Lack of TARGET_ARCH=powerpc support in kgdb from devel/gdb (e.g., -r440115 of /usr/ports): "ABI doesn't support a vmcore target"

Mon May 8 21:36:57 UTC 2017

[I've submitted bugzilla 219153 for this libvm issue of
not handling powerpc's/powerp64's ET_DYN vmcore.* 's and
such.]

On 2017-May-8, at 1:18 PM, Mark Millard <markmi at dsl-only.net> wrote:

> [Mostly: Why THING #2 fails: checks for ET_EXEC
> but the actual vmcore.* 's have ET_DYN instead.]
> 
> On 2017-May-8, at 11:30 AM, John Baldwin <jhb at freebsd.org> wrote:
> 
>> On Saturday, May 06, 2017 10:03:57 PM Mark Millard wrote:
>>> THING #0:
>>> 
>>> It appears that usr.sbin/crashinfo/crashinfo.sh assumes
>>> that /usr/local/bin/gdb will work better for all architectures,
>>> including for kgdb types of activity:
>>> 
>>> find_gdb()
>>> {
>>>       local binary
>>> 
>>>       for binary in /usr/local/bin/gdb /usr/libexec/gdb /usr/bin/gdb; do
>>>               if [ -x ${binary} ]; then
>>>                       GDB=${binary}
>>>                       return
>>>               fi
>>>       done
>>> }
>>> 
>>> But it appears that on powerpc /usr/local/bin/gdb and
>>> /usr/local/bin/kgdb do not support TARGET_ARCH=powerpc
>>> at all for such activity.
>> 
>> Not really.  kgdb on powerpc doesn't work period as neither the base nor ports
>> kgdb can unwind a stack frame.  I spent some time last year trying to get the
>> unwind out of cpu_switch() to work to no avail.  The current hack attempts are
>> here:
>> 
>> https://github.com/bsdjhb/gdb/compare/freebsd-7.11-kgdb...kgdb-ppc
> 
> Interesting.
> 
>>> THING #1:
>>> . . .
>> 
>>> THING #2:
>>> 
>>> /usr/libexec/kgdb (when present) does not support the
>>> powerpc architecture for head either . . .
>>> 
>>> On a head -r317820 powerpc I attempted:
>>> 
>>> # /usr/libexec/kgdb /usr/lib/debug/boot/kernel/kernel.debug /var/crash/vmcore.7
>>> GNU gdb 6.1.1 [FreeBSD]
>>> Copyright 2004 Free Software Foundation, Inc.
>>> GDB is free software, covered by the GNU General Public License, and you are
>>> welcome to change it and/or distribute copies of it under certain conditions.
>>> Type "show copying" to see the conditions.
>>> There is absolutely no warranty for GDB.  Type "show warranty" for details.
>>> This GDB was configured as "powerpc-marcel-freebsd"...
>>> Failed to open vmcore: unsupported architecture
>> 
>> This is a different problem with libkvm.  I would start with 'ps -M' and use
>> a debugger to step through the _powerpc_probe and _powerpc64_probe routines in
>> libkvm to see why the appropriate probe routine isn't claiming the core.
> 
> For THING #2:
> 
> I had to use /usr/libexec/gdb as the debugger because /usr/local/bin/gdb
> segmentation faulted.
> 
> (gdb) list
> 126	int
> 127	_kvm_probe_elf_kernel(kvm_t *kd, int class, int machine)
> 128	{
> 129	
> 130		return (kd->nlehdr.e_ident[EI_CLASS] == class &&
> 131		    kd->nlehdr.e_type == ET_EXEC &&
> 132		    kd->nlehdr.e_machine == machine);
> 133	}
> 
> gets false via: kd->nlehdr.e_type == ET_EXEC
> 
> (gdb) print kd->nlehdr.e_type
> $4 = 3
> 
> but the comparison is for:
> 
> 0x41882fe0 <_kvm_probe_elf_kernel+32>:	cmplwi  cr1,r4,2
> 
> (gdb) disass
> Dump of assembler code for function _kvm_probe_elf_kernel:
> 0x41882fc0 <_kvm_probe_elf_kernel+0>:	stwu    r1,-16(r1)
> 0x41882fc4 <_kvm_probe_elf_kernel+4>:	stw     r31,12(r1)
> 0x41882fc8 <_kvm_probe_elf_kernel+8>:	mr      r31,r1
> 0x41882fcc <_kvm_probe_elf_kernel+12>:	lbz     r6,2076(r3)
> 0x41882fd0 <_kvm_probe_elf_kernel+16>:	crclr   eq
> 0x41882fd4 <_kvm_probe_elf_kernel+20>:	cmplw   cr1,r6,r4
> 0x41882fd8 <_kvm_probe_elf_kernel+24>:	bne-    cr1,0x41882ff0 <_kvm_probe_elf_kernel+48>
> 0x41882fdc <_kvm_probe_elf_kernel+28>:	lhz     r4,2088(r3)
> 0x41882fe0 <_kvm_probe_elf_kernel+32>:	cmplwi  cr1,r4,2
> 0x41882fe4 <_kvm_probe_elf_kernel+36>:	bne-    cr1,0x41882ff0 <_kvm_probe_elf_kernel+48>
> 0x41882fe8 <_kvm_probe_elf_kernel+40>:	lhz     r3,2090(r3)
> 0x41882fec <_kvm_probe_elf_kernel+44>:	cmpw    r3,r5
> 0x41882ff0 <_kvm_probe_elf_kernel+48>:	li      r3,1
> 0x41882ff4 <_kvm_probe_elf_kernel+52>:	beq-    0x41882ffc <_kvm_probe_elf_kernel+60>
> 0x41882ff8 <_kvm_probe_elf_kernel+56>:	li      r3,0
> 0x41882ffc <_kvm_probe_elf_kernel+60>:	lwz     r31,12(r1)
> 0x41883000 <_kvm_probe_elf_kernel+64>:	addi    r1,r1,16
> 0x41883004 <_kvm_probe_elf_kernel+68>:	blr
> 
> powerpc and powerpc64 use position independent kernels
> these days as I remember, making kd->nlehdr.e_type be
> ET_DYN instead of ET_EXEC :
> 
> /* e_type */
> #define	ET_REL		1
> #define	ET_EXEC		2
> #define	ET_DYN		3
> #define	ET_CORE		4
> 
> I do not know if more needs to change than just
> the enabling test since the content is ET_DYN
> type of material.
> 
> It looks like the conversion to position independent
> kernels for powerpc and powerpc64 did not cover all
> the related infrastructure, such as libkvm.

I've submitted bugzilla 219153 for this libvm issue of
not handling powerpc's/powerp64's ET_DYN vmcore.* 's and
such.

It applies to head , stable/11 , and release/11.0.1 :

20150307:
	The 32-bit PowerPC kernel has been changed to a position-independent
	executable. This can only be booted with a version of loader(8)
	newer than January 31, 2015, so make sure to update both world and
	kernel before rebooting.

. . .

20150131:
	The powerpc64 kernel has been changed to a position-independent
	executable. This can only be booted with a new version of loader(8),
	so make sure to update both world and kernel before rebooting.

===
Mark Millard
markmi at dsl-only.net