Re: [Xen-devel] Question regarding suspend in libxc

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, May 10, 2013 at 10:05:45AM +0100, soyer yoo wrote:
> Hi all,
> 
> I was reading source code around saving and restoring domains, and I found 
> the following function in _libxl_save_msgs_helper.c, which looks like causing 
> seg fault.
> 
> int helper_stub_suspend(void *user)
> {
>     unsigned char *buf = 0;
>     int len = 0, allocd = 0;
> 
>     for (;;) {
>         uint16_t_put(buf, &len, 3 /* suspend */);
>         if (buf) break;
>         buf = helper_allocbuf(len, user);
>         assert(buf);
>         allocd = len;
>         len = 0;
>     }
>     assert(len == allocd);
>     helper_transmitmsg(buf, len, user);
>     int r = helper_getreply(user);
>     return r;
> }
> 
> Here, buf is null, and inside the for loop it tries to write '3' into buf, 
> which will cause seg fault.
> Is it intended for suspending domain or am I misunderstanding?
> Also, could you tell me what does '3' do for suspending domain.
> 

uint16_t_put eventually calls bytes_put, which skips memcpy if buf is
NULL.

The number just means different phase. You can see other numbers in the
same files as well.

> On more question:
> 
> helper_transmitmsg in the above writes the 'buf' to file descriptor 1 (i.e., 
> /dev/tty1).
> I have idea what is going on here, and could anybody please enlighten me?
> 

The helper program communicates with the caller via pipe. I'm not sure
what you mean by /dev/tty1 though. You should have a look at
libxl_domain_suspend.


Wei.

> Best,
> Soyer

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