[Devel] Re: [RFC][PATCH 1/4] checkpoint-restart: general infrastructure

[Dave Hansen]

On Fri, 2008-08-08 at 11:46 +0200, Arnd Bergmann wrote:
> On Friday 08 August 2008, Dave Hansen wrote:
> > +	hh->magic = 0x00a2d200;
> > +	hh->major = (LINUX_VERSION_CODE >> 16) & 0xff;
> > +	hh->minor = (LINUX_VERSION_CODE >> 8) & 0xff;
> > +	hh->patch = (LINUX_VERSION_CODE) & 0xff;
...
> > +}
> 
> Do you rely on the kernel version in order to determine the format
> of the binary data, or is it just informational?
> 
> If you think the format can change in incompatible ways, you
> probably need something more specific than the version number
> these days, because there are just so many different trees with
> the same numbers.

Yeah, this is very true.  My guess is that we'll need something like
what we do with modversions. 

> > +/* debugging */
> > +#if 0
> > +#define CR_PRINTK(str, args...)  \
> > +	printk(KERN_ERR "cr@%s#%d: " str, __func__, __LINE__, ##args)
> > +#else
> > +#define CR_PRINTK(...)		do {} while (0)
> > +#endif
> > +
> 
> Please use the existing pr_debug and dev_debug here, instead of creating
> yet another version.

Sure thing.  Will do.

> > +struct cr_hdr_tail {
> > +	__u32 magic;
> > +	__u32 cksum[2];
> > +};
> 
> This structure has an odd multiple of 32-bit members, which means
> that if you put it into a larger structure that also contains
> 64-bit members, the larger structure may get different alignment
> on x86-32 and x86-64, which you might want to avoid.
> I can't tell if this is an actual problem here.

Can't we just declare all these things __packed__ and stop worrying
about aligning them all manually?

> > +
> > +/*
> > + * During restart the code reads in data from the chekcpoint image into a
> > + * temporary buffer (ctx->hbuf). Because operations can be nested, one
> > + * should call cr_hbuf_get() to reserve space in the buffer, and then
> > + * cr_hbuf_put() when it no longer needs that space
> > + */
> > +
> > +#include <linux/version.h>
> > +#include <linux/sched.h>
> > +#include <linux/file.h>
> > +
> > +#include "ckpt.h"
> > +#include "ckpt_hdr.h"
> > +
> > +/**
> > + * cr_hbuf_get - reserve space on the hbuf
> > + * @ctx: checkpoint context
> > + * @n: number of bytes to reserve
> > + */
> > +void *cr_hbuf_get(struct cr_ctx *ctx, int n)
> > +{
> > +	void *ptr;
> > +
> > +	BUG_ON(ctx->hpos + n > CR_HBUF_TOTAL);
> > +	ptr = (void *) (((char *) ctx->hbuf) + ctx->hpos);
> > +	ctx->hpos += n;
> > +	return ptr;
> > +}
> 
> Can (ctx->hpos + n > CR_HBUF_TOTAL) be controlled by the input
> data? If so, this is a denial-of-service attack.

Ugh, this is crappy code anyway.  It needs to return an error and have
someone else handle it.

> > +int cr_kwrite(struct cr_ctx *ctx, void *buf, int count)
> > +{
> > +	mm_segment_t oldfs;
> > +	int ret;
> > +
> > +	oldfs = get_fs();
> > +	set_fs(KERNEL_DS);
> > +	ret = cr_uwrite(ctx, buf, count);
> > +	set_fs(oldfs);
> > +
> > +	return ret;
> > +}
> 
> get_fs()/set_fs() always feels a bit ouch, and this way you have
> to use __force to avoid the warnings about __user pointer casts
> in sparse.
> I wonder if you can use splice_read/splice_write to get around
> this problem.

I have to wonder if this is just a symptom of us trying to do this the
wrong way.  We're trying to talk the kernel into writing internal gunk
into a FD.  You're right, it is like a splice where one end of the pipe
is in the kernel.

Any thoughts on a better way to do this?  

> > +	struct cr_ctx *ctx;
> > +	struct file *file;
> > +	int fput_needed;
> > +	int ret;
> > +
> > +	if (!capable(CAP_SYS_ADMIN))
> > +		return -EPERM;
> > +
> 
> Why do you need CAP_SYS_ADMIN for this? Can't regular users
> be allowed to checkpoint/restart their own tasks?

Yes, eventually.  I think one good point is that we should probably
remove this now so that we *have* to think about security implications
as we add each individual patch.  For instance, what kind of checking do
we do when we restore an mlock()'d VMA?

I'll pull this check out so it causes pain. (the good kind)

> > --- linux-2.6.git/Makefile~handle_a_single_task_with_private_memory_maps	2008-08-05 09:04:27.000000000 -0700
> > +++ linux-2.6.git-dave/Makefile	2008-08-05 09:07:53.000000000 -0700
> > @@ -611,7 +611,7 @@ export mod_strip_cmd
> >  
> >  
> >  ifeq ($(KBUILD_EXTMOD),)
> > -core-y		+= kernel/ mm/ fs/ ipc/ security/ crypto/ block/
> > +core-y		+= kernel/ mm/ fs/ ipc/ security/ crypto/ block/ ckpt/
> >  
> 
> The name 'ckpt' is a bit unobvious, how about naming it 'checkpoint' instead?

Fine with me.  Renamed in new patches, hopefully.

I'll send new patches out later today.

-- Dave

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