linux-kernel - Re: [2.6.27.24] Kernel coredump to a pipe is failing

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Message-Id: <1243372156.7369.119.camel@homebase.localnet>
Date:	Tue, 26 May 2009 17:09:16 -0400
From:	Paul Smith <paul@...-scientist.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [2.6.27.24] Kernel coredump to a pipe is failing

On Tue, 2009-05-26 at 22:31 +0200, Andi Kleen wrote:
> Paul Smith <paul@...-scientist.net> writes:
> > Well, -512 is ERESTARTSYS.  That, to me, seems like a reasonable error
> > code to get when we're trying to dump core to a pipe.  Yes?  No?
> 
> Which signal is it? SIGPIPE?

I'm not sure; I'll have to dig in a little further.  I'm not sure
offhand how to determine which signal it was from inside the kernel but
it shouldn't be hard to find.

> >
> > Shouldn't we be doing some kind of error handling here, at least for
> > basic things like signals?  Should a process that's dumping core be set
> > to ignore signals?  Should dump_write() try again on ERESTARTSYS?
> 
> I think it should block signals. Here's a untested patch.
> 
> It has the disadvantage that it reports the incorrect blocked mask
> in the ELF corefile, but that's probably better than truncated 
> coredumps.

As a quick test I changed dump_write() to retry on ERESTARTSYS after
disabling the pending signal, like:

        static int dump_write(struct file *file, const void *addr, int nr)
        {
                while (1) {
                        int r = file->f_op->write(file, addr, nr, &file->f_pos);
                        if (r != -ERESTARTSYS)
                                return r == nr;
        
                        /* We don't handle signals while dumping core. */
                        clear_thread_flag(TIF_SIGPENDING);
                }
        }

I don't know if this is right, but in some quick tests I ran it did
work: my cores were full size.  I haven't finished testing (and I have
to go to soccer practice right now).

This obviously doesn't reset the signal mask in the dumping process, but
it makes the dump_write() more complex and it may cause other issues so
I can't say whether this is the way to go.

> -
> Block signals during core dump

Cool, I'll test this one as well.

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