[PATCH] ppc64/perf: Fix oops when kthread execs user process

[Naveen N. Rao]

On 2017/06/15 07:16PM, Ravi Bangoria wrote:
> When a kthread makes a call_usermodehelper() call the steps are:
>  a. allocates current->mm
>  b. load_elf_binary()
>  c. populates current->thread.regs
> 
> While doing this, interrupts are not disabled. If there is a perf
> interrupt in the middle of this process (i.e. step 'a' has completed
> but not yet reached to step 'c') and if perf tries to read userspace
> regs, kernel oops with following log:
> 
>   [  131.217172] Unable to handle kernel paging request for data at address 0x00000000
>   [  131.217731] Faulting instruction address: 0xc0000000000da0fc
>   ...
>   [  131.235555] Call Trace:
>   [  131.235714] [c0000000bbaaad60] [c00000000025dedc] perf_output_sample_regs+0x6c/0xd0
>   [  131.236020] [c0000000bbaaadb0] [c000000000269b44] perf_output_sample+0x4e4/0x830
>   [  131.236362] [c0000000bbaaae40] [c00000000026a354] perf_event_output_forward+0x64/0x90
>   [  131.236668] [c0000000bbaaaeb0] [c00000000026298c] __perf_event_overflow+0x8c/0x1e0
>   [  131.236979] [c0000000bbaaaf00] [c0000000000dc330] record_and_restart+0x220/0x5c0
>   [  131.237306] [c0000000bbaab230] [c0000000000dd1d8] perf_event_interrupt+0x2d8/0x4d0
>   [  131.237611] [c0000000bbaab320] [c0000000000294a4] performance_monitor_exception+0x54/0x70
>   [  131.237891] [c0000000bbaab350] [c00000000000a0a8] performance_monitor_common+0x158/0x160
>   [  131.238208] --- interrupt: f01 at avtab_search_node+0x150/0x1a0
>   [  131.238208]     LR = avtab_search_node+0x100/0x1a0
>   [  131.238617] [c0000000bbaab640] [c000000000526770] context_struct_compute_av+0x220/0x5b0 (unreliable)
>   [  131.238948] [c0000000bbaab730] [c0000000005278b4] security_compute_av+0x174/0x390
>   [  131.239231] [c0000000bbaab7e0] [c0000000005050e4] avc_compute_av+0x84/0x260
>   [  131.239471] [c0000000bbaab890] [c000000000506198] avc_has_perm+0xf8/0x1c0
>   [  131.239708] [c0000000bbaab980] [c00000000050f32c] file_has_perm+0x6c/0xd0
>   [  131.239972] [c0000000bbaab9e0] [c0000000004ff0fc] security_mmap_file+0xac/0x140
>   [  131.240256] [c0000000bbaaba50] [c0000000002b1fc0] vm_mmap_pgoff+0x80/0x160
>   [  131.240532] [c0000000bbaabb30] [c0000000003f7db4] elf_map+0xa4/0x180
>   [  131.240771] [c0000000bbaabb90] [c0000000003f9a48] load_elf_binary+0x6e8/0x15a0
>   [  131.241060] [c0000000bbaabc90] [c000000000374f58] search_binary_handler+0xe8/0x290
>   [  131.241347] [c0000000bbaabd20] [c000000000375c14] do_execveat_common.isra.14+0x5f4/0x840
>   [  131.241631] [c0000000bbaabdf0] [c00000000010be70] call_usermodehelper_exec_async+0x170/0x210
>   [  131.241955] [c0000000bbaabe30] [c00000000000bae0] ret_from_kernel_thread+0x5c/0x7c
> 
> Fix it by setting abi to PERF_SAMPLE_REGS_ABI_NONE when userspace
> pt_regs are not set. 

So, this only shows up with --call-graph=dwarf. This should be:
Fixes: 17ed7c38427ff8 ("powerpc: Add HAVE_PERF_USER_STACK_DUMP support")

> 
> Signed-off-by: Ravi Bangoria <ravi.bangoria at linux.vnet.ibm.com>
> ---
> Note: this should go to stable as well. I've not checked below 4.4
> kernel but I'm able to reproduce it with 4.4 kernel.

Hmm... are you sure it's the same issue? The above commit only went into 
v4.7, which means we weren't able to use --call-graph=dwarf till v4.7.

Apart from that:
Acked-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>


- Naveen

> 
>  arch/powerpc/perf/perf_regs.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/perf/perf_regs.c b/arch/powerpc/perf/perf_regs.c
> index cbd82fd..09ceea6 100644
> --- a/arch/powerpc/perf/perf_regs.c
> +++ b/arch/powerpc/perf/perf_regs.c
> @@ -101,5 +101,6 @@ void perf_get_regs_user(struct perf_regs *regs_user,
>  			struct pt_regs *regs_user_copy)
>  {
>  	regs_user->regs = task_pt_regs(current);
> -	regs_user->abi  = perf_reg_abi(current);
> +	regs_user->abi = (regs_user->regs) ? perf_reg_abi(current) :
> +			 PERF_SAMPLE_REGS_ABI_NONE;
>  }
> -- 
> 2.9.4
>