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

[Andrew Vagin]

On Tue, Sep 16, 2014 at 08:56:38PM +0400, Pavel Emelyanov wrote:
> On 09/16/2014 03:46 PM, Andrew Vagin wrote:
> > 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.
> 
> How to treat the "[PATCH 3/3] x86: use breakpoints instead of tracing syscalls"
> sent 3 hours (and 15 minutes) after this?

I explain there, that sigreturn() doesn't restart system calls. They are
restarted by other code, which adjusts eip to call __NR_restart_syscall.
Look at get_task_regs() for more details.

> 
> > Sorry for talking with myself.
> 
> You're always welcome!
> 
> >>>
> >>> 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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> > 
>