Powerpc64: Fixup oops when debug programs with CONFIG_RELOCATABLE=y

From: LiuHailong <liu.hailong6@zte.com.cn>

Debug interrupts can be taken during regular program or a standard
interrupt, the EA of the instruction causing the interrupt will be
kept in DSRR0.
Kernel will check if this value is between [interrupt_base_book3e,
__end_interrupts].
However, when the kernel build with CONFIG_RELOCATABLE, it can't get
EA of those lables by LOAD_REG_IMMEDIATE(r14,interrupt_base_book3e)
and LOAD_REG_IMMEDIATE(r15,__end_interrupts),then it cases problems
later.
At the same time, r2(toc) are not usable here, so LOAD_REG_ADDR()
dosen't work neither. So we use the *name@got* to get the EV of two
lables directly.
This patch can fix the problem and remove the oops when we gdb a
program with single-step.

Test programs test.c shows as follows:
#include <fcntl.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
	if (access("/proc/sys/kernel/perf_event_paranoid", F_OK) == -1)
		printf("Kernel doesn't have perf_event support\n");
}

Steps to reproduce the bug, for example:
 1) ./gdb ./test
 2) (gdb) b access
 3) (gdb) r
 4) (gdb) s

Then will trigger the oops, it looks like:
(gdb) s
Single stepping Oops: Exception in kernel mode, sig: 5 [#2]
PREEMPT CoreNet Generic
Modules linked in:
CPU: 0 PID: 1135 Comm: test Tainted: G    D    Linux (none) 4.9.5 #79
task: c000000079199580 ti: c00000007ffc4000 task.ti: c000000074064000
NIP: c00000000001a1e4 LR: 000000001000103c CTR: 000000001000100c
REGS: c00000007ffc7cf0 TRAP: 0d08   Tainted: G   D  (Linux (none) 4.9.5)
MSR: 0000000080021000 <CE,ME>  CR: 24000442  XER: 00000000
SOFTE: 1
GPR00: 0000000010001274 00000000ffffeba0 00000000100ab4b0 00000000100764a4
GPR04: 0000000000000000 00000000ffffee2c 00000000ffffee54 00000000100a44c8
GPR08: 0000000000000001 0000000010070000 00000000100a0000 0000000000000001
GPR12: 000000004347432f 00000000100aa648 0000000000000000 0000000000000000
GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR24: 0000000000000000 0000000010001950 0000000010001850 0000000000000000
GPR28: 0000000000000000 00000000100000f4 0000000000000000 00000000ffffeba0
NIP [c00000000001a1e4] interrupt_base_book3e+0x1e4/0x348
LR [000000001000103c] 0x1000103c
Call Trace:
Instruction dump:
00000000 00000000 00000000 60000000 4800e600 00000000 00000000 00000000
00000000 00000000 00000000 60000000 <4800e588> 00000000 00000000 00000000

Signed-off-by: Liu Hailong <liu.hailong6@zte.com.cn>
Signed-off-by: Jiang Xuexin <jiang.xuexin@zte.com.cn>
Reviewed-by: Jiang Biao <jiang.biao2@zte.com.cn>
Reviewed-by: Liu Song <liu.song11@zte.com.cn>
Reviewed-by: Huang Jian <huang.jian@zte.com.cn>
---
 arch/powerpc/kernel/exceptions-64e.S |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)
 mode change 100644 => 100755 arch/powerpc/kernel/exceptions-64e.S

--
1.7.1

On Tue, Feb 07, 2017 at 10:35:52AM +0800, Liu Hailong wrote:
> From: LiuHailong <liu.hailong6@zte.com.cn>
> 
> Debug interrupts can be taken during regular program or a standard
> interrupt, the EA of the instruction causing the interrupt will be
> kept in DSRR0.
> Kernel will check if this value is between [interrupt_base_book3e,
> __end_interrupts].
> However, when the kernel build with CONFIG_RELOCATABLE, it can't get
> EA of those lables by LOAD_REG_IMMEDIATE(r14,interrupt_base_book3e)
> and LOAD_REG_IMMEDIATE(r15,__end_interrupts),then it cases problems
> later.
> At the same time, r2(toc) are not usable here, so LOAD_REG_ADDR()
> dosen't work neither. So we use the *name@got* to get the EV of two
> lables directly.
> This patch can fix the problem and remove the oops when we gdb a
> program with single-step.
> 
> Test programs test.c shows as follows:
> #include <fcntl.h>
> #include <stdio.h>
> int main(int argc, char *argv[])
> {
> 	if (access("/proc/sys/kernel/perf_event_paranoid", F_OK) == -1)
> 		printf("Kernel doesn't have perf_event support\n");
> }
> 
> Steps to reproduce the bug, for example:
>  1) ./gdb ./test
>  2) (gdb) b access
>  3) (gdb) r
>  4) (gdb) s
> 
> Then will trigger the oops, it looks like:
> (gdb) s
> Single stepping Oops: Exception in kernel mode, sig: 5 [#2]
> PREEMPT CoreNet Generic
> Modules linked in:
> CPU: 0 PID: 1135 Comm: test Tainted: G    D    Linux (none) 4.9.5 #79
> task: c000000079199580 ti: c00000007ffc4000 task.ti: c000000074064000
> NIP: c00000000001a1e4 LR: 000000001000103c CTR: 000000001000100c
> REGS: c00000007ffc7cf0 TRAP: 0d08   Tainted: G   D  (Linux (none) 4.9.5)
> MSR: 0000000080021000 <CE,ME>  CR: 24000442  XER: 00000000
> SOFTE: 1

I apologize for not getting to this earlier...

Does it really produce an oops, rather than a hang?  It looks like
without this fix, flow would go to kernel_dbg_exc which is a
branch-to-self.  Do you have other changes in your tree that affected
this?

If so, have you tested the patch on an unmodified top-of-tree kernel?  I
can't test this at the moment as I don't currently have hardware and QEMU
doesn't emulate the booke debug registers.

That said, the patch looks correct, and the bug is even worse if it's a
hang rather than merely noisily killing the debugged process.  It should
go to stable for 4.4+ (when support for relocatable e500 was added) and
probably to Linus this week (though I'd feel more comfortable knowing it
got testing on the current tree).  OTOH, I believe this bug will only
trigger if a relocation actually happened, which on e500 is an unusual
case outside of a kdump crash kernel, since the kernel is normally loaded
at zero.  But maybe you've got a different use case for relocatable?

-Scott