it... So any extra tests (both
for correctness and timing) would be very much appreciated...