Re: [PATCH v5 8/8] x86/crash: Add x86 crash hotplug support for kexec_load

[Eric DeVolder]

On 3/31/22 06:10, Baoquan He wrote:
> On 03/03/22 at 11:27am, Eric DeVolder wrote:
> > For kexec_file_load support, the loading of the crash kernel occurs
> > entirely within the kernel, and as such the elfcorehdr is readily
> > identified (so that it can be modified upon hotplug events).
> >
> > This change enables support for kexec_load by identifying the
> > elfcorehdr segment in the arch_crash_hotplug_handler(), if it has
> > not already been identified.
> >
> > In general, support for kexec_load requires corresponding changes
> > to the userspace kexec-tools utility. It is the responsibility of
> > the userspace kexec utility to ensure that:
> >   - the elfcorehdr segment is sufficiently large enough to accommodate
> >     hotplug changes, ala CRASH_HOTPLUG_ELFCOREHDR_SZ.
> >   - provides a purgatory that excludes the elfcorehdr from its list of
> >     run-time segments to check.
> > These changes to the userspace kexec utility are available, but not
> > yet accepted upstream.
>
> These backgroud information should be put in cover letter, or in patch
> where they won't be grabbed into the final commit log.
Removed.

> > Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
> > ---
> Here, if you want to put some status or backgroud information and expect
> they are not got into log, this is a appropriate place.
>
>
> >   arch/x86/kernel/crash.c | 26 ++++++++++++++++++++++++++
> >   1 file changed, 26 insertions(+)
> >
> > diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> > index 5da30e2bc780..45cc6e3af63e 100644
> > --- a/arch/x86/kernel/crash.c
> > +++ b/arch/x86/kernel/crash.c
> > @@ -486,6 +486,32 @@ void arch_crash_hotplug_handler(struct kimage *image,
> >   	void *elfbuf = NULL;
> >   	unsigned long mem, memsz;
> >   
> > +	/*
> > +	 * When the struct kimage is alloced, it is wiped to zero, so
> > +	 * the elf_index_valid defaults to false. It is set on the
> > +	 * kexec_file_load path, or here for kexec_load.
>
>           I would make the last line as:
> 	 kexec_file_load path, or here for kexec_load if not already
>           identified..
done

> > +	 */
> > +	if (!image->elf_index_valid) {
> > +		unsigned int n;
> > +
> > +		for (n = 0; n < image->nr_segments; n++) {
> > +			mem = image->segment[n].mem;
> > +			memsz = image->segment[n].memsz;
> > +			ptr = map_crash_pages(mem, memsz);
> > +			if (ptr) {
> > +				/* The segment containing elfcorehdr */
> > +				if ((ptr[0] == 0x7F) &&
> > +					(ptr[1] == 'E') &&
> > +					(ptr[2] == 'L') &&
> > +					(ptr[3] == 'F')) {
>
> Can it be like memcmp(ptr, ELFMAG, SELFMAG) as we have done in
> parse_crash_elf_headers()? With that, one line is taken.
done

> > +					image->elf_index = (int)n;
> > +					image->elf_index_valid = true;
> > +				}
> > +			}
> > +			unmap_crash_pages((void **)&ptr);
> > +		}
> > +	}
> > +
> >   	/* Must have valid elfcorehdr index */
> >   	if (!image->elf_index_valid) {
> >   		pr_err("crash hp: unable to locate elfcorehdr segment");
> > --
> > 2.27.0