Cannot step through kernel code (ARM Cortex-A9)

Open source ARM Debugger

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Vmcsnekke
Posts: 1
Joined: Wed Jan 07, 2015 12:48 am

Cannot step through kernel code (ARM Cortex-A9)

Post by Vmcsnekke » Wed Jan 07, 2015 12:50 am

I cannot step through kernel code (ARM Cortex-A9), hopefully someone knows why.

I have a ARM Cortex-A9 target which is being debugged with openocd
(version: 0.9.0-dev-00186-g30203b3-dirty (2014-11-11-17:05)) and an
Olimex ARM-USB-OCD-H.
The problem is I cannot step through Linux kernel code, I'm probably doing
something wrong or missing something. To be precise: after setting a
hardware breakpoint and when it is hit, it is not possible to step through
the Linux kernel code.

This is the exact scenario (including gdb tracing):

run linux kernel on ARM Cortex-A9 target

start openocd [target is halted]

start arm-linux-gdb and enter following commands:
(gdb) symbol-file vmlinux
(gdb) target remote localhost:3333
Remote debugging using localhost:3333
cpu_v7_do_idle () at arch/arm/mm/proc-v7.S:74
74 mov pc, lr
(gdb) hbreak sys_sync
Hardware assisted breakpoint 1 at 0xc00a14dc: file fs/sync.c, line 103.
(gdb) c


So far so good, the target is running. When executing 'sync' the breakpoint is
hit and I can continue with the gdb continue command. I can repeat this a couple
of times, it keeps working. Here's the corresponding ("set debug remote 1") trace:


Continuing.
Sending packet: $qTStatus#49...Packet received:
Sending packet: $Z1,c00a14dc,4#67...Packet received: OK
Packet Z1 (hardware-breakpoint) is supported
Sending packet: $vCont?#49...Packet received:
Packet vCont (verbose-resume) is NOT supported
Sending packet: $Hc0#db...Packet received: OK
Sending packet: $c#63...Packet received: T05
Sending packet: $g#67...Packet received: 00000000945e9fbe01000000399d050001000000945e9fbec000000024000000a49200c0004070c100000000000000007d3cc510a85f70c1409100c0dc140ac013000080
Sending packet: $z1,c00a14dc,4#87...Packet received: OK

Sending packet: $mc00a14dc,4#1d...Packet received: 0dc0a0e1
Sending packet: $mc00a14d8,4#f2...Packet received: 6c4930c0
Sending packet: $mc00a14dc,4#1d...Packet received: 0dc0a0e1
Sending packet: $mc00a14d8,4#f2...Packet received: 6c4930c0
Sending packet: $mc00a14dc,4#1d...Packet received: 0dc0a0e1
Sending packet: $mc00a14d8,4#f2...Packet received: 6c4930c0
Sending packet: $mc00a14dc,4#1d...Packet received: 0dc0a0e1
Sending packet: $mc00a14d8,4#f2...Packet received: 6c4930c0
Sending packet: $mc00a14dc,4#1d...Packet received: 0dc0a0e1
Sending packet: $mc00a14dc,4#1d...Packet received: 0dc0a0e1
Sending packet: $mc00a14dc,4#1d...Packet received: 0dc0a0e1
Breakpoint 1, sys_sync () at fs/sync.c:103
103 {
(gdb) c
Continuing.
Sending packet: $mc00a14dc,4#1d...Packet received: 0dc0a0e1
Sending packet: $mc00a14dc,4#1d...Packet received: 0dc0a0e1
Sending packet: $Z0,c00a14e0,4#34...Packet received: OK
Packet Z0 (software-breakpoint) is supported
Sending packet: $Hc0#db...Packet received: OK
Sending packet: $c#63...Packet received: T05
Sending packet: $g#67...Packet received: 0000000094dee8be01000000399d05000100000094dee8bec000000024000000a49200c0004070c10000000000000000a85f70c1a85f70c1409100c0e0140ac013000080
Sending packet: $z0,c00a14e0,4#54...Packet received: OK
Sending packet: $qTStatus#49...Packet received:
Sending packet: $Z1,c00a14dc,4#67...Packet received: OK
Sending packet: $Hc0#db...Packet received: OK
Sending packet: $c#63...Packet received: T05
Sending packet: $g#67...Packet received: 0000000094dee8be01000000399d05000100000094dee8bec000000024000000a49200c0004070c10000000000000000a85f70c1a85f70c1409100c0e0140ac013000080

