Re: [Xen-devel] Fwd: NetBSD xl core-dump not working... Memory fault (core dumped)

[John Nemeth <jnemeth@xxxxxxxxx>]

On Nov 8, 10:29am, Ian Campbell wrote:
} On Thu, 2013-11-07 at 21:04 +0000, Miguel C. wrote:
} > yes its 4.2 from pkgsrc.
} 
} Thanks, that might be enough.

     More specifically, it's 4.2.3.

} >  how can i get the changeset id?
} 
} that'd be one for the port-xen folks I think. It might be printed in the
} xen dmesg, but that depends on how it was built and 4.2 may be too old
} to have such functionalilty.

     xl dmesg says:

(XEN) Latest ChangeSet: unavailable

The package was built using this tarball:

http://bits.xensource.com/oss-xen/release/4.2.3/xen-4.2.3.tar.gz

And, just for reference, this is the info we have on the tarball:

SHA1 (xen-4.2.3.tar.gz) = 7c72e1aa870cc938afdc50bd9f2d879118aa8b99
RMD160 (xen-4.2.3.tar.gz) = da0fbb7bbc0796bd83c223f7d21015ce0d4c8553
Size (xen-4.2.3.tar.gz) = 15613235 bytes

} > Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
} > >On Mon, 2013-11-04 at 22:13 +0000, Mike C. wrote:
} > >> On 31.10.13 04:34, Miguel Clara wrote:
} > >> 
} > >> > I was trying to get a core-dump for a domU with xl and got this
} > >error:
} > >> >
} > >> > # xl dump-core 20 test.core
} > >> > Memory fault
} > >> >
} > >> > GDB shows this:
} > >> >
} > >> > a# gdb xl xl.core
} > >> > GNU gdb (GDB) 7.3.1
} > >> > Copyright (C) 2011 Free Software Foundation, Inc.
} > >> > License GPLv3+: GNU GPL version 3 or
} > >later<http://gnu.org/licenses/gpl.html>
} > >> > This is free software: you are free to change and redistribute it.
} > >> > There is NO WARRANTY, to the extent permitted by law.  Type "show
} > >copying"
} > >> > and "show warranty" for details.
} > >> > This GDB was configured as "x86_64--netbsd".
} > >> > For bug reporting instructions, please see:
} > >> > <http://www.gnu.org/software/gdb/bugs/>...
} > >> > Reading symbols from /usr/sbin/xl...done.
} > >> > [New process 1]
} > >> > Core was generated by `xl'.
} > >> > Program terminated with signal 11, Segmentation fault.
} > >> > #0  0x00007f7ff7007b45 in xc_domain_dumpcore_via_callback
} > >> > (xch=0x7f7ff7b0d800, domid=20, args=0x7f7fffffdae0,
} > >> > dump_rtn=0x7f7ff700632c<local_file_dump>)
} > >> >      at xc_core.c:860
} > >
} 
} In 4.2.0 this corresponds to
}  memcpy(dump_mem, vaddr, PAGE_SIZE);
} which is a plausible source of a segfault.
} 
} xc_core.c has only changed in immaterial ways (although ways which
} caused all the line numbers to shift) since 4.2.0 AFAICT so it is likely
} that this bug is still present.
} 
} Can you tell via gdb what the faulting address was and whether it
} corresponds to dump_mem or vaddr? gdb's "info locals" might give you at
} least some of that? Also you can use disas to identify the precise
} instruction at 0x00007f7ff7007b45, which will show you the registers
} which might lead you to the faulting address.
} 
} vaddr is certainly not NULL, it's checked right before. It could be
} non-NULL and still invalid if xc_map_foreign_range were buggy on NetBSD,
} but that is surely used elsewhere? I suppose it might have mapped an MFN
} which was either invalid (or became invalid, but your bug is
} deterministic, right?. IIRC NetBSD's privcmd foreign mappings are
} populated lazily and not immediately like on Linux? If that were the
} case (and I'm only vaguely aware of how NetBSD operates) then it would
} be plausible that xc_map_foreign_range would succeed but that a
} subsequent attempt to access the region would fault?
} 
} dump_mem isn't NULL, it's a pointer into the dump_mem_start array which
} has a check for failure when it is allocated. Since dump_mem is just
} normal process memory and vaddr is a magic foreign mapping I'd be
} inclined to suspect vaddr was not right in some way...
} 
} Does "xl -vvv core-dump" give any useful additional logging?
} 
} Unfortunately I don't think anyone has done valgrind support for
} debugging processes which use Xen hypercalls for *BSD (if you were very
} keen you could probably follow what was done for Linux
} 
http://blog.xen.org/index.php/2013/01/18/using-valgrind-to-debug-xen-toolstacks/
} and wire up the BSD privcmd ioctl to the generic Xen hypercall code I
} added)
} 
} I fear this bug is going to take someone on the ground with a NetBSD
} system and the ability to dive into BSD kernel internals to get to the
} bottom of...
} 
} Ian.
} 
}-- End of excerpt from Ian Campbell



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