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

[Pavel Emelyanov]

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?

> 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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