Program received signal SIGTRAP, Trace/breakpoint trap.
Sending packet: $z1,c00a14dc,4#87...Packet received: OK
Sending packet: $mc00a14e0,4#eb...Packet received: 10d82de9
Sending packet: $mc00a14dc,4#1d...Packet received: 0dc0a0e1
Sending packet: $mc00a14e0,4#eb...Packet received: 10d82de9
Sending packet: $mc00a14dc,4#1d...Packet received: 0dc0a0e1
Sending packet: $mc00a14e0,4#eb...Packet received: 10d82de9
Sending packet: $mc00a14dc,4#1d...Packet received: 0dc0a0e1
Sending packet: $mc00a14e0,4#eb...Packet received: 10d82de9
Sending packet: $mc00a14dc,4#1d...Packet received: 0dc0a0e1
Sending packet: $mc00a14e0,4#eb...Packet received: 10d82de9
Sending packet: $mc00a14e0,4#eb...Packet received: 10d82de9
Sending packet: $mc00a14e0,4#eb...Packet received: 10d82de9



The problem occurs when I want to step through the code with stepi, step or next.
In this case gdb & target seems to hang. Here's the corresponding ("set debug remote 1") trace:



0xc00a14e0 in sys_sync () at fs/sync.c:103
103 {
(gdb) step
Sending packet: $qTStatus#49...Packet received:
Sending packet: $Z1,c00a14dc,4#67...Packet received: OK
Sending packet: $mc00a14e0,4#eb...Packet received: 10d82de9
Sending packet: $mc00a14e0,4#eb...Packet received: 10d82de9
Sending packet: $Z0,c00a14e4,4#38...Packet received: OK
Sending packet: $c#63...Packet received: T05
Sending packet: $g#67...Packet received: 0000000094dee8be01000000399d05000100000094dee8bec000000024000000a49200c0004070c10000000000000000a85f70c1945f70c1409100c0e4140ac013000080
Sending packet: $z0,c00a14e4,4#58...Packet received: OK
Sending packet: $qTStatus#49...Packet received:
Sending packet: $mc00a14e4,4#ef...Packet received: 04b04ce2
Sending packet: $mc00a14e4,4#ef...Packet received: 04b04ce2
Sending packet: $Z0,c00a14e8,4#3c...Packet received: OK
Sending packet: $c#63...Packet received: T05
Sending packet: $g#67...Packet received: 0000000094dee8be01000000399d05000100000094dee8bec000000024000000a49200c0004070c10000000000000000a85f70c1945f70c1409100c0e4140ac013000080
Sending packet: $z0,c00a14e8,4#5c...Packet received: OK
Sending packet: $qTStatus#49...Packet received:
Sending packet: $mc00a14e4,4#ef...Packet received: 04b04ce2
Sending packet: $mc00a14e4,4#ef...Packet received: 04b04ce2
Sending packet: $Z0,c00a14e8,4#3c...Packet received: OK
Sending packet: $c#63...Packet received: T05
Sending packet: $g#67...Packet received: 0000000094dee8be01000000399d05000100000094dee8bec000000024000000a49200c0004070c10000000000000000a85f70c1945f70c1409100c0e4140ac013000080
.
.
.
this keeps repeating.


Btw. when using a software breakpoint it doesn't work at all, after hitting the
breakpoint for the first time, the gdb continue command always results in a new
"breakpoint hit".

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