Re: [CRIU] [PATCH 3/3] signalfd: add ability to read siginfo-s without dequeuing signals (v2)

On Mon, Feb 11, 2013 at 10:29:50AM +0100, Denys Vlasenko wrote:

On Friday 08 February 2013 21:15, Michael Kerrisk (man-pages) wrote:

quoted

quoted

quoted

quoted

Damn. But after I wrote this email I realized that llseek() probably can't
work. Because peek_offset/f_pos/whatever has to be shared with all processes
which have this file opened.

Yes. but I thought you decided to ignore this oddity ;)

quoted

So I want to suggest a way how to forbid read() for SIGNALFD_PEEK.
file->f_pos can be initialized to -1. read() returns EINVAL in this
case. In a man page we will write that signals can be dumped only with
help pread(). Is it overload or too ugly?

Well. I do not know. Up to you and Michael.

But honestly, I can't say this all looks really nice. And why do we
need SIGNALFD_PEEK then?

It surely is no beauty. The hope is at least to make it less ugly than it was.

quoted

Seriously, perhaps we should simply add signalfd_fops->ioctl() for PEEK.
Or add PTRACE_{PEEK,POKE}_SIGNAL which looks even logical and useful...
And much simpler/straightforward.

But I am not going to argue.

ptrace interface might find some use in debuggers.
Not that any of them expressed such desires so far,
but just maybe.

However, it needs coding in C to read it,
which brings me to:

quoted

I suppose I had wondered along similar lines, but in a slightly
different direction: would the use of a /proc interface to get the
queued signals make some sense?

I think that /proc interface beats adding magic flags and magic semantic
to [p]read.

It also has the benefit of being human-readable. You don't need
to write a special C program to "cat /proc/$$/foo".

Andrey, I know that it is hard to let go of the code you invested time
and efforts in creating. But this isn't the last patch, is it?
You will need to retrieve yet more data for process checkpointing.
When you start working on the next patch for it, consider trying
/proc approach.

I don't think that we need to convert siginfo into a human readable format
in kernel. For example siginfo with a negative si_code contains binary data.
A kernel already has got signalfd_siginfo and we can see, that it's not
good.
I think would be better to print siginfo in a human readable format from
userspace (e.g. gdb) and implement a generic interface in kernel (e.g.
ptrace).

quoted

-- 

vda
_______________________________________________
CRIU mailing list
CRIU@openvz.org
https://lists.openvz.org/mailman/listinfo/criu