Re: [PATCH -mm -v2] x86 boot : export boot_params via sysfs

["H. Peter Anvin" <hpa@zytor.com>]

Greg KH wrote:
> On Wed, Dec 12, 2007 at 11:05:45AM -0800, H. Peter Anvin wrote:
> > Greg KH wrote:
> > > > This is a binary structure defined by protocol;
> > > What protocol?  Is this a "standard" documented somewhere?
> > Yes, see Documentation/i386/* (although some of it is documented by 
> > reference to include/asm-x86/boot_params.h).
>
> Ah, so the structure has changed over the years, making this not so much
> a "firmware" field as originally thought :)

The structure has been *amended* to over the years.  That's why the 
format is so awkward...

> > > > in that way it's not significantly different from something passed
> > > > from the firmware (in fact, it might very well *be* passed from the
> > > > firmware.)  We have in the past found platform bugs by looking at the
> > > > contents of the whole structure, e.g.  to find that part of it has
> > > > been inappropriately clobbered.
> > > For debugging things, then just export it through debugfs.
> > Fair enough, however...
> >
> > > > It is also in the form needed by e.g. kexec to operate.
> > > Does kexec need this today to work properly?  Or is this something new?
> > I believe kexec currently tries to reconstitute it from what data is 
> > available to it.  This is incomplete, though, and has been flagged as a 
> > problem for kexec.
>
> Can't kexec get this from within the kernel itself when it is running?
> It doesn't need to get this from userspace, does it?

It probably could, but it probably needs to modify some fields.

> > > What userspace program is going to know the exact data format of this
> > > blob, and where is it going to know that format from?  The kernel header
> > > files in sanitized form?  Or something else?
> > It can pick it up from <asm/boot_params.h> (which is now userspace-safe); 
> > or it can decode it itself.  Programs like kexec can pass through most of 
> > the data without examining it, this is the main reason for having it as a 
> > blob.
>
> I just don't want kernel structures exported in binary fashions in
> sysfs.  If there are specific portions that kexec currently can't figure
> out, why not just export those fields?
>
> And, by exporting the different fields (yeah, lots of files, no big
> deal), you can handle the change in the structure over time much easier
> than trying to "know" the layout of the binary blob.

But if you kexec an older kernel from a newer kernel, the potential 
additional information will be lost for a subsequent uprevision kexec, 
for example.

	-hpa

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