[PATCH v3 6/8] x86: Split syscall_trace_enter into two phases

Andy Lutomirski luto at amacapital.net
Tue Jul 29 10:01:47 PDT 2014

On Tue, Jul 29, 2014 at 9:54 AM, Oleg Nesterov <oleg at redhat.com> wrote:
> On 07/28, Andy Lutomirski wrote:
>> On Mon, Jul 28, 2014 at 10:37 AM, Oleg Nesterov <oleg at redhat.com> wrote:
>> > 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!
>> > 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.
>> 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.


>> > 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

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