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    SubjectRe: [PATCH v3 6/8] x86: Split syscall_trace_enter into two phases
    On Mon, Jul 28, 2014 at 10:37 AM, Oleg Nesterov <oleg@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.

    >
    > On 07/21, Andy Lutomirski wrote:
    >>
    >> +long syscall_trace_enter_phase2(struct pt_regs *regs, u32 arch,
    >> + unsigned long phase1_result)
    >> {
    >> long ret = 0;
    >> + u32 work = ACCESS_ONCE(current_thread_info()->flags) &
    >> + _TIF_WORK_SYSCALL_ENTRY;
    >> +
    >> + BUG_ON(regs != task_pt_regs(current));
    >>
    >> user_exit();
    >>
    >> @@ -1458,17 +1562,20 @@ long syscall_trace_enter(struct pt_regs *regs)
    >> * do_debug() and we need to set it again to restore the user
    >> * state. If we entered on the slow path, TF was already set.
    >> */
    >> - if (test_thread_flag(TIF_SINGLESTEP))
    >> + if (work & _TIF_SINGLESTEP)
    >> regs->flags |= X86_EFLAGS_TF;
    >
    > This looks suspicious, but perhaps I misread this change.
    >
    > 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. I'll update the code to call user_exit iff TIF_NOHZ is
    set. If that's still wrong, then I don't see how the current code is
    correct either.

    >
    > 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, and phase2 will set TF.

    I admit I don't really understand all the TF machinations.

    --Andy


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