On Tue, Jul 29, 2014 at 9:54 AM, Oleg Nesterov [off-list ref] wrote:

On 07/28, Andy Lutomirski wrote:

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On Mon, Jul 28, 2014 at 10:37 AM, Oleg Nesterov [off-list ref] wrote:

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Hi Andy,

I am really sorry for delay.

This is on top of the recent change from Kees, right? Could me remind me
where can I found the tree this series based on? So that I could actually
apply these changes...

https://git.kernel.org/cgit/linux/kernel/git/kees/linux.git/log/?h=seccomp/fastpath

The first four patches are already applied there.

Thanks!

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If I understand correctly, syscall_trace_enter() can avoid _phase2() above.
But we should always call user_exit() unconditionally?

Damnit.  I read that every function called by user_exit, and none of
them give any indication of why they're needed for traced syscalls but
not for untraced syscalls.  On a second look, it seems that TIF_NOHZ
controls it.

Yes, just to trigger the slow path, I guess.

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I'll update the code to call user_exit iff TIF_NOHZ is
set.

Or perhaps it would be better to not add another user of this (strange) flag
and just call user_exit() unconditionally(). But, yes, you need to use
from "work = flags & (_TIF_WORK_SYSCALL_ENTRY & ~TIF_NOHZ)" then.\

user_exit looks slow enough to me that a branch to try to avoid it may
be worthwhile.  I bet that explicitly checking the flag is
actually both faster and clearer.  That's what I did for v4.

--Andy

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And we should always set X86_EFLAGS_TF if TIF_SINGLESTEP? IIRC, TF can be
actually cleared on a 32bit kernel if we step over sysenter insn?

I don't follow.  If TIF_SINGLESTEP, then phase1 will return a nonzero
value,

Ah yes, thanks, I missed this.

Oleg.

-- 
Andy Lutomirski
AMA Capital Management, LLC