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