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

[Will Deacon]

On Thu, May 30, 2019 at 03:57:36PM +0530, Ravi Bangoria wrote:
> 
> 
> On 5/30/19 2:08 PM, Ravi Bangoria wrote:
> >> ---
> >> Subject: perf: Fix perf_sample_regs_user()
> >> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >> Date: Wed May 29 14:37:24 CEST 2019
> >>
> >> perf_sample_regs_user() uses 'current->mm' to test for the presence of
> >> userspace, but this is insufficient, consider use_mm().
> >>
> >> A better test is: '!(current->flags & PF_KTHREAD)', exec() clears
> >> PF_KTHREAD after it sets the new ->mm but before it drops to userspace
> >> for the first time.
> > 
> > This looks correct. I'll give it a try.
> > 
> >>
> >> Possibly obsoletes: bf05fc25f268 ("powerpc/perf: Fix oops when kthread execs user process")
> >>
> >> Reported-by: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxxxxxxx>
> >> Reported-by: Young Xiao <92siuyang@xxxxxxxxx>
> >> Cc: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxxxxxxx>
> >> Cc: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
> >> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> >> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> >> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> >> Cc: Stephane Eranian <eranian@xxxxxxxxxx>
> >> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> >> Acked-by: Will Deacon <will.deacon@xxxxxxx>
> >> Fixes: 4018994f3d87 ("perf: Add ability to attach user level registers dump to sample")
> >> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> >> ---
> >>  kernel/events/core.c |    2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> --- a/kernel/events/core.c
> >> +++ b/kernel/events/core.c
> >> @@ -5923,7 +5923,7 @@ static void perf_sample_regs_user(struct
> >>  	if (user_mode(regs)) {
> >>  		regs_user->abi = perf_reg_abi(current);
> >>  		regs_user->regs = regs;
> >> -	} else if (current->mm) {
> >> +	} else if (!(current->flags & PF_KTHREAD)) {
> 
> With this patch applied and my patch reverted, I still see it crashing
> because current->flags does not have PF_KTHREAD set. Sample trace with
> v5.0 kernel:
> 
> 
>    BUG: Kernel NULL pointer dereference at 0x00000000
>    Faulting instruction address: 0xc0000000000f1a6c
>    Oops: Kernel access of bad area, sig: 11 [#1]
>    LE SMP NR_CPUS=2048 NUMA pSeries
>    CPU: 17 PID: 3241 Comm: systemd-cgroups Kdump: loaded Not tainted 5.0.0+ #7
>    NIP:  c0000000000f1a6c LR: c0000000002acc7c CTR: c0000000002a8f90
>    REGS: c0000001e80469a0 TRAP: 0300   Not tainted  (5.0.0+)
>    MSR:  8000000000001033 <SF,ME,IR,DR,RI,LE>  CR: 48022448  XER: 20000000
>    CFAR: c00000000000deb4 DAR: 0000000000000000 DSISR: 40000000 IRQMASK: 1 
>    GPR00: c0000000002acc7c c0000001e8046c30 c000000001607500 0000000000000000 
>    GPR04: 0000000000000000 0000000000000000 0000000000000000 c000000000128618 
>    GPR08: 000007ffffffffff 0000000000000000 ffffffffffffffff c00000000001cd40 
>    GPR12: c000000000446fd8 c00000003ffdf080 00000000ffff0000 0000000000000007 
>    GPR16: c0000001edd74988 c0000001edd60400 00007fff89801130 000000000005e1b0 
>    GPR20: c0000001edb77a08 c0000001e8047208 c0000001f03d9800 c0000001e8046e00 
>    GPR24: 000000000000b1af c000000001126938 c0000001f03d9b28 0000000000010000 
>    GPR28: c0000001e8046d30 0000000000000000 0000000000000000 0000000000000000 
>    NIP [c0000000000f1a6c] perf_reg_value+0x5c/0xc0
>    LR [c0000000002acc7c] perf_output_sample_regs+0x6c/0xd0
>    Call Trace:
>    [c0000001e8046c30] [c0000000002acc7c] perf_output_sample_regs+0x6c/0xd0 (unreliable)
>    [c0000001e8046c80] [c0000000002b9cd0] perf_output_sample+0x620/0x8c0
>    [c0000001e8046d10] [c0000000002ba6b4] perf_event_output_forward+0x64/0x90
>    [c0000001e8046d80] [c0000000002b2908] __perf_event_overflow+0x88/0x1e0
>    [c0000001e8046dd0] [c0000000000f3d18] record_and_restart+0x288/0x670
>    [c0000001e8047180] [c0000000000f4c18] perf_event_interrupt+0x2b8/0x4b0
>    [c0000001e8047280] [c00000000002b380] performance_monitor_exception+0x50/0x70
>    [c0000001e80472a0] [c000000000009ca0] performance_monitor_common+0x110/0x120
>    --- interrupt: f01 at slice_scan_available+0x20/0xc0
>        LR = slice_find_area+0x174/0x210
>    [c0000001e8047630] [c000000000083ea0] slice_get_unmapped_area+0x3d0/0x7f0
>    [c0000001e8047ae0] [c00000000032d5b0] get_unmapped_area+0xa0/0x170
>    [c0000001e8047b10] [c00000000001cd40] arch_setup_additional_pages+0xc0/0x1c0
>    [c0000001e8047b60] [c000000000446fd8] load_elf_binary+0xb48/0x1580
>    [c0000001e8047c80] [c0000000003c3938] search_binary_handler+0xe8/0x2a0
>    [c0000001e8047d10] [c0000000003c42f4] __do_execve_file.isra.13+0x694/0x980
>    [c0000001e8047de0] [c000000000128618] call_usermodehelper_exec_async+0x248/0x290
>    [c0000001e8047e20] [c00000000000b65c] ret_from_kernel_thread+0x5c/0x80
>    Instruction dump:
>    7c9e2378 7c7f1b78 f8010010 f821ffd1 419e0044 3d22ff6b 7bc41764 3929ae10 
>    7d29202e 2b890150 419d003c 38210030 <7c7f482a> e8010010 ebc1fff0 ebe1fff8 
>    ---[ end trace 54f3492ad1d403d8 ]---

Oh, nice! I think this happens because Power doesn't actually initialise
the regs after a kthread execs() until late in start_thread(). But the plot
thickens somewhat, since current_pt_regs() is different to
task_pt_regs(current) on Power (the former cannot return NULL).

So a really hideous hack on top of Peter's patch might be:

diff --git a/arch/arm64/kernel/perf_regs.c b/arch/arm64/kernel/perf_regs.c
index 0bbac612146e..5bde866024b6 100644
--- a/arch/arm64/kernel/perf_regs.c
+++ b/arch/arm64/kernel/perf_regs.c
@@ -57,6 +57,6 @@ void perf_get_regs_user(struct perf_regs *regs_user,
 			struct pt_regs *regs,
 			struct pt_regs *regs_user_copy)
 {
-	regs_user->regs = task_pt_regs(current);
+	regs_user->regs = current_pt_regs();
 	regs_user->abi = perf_reg_abi(current);
 }

Will