Re: [PATCH v8 7/7] x86/crash: Add x86 crash hotplug support for kexec_load

[Eric DeVolder]

On 5/25/22 09:26, Sourabh Jain wrote:
> Hello Eric,
>
> On 06/05/22 00:16, 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_handle_hotplug_event(),
> > if it has not already been identified.
> >
> > Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
> > Acked-by: Baoquan He <bhe@xxxxxxxxxx>
> > ---
> >   arch/x86/kernel/crash.c | 24 ++++++++++++++++++++++++
> >   1 file changed, 24 insertions(+)
> >
> > diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> > index 951ef365f0a7..845d7c77854d 100644
> > --- a/arch/x86/kernel/crash.c
> > +++ b/arch/x86/kernel/crash.c
> > @@ -485,6 +485,30 @@ void arch_crash_handle_hotplug_event(struct kimage *image,
> >       void *elfbuf = NULL;
> >       unsigned long mem, memsz;
> > +    /*
> > +     * When the struct kimage is alloced, it is wiped to zero, so
> > +     * the elfcorehdr_index_valid defaults to false. It is set on the
> > +     * kexec_file_load path, or here for kexec_load, if not already
> > +     * identified.
> > +     */
> > +    if (!image->elfcorehdr_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 (memcmp(ptr, ELFMAG, SELFMAG) == 0) {
> > +                    image->elfcorehdr_index = (int)n;
> > +                    image->elfcorehdr_index_valid = true;
>
> How about finding elfcorehdr index on kexec_load path post kimage_load_segment call in
> do_kexec_load (kernel/kexec.c) or other suitable place? This way we can avoid checking for
> elfcorehdr index for every hotplug. Also we might not need image->elfcorehdr_index_valid.

That would be a viable place to put it. However, a couple of notes.

This code actually works for both kexec_load and kexec_file_load paths (how I originally used this 
code); so if we were to move this to a more common location, I'd be in favor of a location that 
serves both paths (ie handle_hotplug_event() makes sense to me). That would eliminate the need for 
setting the index in the kexec_file_load paths.

Also, I don't see a scenario where elfcorehdr_index_valid can be eliminated; the index isn't always 
valid. What are you thinking?

And the check is rather low cost being the check of a boolean.

eric
> Thanks,
> Sourabh Jain