[PATCH] powerpc/perf: Fix crash with 'is_sier_available' when pmu is not set

[Michael Ellerman]

Hi Athira,

Athira Rajeev <atrajeev at linux.vnet.ibm.com> writes:
> On systems without any platform specific PMU driver support registered or
> Generic Compat PMU support registered,

The compat PMU is registered just like other PMUs, so I don't see how we
can crash like this if the compat PMU is active?

ie. if we're using the compat PMU then ppmu will be non-NULL and point
to generic_compat_pmu.

> running 'perf record' with
> —intr-regs  will crash ( perf record -I <workload> ).
>
> The relevant portion from crash logs and Call Trace:
>
> Unable to handle kernel paging request for data at address 0x00000068
> Faulting instruction address: 0xc00000000013eb18
> Oops: Kernel access of bad area, sig: 11 [#1]
> CPU: 2 PID: 13435 Comm: kill Kdump: loaded Not tainted 4.18.0-193.el8.ppc64le #1
> NIP:  c00000000013eb18 LR: c000000000139f2c CTR: c000000000393d80
> REGS: c0000004a07ab4f0 TRAP: 0300   Not tainted  (4.18.0-193.el8.ppc64le)
> NIP [c00000000013eb18] is_sier_available+0x18/0x30
> LR [c000000000139f2c] perf_reg_value+0x6c/0xb0
> Call Trace:
> [c0000004a07ab770] [c0000004a07ab7c8] 0xc0000004a07ab7c8 (unreliable)
> [c0000004a07ab7a0] [c0000000003aa77c] perf_output_sample+0x60c/0xac0
> [c0000004a07ab840] [c0000000003ab3f0] perf_event_output_forward+0x70/0xb0
> [c0000004a07ab8c0] [c00000000039e208] __perf_event_overflow+0x88/0x1a0
> [c0000004a07ab910] [c00000000039e42c] perf_swevent_hrtimer+0x10c/0x1d0
> [c0000004a07abc50] [c000000000228b9c] __hrtimer_run_queues+0x17c/0x480
> [c0000004a07abcf0] [c00000000022aaf4] hrtimer_interrupt+0x144/0x520
> [c0000004a07abdd0] [c00000000002a864] timer_interrupt+0x104/0x2f0
> [c0000004a07abe30] [c0000000000091c4] decrementer_common+0x114/0x120
>
> When perf record session started with "-I" option, capture registers
                          ^
                          is

> via intr-regs,

"intr-regs" is just the full name for the -I option, so that kind of
repeats itself.

> on each sample ‘is_sier_available()'i is called to check
                                      ^
                                      extra i

The single quotes around is_sier_available() aren't necessary IMO.

> for the SIER ( Sample Instruction Event Register) availability in the
                ^
                stray space
> platform. This function in core-book3s access 'ppmu->flags'. If platform
                                               ^                 ^
                                               es                a
> specific pmu driver is not registered, ppmu is set to null and accessing
           ^                                            ^
           PMU                                          NULL
> its members results in crash. Patch fixes this by returning false in
                        ^
                        a
> 'is_sier_available()' if 'ppmu' is not set.

Use the imperative mood for the last sentence which says what the patch
does:

  Fix the crash by returning false in is_sier_available() if ppmu is not set.


> Fixes: 333804dc3b7a ("powerpc/perf: Update perf_regs structure to include SIER")
> Reported-by: Sachin Sant <sachinp at linux.vnet.ibm.com>
> Signed-off-by: Athira Rajeev <atrajeev at linux.vnet.ibm.com>
> ---
>  arch/powerpc/perf/core-book3s.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
> index 08643cb..1de4770 100644
> --- a/arch/powerpc/perf/core-book3s.c
> +++ b/arch/powerpc/perf/core-book3s.c
> @@ -137,6 +137,9 @@ static void pmao_restore_workaround(bool ebb) { }
>  
>  bool is_sier_available(void)
>  {
> +	if (!ppmu)
> +		return false;
> +
>  	if (ppmu->flags & PPMU_HAS_SIER)
>  		return true;
>  
> -- 
> 1.8.3.1


cheers