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

[Alexander Gordeev]

On Wed, Sep 04, 2024 at 06:12:59PM +0200, Philipp Rudo wrote:

Hi Philipp,

> 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.

Yes, I think your analysis is correct.

> 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?

I will take a deeper look at it.

> Thanks
> Philipp

Thanks for reporting!

> 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.

It will be TRUE when readmem() succeeded and read out zero.
In fact, || !addr condition is redundant. Do you want to send a patch?

Thanks!