[Devel] Re: build breaks when checkpoint unimplemented by arch

[Nathan Lynch]

Oren Laadan <orenl at cs.columbia.edu> writes:
> On Tue, 7 Jul 2009, Nathan Lynch wrote:
>
>> Oren Laadan <orenl at cs.columbia.edu> writes:
>> > That's what I tried initially, but the problem is that sigset_t may
>> > be defined differently for userspace - see /usr/include/asm/sigset_t.h.
>> > In fact, for x86_32, it it is different, defined as 'unsigned long' 
>> > (and NSIG defined as 32, so only 32 bits).
>> 
>> I noticed this, but I figured only the kernel definition was salient.
>> Apart from debugging checkpoint/restart, why would userspace need the
>> definition of struct ckpt_hdr_sigset?
>
> I expect user space tools to at least:
>
> - Assist in debugging c/r
>
> - Assist users in reporting problems with c/r (especially since they
>   themselves do not debug or hack)
>
> - Convert checkpoint images from one kernel version to another
>
> - Provide information about a checkpoint image, and even allow its
>   manipulation.  This can assist developers in debugging their programs
>   (e.g. to debug a crash you need to run a program for 30 minutes so it 
>   ets up its state; instead of repeatedly running it, you run it once, 
>   checkpoint, and then debug from a restarted version. A tool could 
>   allow you to peek/poke inside the checkpoint and even modify data in 
>   it).
>
> - Or a tool that converts a checkpoint image to a core dump so it
>   can be inspected with gdb.
>
> I'm pretty sure others will find other uses to it...

But I asked specifically about ckpt_hdr_sigset.


>> For that matter, why would userspace need the definitions of most of the
>> structures in checkpoint_hdr.h?  (Again, debugging purposes don't count:
>> ckptinfo or similar developer utilities can be included with the
>> kernel.)
>
> Keeping the checkpoint header format understandable by user space (and 
> immune to 32-64 variations) has been a requirement since day 1.

I guess I wasn't around that day.  It seems backwards to expose the
format of every checkpoint record in the ABI regardless of whether
plausible use cases exist.  Linux has a well-established pattern of
introducing interfaces without sufficient testing or documentation[1],
and I expect C/R will adhere to tradition.  Making the ABI obese in the
hope of anticipating every conceivable use will just provide more
opportunities to screw up.

[1] http://userweb.kernel.org/~mtk/papers/lce2007/What_we_lose_without_words.pdf


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