[CRIU] [PATCH] ptrace: Skip GETREGS on exits from syscalls when possible

[Andrew Vagin]

On Tue, Sep 16, 2014 at 01:26:15PM +0400, Andrew Vagin wrote:
> On Tue, Sep 16, 2014 at 12:27:07AM +0400, Pavel Emelyanov wrote:
> > From: Pavel Emelyanov <xemul at parallels.com>
> > 
> > The PTRACE_SYSCALL traps task twice -- first on enter into
> > and then on exit from syscall. If we trace a single task (and
> > we do it on dump two times per task) we may skip half of all
> > getregs calls -- on exit we don't need them.
> 
> Acked-by: Andrew Vagin <avagin at parallels.com>
> 
> Can we use breakpoints to avoid step-by-step tracing?

No, We can't. If sigreturn restarts syscalls, we doesn't have a place
for breakpoint.

Sorry for talking with myself.

> > 
> > Signed-off-by: Pavel Emelyanov <xemul at parallels.com>
> > ---
> >  parasite-syscall.c | 22 +++++++++++++++++++++-
> >  1 file changed, 21 insertions(+), 1 deletion(-)
> > 
> > diff --git a/parasite-syscall.c b/parasite-syscall.c
> > index 6f56617..266ef0b 100644
> > --- a/parasite-syscall.c
> > +++ b/parasite-syscall.c
> > @@ -886,6 +886,10 @@ static int parasite_fini_seized(struct parasite_ctl *ctl)
> >  	return 0;
> >  }
> >  
> > +#define TRACE_ALL	1
> > +#define TRACE_ENTER	2
> > +#define TRACE_EXIT	3
> > +
> >  /*
> >   * Trap tasks on the exit from the specified syscall
> >   *
> > @@ -897,6 +901,13 @@ int parasite_stop_on_syscall(int tasks, const int sys_nr)
> >  	user_regs_struct_t regs;
> >  	int status, ret;
> >  	pid_t pid;
> > +	/*
> > +	 * The PTRACE_SYSCALL will trap task twice -- on
> > +	 * enter into and on exit from syscall. If we trace
> > +	 * a single task, we may skip half of all getregs
> > +	 * calls -- on exit we don't need them.
> > +	 */
> > +	int trace = (tasks == 1 ? TRACE_ENTER : TRACE_ALL);
> >  
> >  	/* Stop all threads on the enter point in sys_rt_sigreturn */
> >  	while (tasks) {
> > @@ -916,6 +927,15 @@ int parasite_stop_on_syscall(int tasks, const int sys_nr)
> >  			pr_err("%d\n", status);
> >  			return -1;
> >  		}
> > +
> > +		if (trace == TRACE_EXIT) {
> > +			trace = TRACE_ENTER;
> > +			pr_debug("`- Expecting exit\n");
> > +			goto goon;
> > +		}
> > +		if (trace == TRACE_ENTER)
> > +			trace = TRACE_EXIT;
> > +
> >  		ret = ptrace_get_regs(pid, &regs);
> >  		if (ret) {
> >  			pr_perror("ptrace");
> > @@ -949,7 +969,7 @@ int parasite_stop_on_syscall(int tasks, const int sys_nr)
> >  			tasks--;
> >  			continue;
> >  		}
> > -
> > +goon:
> >  		ret = ptrace(PTRACE_SYSCALL, pid, NULL, NULL);
> >  		if (ret) {
> >  			pr_perror("ptrace");
> > -- 
> > 1.8.4.2
> > 
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