[PATCH] powerpc/sysrq: Fix oops whem ppmu is not registered

Kernel crashes if power pmu is not registered and user tries to dump
regs with 'echo p > /proc/sysrq-trigger'. Sample log:

  Unable to handle kernel paging request for data at address 0x00000008
  Faulting instruction address: 0xc0000000000d52f0

  NIP [c0000000000d52f0] perf_event_print_debug+0x10/0x230
  LR [c00000000058a938] sysrq_handle_showregs+0x38/0x50
  Call Trace:
   printk+0x38/0x4c (unreliable)
   __handle_sysrq+0xe4/0x270
   write_sysrq_trigger+0x64/0x80
   proc_reg_write+0x80/0xd0
   __vfs_write+0x40/0x200
   vfs_write+0xc8/0x240
   SyS_write+0x60/0x110
   system_call+0x58/0x6c

Signed-off-by: Ravi Bangoria <redacted>
Reviewed-by: Kamalesh Babulal <redacted> 
---
 arch/powerpc/perf/core-book3s.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 2e3eb74..9e3da16 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c

@@ -793,6 +793,11 @@ void perf_event_print_debug(void)
 	u32 pmcs[MAX_HWEVENTS];
 	int i;
 
+	if (!ppmu) {
+		pr_info("Performance monitor hardware not registered.\n");
+		return;
+	}
+
 	if (!ppmu->n_counter)
 		return;
 

-- 
1.8.3.1

Ravi Bangoria [off-list ref] writes:

Kernel crashes if power pmu is not registered and user tries to dump
regs with 'echo p > /proc/sysrq-trigger'. Sample log:

  Unable to handle kernel paging request for data at address 0x00000008
  Faulting instruction address: 0xc0000000000d52f0

  NIP [c0000000000d52f0] perf_event_print_debug+0x10/0x230
  LR [c00000000058a938] sysrq_handle_showregs+0x38/0x50
  Call Trace:
   printk+0x38/0x4c (unreliable)
   __handle_sysrq+0xe4/0x270
   write_sysrq_trigger+0x64/0x80
   proc_reg_write+0x80/0xd0
   __vfs_write+0x40/0x200
   vfs_write+0xc8/0x240
   SyS_write+0x60/0x110
   system_call+0x58/0x6c

Thanks.

I added:

  Fixes: 5f6d0380c640 ("powerpc/perf: Define perf_event_print_debug() to print PMU register values")

But I don't think I'll Cc stable, I can't think why we'd ever *not* have
a ppmu registered. How did you hit it?

cheers

On Tuesday 19 September 2017 03:30 PM, Michael Ellerman wrote:

Ravi Bangoria [off-list ref] writes:

quoted

Kernel crashes if power pmu is not registered and user tries to dump
regs with 'echo p > /proc/sysrq-trigger'. Sample log:

   Unable to handle kernel paging request for data at address 0x00000008
   Faulting instruction address: 0xc0000000000d52f0

   NIP [c0000000000d52f0] perf_event_print_debug+0x10/0x230
   LR [c00000000058a938] sysrq_handle_showregs+0x38/0x50
   Call Trace:
    printk+0x38/0x4c (unreliable)
    __handle_sysrq+0xe4/0x270
    write_sysrq_trigger+0x64/0x80
    proc_reg_write+0x80/0xd0
    __vfs_write+0x40/0x200
    vfs_write+0xc8/0x240
    SyS_write+0x60/0x110
    system_call+0x58/0x6c

Thanks.

I added:

   Fixes: 5f6d0380c640 ("powerpc/perf: Define perf_event_print_debug() to print PMU register values")

But I don't think I'll Cc stable, I can't think why we'd ever *not* have
a ppmu registered. How did you hit it?

mpe,

We hit this in a compact mode setup where the distro kernel did not have 
the
pmu support for the base pvr.

Maddy

cheers

On Tue, 2017-09-19 at 06:17:06 UTC, Ravi Bangoria wrote:

Kernel crashes if power pmu is not registered and user tries to dump
regs with 'echo p > /proc/sysrq-trigger'. Sample log:

  Unable to handle kernel paging request for data at address 0x00000008
  Faulting instruction address: 0xc0000000000d52f0

  NIP [c0000000000d52f0] perf_event_print_debug+0x10/0x230
  LR [c00000000058a938] sysrq_handle_showregs+0x38/0x50
  Call Trace:
   printk+0x38/0x4c (unreliable)
   __handle_sysrq+0xe4/0x270
   write_sysrq_trigger+0x64/0x80
   proc_reg_write+0x80/0xd0
   __vfs_write+0x40/0x200
   vfs_write+0xc8/0x240
   SyS_write+0x60/0x110
   system_call+0x58/0x6c

Signed-off-by: Ravi Bangoria <redacted>
Reviewed-by: Kamalesh Babulal <redacted>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/4917fcb58cc73f6b81455e3c5f9601

cheers