[RFA] makedumpfile: fix access to os_info for /proc/kcore

[Philipp Rudo]

Hi Alex,

our QE found a problem when trying to run makedumpfile with /proc/kcore
on s390. For example

	# makedumpfile --mem-usage /proc/kcore
	s390x_init_vm: Can't get s390x os_info ptr.

The exact options passed to makedumpfile don't matter. The error is
always the same. Trying the same on a dump file created from
/proc/vmcore works fine. As the function in question was introduced
with you commit 6f8325d ("[PATCH v2 2/2] s390x: uncouple virtual and
physical address spaces") I'm reaching out to you.

Looking at /proc/kcore with crash I noticed that
abs_lowcore->os_info (aka. address 0xe18) is zero. Hence the check

	if (!readmem(PADDR, S390X_LC_OS_INFO, &addr,
			sizeof(addr)) || !addr) {
		ERRMSG("Can't get s390x os_info ptr.\n");
		return FALSE;
	}

at the beginning of s390x_init_vm fails. My theory is that when trying
to access the absolute lowcore via /proc/kcore the read gets prefixed
and thus ends up in the per-cpu lowcore. As the os_info field isn't set
in the per-cpu lowcore the read returns 0, triggering the error.

I played around with crash trying to access the absolute lowcore via
__abs_lowcore and lowcore_ptr but failed. I always ended up in the
per-cpu lowcore. I also tried to get the address of os_info from the
dwarf information but that only returnes a virtual address which cannot
be used in the function that sets up vm...

Any idea how this problem could be fixed?

Thanks
Philipp

P.S. While looking at the function I found one nit. Right after the
check mentioned above there's an other check for

	if (addr == 0)
		return TRUE;

which can never be true as the !addr from above already handles this
case.