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

From: Michael Ellerman
Date: Tue Sep 19 2017 - 06:00:56 EST

Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxxxxxxx> 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


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?