On Thu, 4 Jul 2019 20:04:41 +0530 "Naveen N. Rao" <naveen.n....@linux.vnet.ibm.com> wrote:
> LTP testsuite on powerpc results in the below crash: > > Unable to handle kernel paging request for data at address 0x00000000 > Faulting instruction address: 0xc00000000029d800 > Oops: Kernel access of bad area, sig: 11 [#1] > LE SMP NR_CPUS=2048 NUMA PowerNV > ... > CPU: 68 PID: 96584 Comm: cat Kdump: loaded Tainted: G W > NIP: c00000000029d800 LR: c00000000029dac4 CTR: c0000000001e6ad0 > REGS: c0002017fae8ba10 TRAP: 0300 Tainted: G W > MSR: 9000000000009033 <SF,HV,EE,ME,IR,DR,RI,LE> CR: 28022422 XER: > 20040000 > CFAR: c00000000029d90c DAR: 0000000000000000 DSISR: 40000000 IRQMASK: 0 > ... > NIP [c00000000029d800] t_probe_next+0x60/0x180 > LR [c00000000029dac4] t_mod_start+0x1a4/0x1f0 > Call Trace: > [c0002017fae8bc90] [c000000000cdbc40] _cond_resched+0x10/0xb0 (unreliable) > [c0002017fae8bce0] [c0000000002a15b0] t_start+0xf0/0x1c0 > [c0002017fae8bd30] [c0000000004ec2b4] seq_read+0x184/0x640 > [c0002017fae8bdd0] [c0000000004a57bc] sys_read+0x10c/0x300 > [c0002017fae8be30] [c00000000000b388] system_call+0x5c/0x70 > > The test (ftrace_set_ftrace_filter.sh) is part of ftrace stress tests > and the crash happens when the test does 'cat > $TRACING_PATH/set_ftrace_filter'. > > The address points to the second line below, in t_probe_next(), where > filter_hash is dereferenced: > hash = iter->probe->ops.func_hash->filter_hash; > size = 1 << hash->size_bits; > > This happens due to a race with register_ftrace_function_probe(). A new > ftrace_func_probe is created and added into the func_probes list in > trace_array under ftrace_lock. However, before initializing the filter, > we drop ftrace_lock, and re-acquire it after acquiring regex_lock. If > another process is trying to read set_ftrace_filter, it will be able to > acquire ftrace_lock during this window and it will end up seeing a NULL > filter_hash. > > Fix this by just checking for a NULL filter_hash in t_probe_next(). If > the filter_hash is NULL, then this probe is just being added and we can > simply return from here. Hmm, this is very subtle. I'll take a deeper look at this to see if we can keep the race from happening. Thanks! -- Steve > > Signed-off-by: Naveen N. Rao <naveen.n....@linux.vnet.ibm.com> > --- > kernel/trace/ftrace.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c > index 7b037295a1f1..0791eafb693d 100644 > --- a/kernel/trace/ftrace.c > +++ b/kernel/trace/ftrace.c > @@ -3093,6 +3093,10 @@ t_probe_next(struct seq_file *m, loff_t *pos) > hnd = &iter->probe_entry->hlist; > > hash = iter->probe->ops.func_hash->filter_hash; > + > + if (!hash) > + return NULL; > + > size = 1 << hash->size_bits; > > retry: