[Crash-utility] [PATCH v3] Add support for kASLR for offline vmcore files

[Dave Anderson]

----- Original Message -----
> On Wed, Jan 22, 2014 at 10:42 AM, Dave Anderson <anderson at redhat.com> wrote:
> >
> >
> > ----- Original Message -----
> >> >> >
> >> >> > Then, my questions are:
> >> >> >
> >> >> >  (1) on a live system, how would a root user determine the offset
> >> >> >  from userspace?
> >> >>
> >> >> AFAICT, it can be calculated from /proc/kallsyms.
> >> >
> >> > Will /proc/kallsyms contain the relocated addresses?  Andy had mentioned
> >> > that
> >> > the offset would be in the dmesg buffer but that can be overwritten.
> >>
> >> Yeah, kallsyms should show the current actual locations. It should
> >> only show up in dmesg on a crash.
> >>
> >> >> >  (2) given a random vmlinux/vmcore pair, how would any user determine
> >> >> >  the offset?
> >> >>
> >> >> It'd be nice for the vmcore to contain offset details.
> >> >
> >> > Right -- Andy mentioned that it would be put in a VMCOREINFO item:
> >> >
> >> >   https://www.redhat.com/archives/crash-utility/2013-October/msg00043.html
> >> >
> >> > But I'm presuming that wasn't part of your patchset.
> >>
> >> It was not, no. What's needed to get that added?
> >
> > Since kASLR is x86 only (right?), I believe it would simply require an
> > addition to "arch/x86/kernel/machine_kexec_64.c" here:
> >
> > void arch_crash_save_vmcoreinfo(void)
> > {
> >         VMCOREINFO_SYMBOL(phys_base);
> >         VMCOREINFO_SYMBOL(init_level4_pgt);
> >
> > #ifdef CONFIG_NUMA
> >         VMCOREINFO_SYMBOL(node_data);
> >         VMCOREINFO_LENGTH(node_data, MAX_NUMNODES);
> > #endif
> > }
> >
> > Since it's the offset value that we're interested in, something
> > like this should suffice:
> >
> >         VMCOREINFO_NUMBER(<name_of_symbol_containing_offset_value>)
> >
> > with an appropriate header inclusion that declares the symbol,
> > and enclosed by whatever "CONFIG_<kASLR>" you've got in place.
> >
> > The macro looks like this:
> >
> > #define VMCOREINFO_NUMBER(name) \
> >         vmcoreinfo_append_str("NUMBER(%s)=%ld\n", #name, (long)name)
> >
> > Of course that presumes you've got a symbol in place that holds the offset?
> >
> > On a related note, the VMCOREINFO_SYMBOL(phys_base) above is completely
> > useless, and should also have been introduced as
> > VMCOREINFO_NUMBER(phys_base).
> > The makedumpfile maintainers on this list can confirm it, but I
> > don't believe that they use it either, but just pass it on to the
> > crash utility to ignore.  Both the crash utility and makedumpfile
> > jump through hoops to figure out the phys_base value when it could
> > simply be passed in the vmcoreinfo data.
> 
> FWIW, the offset reported during a panic to dmesg is:
>     (unsigned long)&_text - __START_KERNEL
> 
> I think this is what Andrew was looking at for pushing into the vmcore info.
> 
> -Kees

OK good.  Since VMCOREINFO_NUMBER() won't work without a variable, maybe Andrew
was planning to create something like a new VMCOREINFO_KASLR_OFFSET() in 
include/linux/kexec.h?  BTW, is he still going to work on this or has he moved
on to other things?

Dave