linux-kernel - Re: [PATCH 10/11] kexec: Support for loading ELF x86

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Message-ID: <20140307183959.GE5255@pd.tnic>
Date:	Fri, 7 Mar 2014 19:39:59 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
	ebiederm@...ssion.com, hpa@...or.com, mjg59@...f.ucam.org,
	greg@...ah.com, jkosina@...e.cz
Subject: Re: [PATCH 10/11] kexec: Support for loading ELF x86_64 images

On Fri, Mar 07, 2014 at 06:12:33PM +0100, Borislav Petkov wrote:
> > > > +/* Fill in fields which are usually present in bzImage */
> > > > +static int init_linux_parameters(struct boot_params *params)
> > > > +{
> > > > +	/*
> > > > +	 * FIXME: It is odd that the information which comes from kernel
> > > > +	 * has to be faked by loading kernel. I guess it is limitation of
> > > > +	 * ELF format. Right now keeping it same as kexec-tools
> > > > +	 * implementation. But this most likely needs fixing.
> > > > +	 */
> > > > +	memcpy(&params->hdr.header, "HdrS", 4);
> > > > +	params->hdr.version = 0x0206;
> > > > +	params->hdr.initrd_addr_max = 0x37FFFFFF;
> > > > +	params->hdr.cmdline_size = 2048;
> > > > +	return 0;
> > > > +}

...

> Well, this is mandated by the boot protocol, no?
> 
> "If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
> the boot protocol version is "old".  Loading an old kernel, the
> following parameters should be assumed:
> 
>         Image type = zImage
>         initrd not supported
>         Real-mode kernel must be located at 0x90000."
> 
> About version 0x0206:
> 
> Field name:     cmdline_size
> Type:           read
> Offset/size:    0x238/4
> Protocol:       2.06+
> 
>   The maximum size of the command line without the terminating
>   zero. This means that the command line can contain at most
>   cmdline_size characters. With protocol version 2.05 and earlier, the
>   maximum size was 255.
> 
> So according to the protocol, cmdline_size should be set by
> kexec_file_load and not hardcoded to 2K, if we're mandating protocol
> version 2.06.

Btw, all those fields above are of type "read", which means,

"For each field, some are information from the kernel to the bootloader
("read"),... "

Now, if we assume that the first kernel is the bootloader and it loads
the second kernel, then the first kernel shouldn't be setting all those
fields *at* *all* because it violates the boot protocol. And that's a
no-no.

Generally, how important is that use case for loading ELF images? If no
one is actually using it, why even bother?

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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