Re: [PATCH bpf-next v6 1/2] bpf: Fix attaching fentry/fexit/fmod_ret/lsm to modules

[Petr Mladek <pmladek@suse.com>]

On Fri 2023-03-31 17:15:56, Leizhen (ThunderTown) wrote:
> 
> 
> On 2023/3/31 16:31, Petr Mladek wrote:
> > On Thu 2023-03-30 22:59:12, Jiri Olsa wrote:
> >> On Thu, Mar 30, 2023 at 08:26:41PM +0800, Leizhen (ThunderTown) wrote:
> >>>
> >>>
> >>> On 2023/3/30 15:29, Jiri Olsa wrote:
> >>>> ping,
> >>>>
> >>>> Petr, Zhen, any comment on discussion below?
> >>>>
> >>>> thanks,
> >>>> jirka
> >>>>
> >>>> On Thu, Mar 23, 2023 at 03:00:25PM +0100, Jiri Olsa wrote:
> >>>>> On Wed, Mar 22, 2023 at 09:03:46AM -0700, Alexei Starovoitov wrote:
> >>>>>> On Wed, Mar 22, 2023 at 5:14 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> >>>>>>>
> >>>>>>> On Wed, Mar 22, 2023 at 10:49:38AM +0100, Artem Savkov wrote:
> >>>>>>>
> >>>>>>> SNIP
> >>>>>>>
> >>>>>>>>>> Hm, do we even need to preempt_disable? IIUC, preempt_disable is used
> >>>>>>>>>> in module kallsyms to prevent taking the module lock b/c kallsyms are
> >>>>>>>>>> used in the oops path. That shouldn't be an issue here, is that correct?
> >>>>>>>>>
> >>>>>>>>> btf_try_get_module calls try_module_get which disables the preemption,
> >>>>>>>>> so no need to call it in here
> >>>>>>>>
> >>>>>>>> It does, but it reenables preemption right away so it is enabled by the
> >>>>>>>> time we call find_kallsyms_symbol_value(). I am getting the following
> >>>>>>>> lockdep splat while running module_fentry_shadow test from test_progs.
> >>>>>>>>
> >>>>>>>> [   12.017973][  T488] =============================
> >>>>>>>> [   12.018529][  T488] WARNING: suspicious RCU usage
> >>>>>>>> [   12.018987][  T488] 6.2.0.bpf-test-13063-g6a9f5cdba3c5 #804 Tainted: G           OE
> >>>>>>>> [   12.019898][  T488] -----------------------------
> >>>>>>>> [   12.020391][  T488] kernel/module/kallsyms.c:448 suspicious rcu_dereference_check() usage!
> >>>>>>>> [   12.021335][  T488]
> >>>>>>>> [   12.021335][  T488] other info that might help us debug this:
> >>>>>>>> [   12.021335][  T488]
> >>>>>>>> [   12.022416][  T488]
> >>>>>>>> [   12.022416][  T488] rcu_scheduler_active = 2, debug_locks = 1
> >>>>>>>> [   12.023297][  T488] no locks held by test_progs/488.
> >>>>>>>> [   12.023854][  T488]
> >>>>>>>> [   12.023854][  T488] stack backtrace:
> >>>>>>>> [   12.024336][  T488] CPU: 0 PID: 488 Comm: test_progs Tainted: G           OE      6.2.0.bpf-test-13063-g6a9f5cdba3c5 #804
> >>>>>>>> [   12.025290][  T488] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.1-2.fc37 04/01/2014
> >>>>>>>> [   12.026108][  T488] Call Trace:
> >>>>>>>> [   12.026381][  T488]  <TASK>
> >>>>>>>> [   12.026649][  T488]  dump_stack_lvl+0xb4/0x110
> >>>>>>>> [   12.027060][  T488]  lockdep_rcu_suspicious+0x158/0x1f0
> >>>>>>>> [   12.027541][  T488]  find_kallsyms_symbol_value+0xe8/0x110
> >>>>>>>> [   12.028028][  T488]  bpf_check_attach_target+0x838/0xa20
> >>>>>>>> [   12.028511][  T488]  check_attach_btf_id+0x144/0x3f0
> >>>>>>>> [   12.028957][  T488]  ? __pfx_cmp_subprogs+0x10/0x10
> >>>>>>>> [   12.029408][  T488]  bpf_check+0xeec/0x1850
> >>>>>>>> [   12.029799][  T488]  ? ktime_get_with_offset+0x124/0x1d0
> >>>>>>>> [   12.030247][  T488]  bpf_prog_load+0x87a/0xed0
> >>>>>>>> [   12.030627][  T488]  ? __lock_release+0x5f/0x160
> >>>>>>>> [   12.031010][  T488]  ? __might_fault+0x53/0xb0
> >>>>>>>> [   12.031394][  T488]  ? selinux_bpf+0x6c/0xa0
> >>>>>>>> [   12.031756][  T488]  __sys_bpf+0x53c/0x1240
> >>>>>>>> [   12.032115][  T488]  __x64_sys_bpf+0x27/0x40
> >>>>>>>> [   12.032476][  T488]  do_syscall_64+0x3e/0x90
> >>>>>>>> [   12.032835][  T488]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
> >>>>>>>
> >>>>>>> --- a/kernel/module/kallsyms.c
> >>>>>>> +++ b/kernel/module/kallsyms.c
> >>> Commit 91fb02f31505 ("module: Move kallsyms support into a separate file") hides
> >>> the answer. find_kallsyms_symbol_value() was originally a static function, and it
> >>> is only called by module_kallsyms_lookup_name() and is preemptive-protected.
> >>>
> >>> Now that we've added a call to function find_kallsyms_symbol_value(), it seems like
> >>> we should do the same thing as function module_kallsyms_lookup_name().
> >>>
> >>> Like this?
> >>> +				mod = btf_try_get_module(btf);
> >>> +				if (mod) {
> >>> +					preempt_disable();
> >>> +					addr = find_kallsyms_symbol_value(mod, tname);
> >>> +					preempt_enable();
> >>> +				} else
> >>> +					addr = 0;
> >>
> >> yes, that's what I did above, but I was just curious about the strange
> >> RCU usage Alexei commented on earlier:
> >>
> >> 	>>> +unsigned long find_kallsyms_symbol_value(struct module *mod, const char *name)
> >> 	>>> +{
> >> 	>>> +       unsigned long ret;
> >> 	>>> +
> >> 	>>> +       preempt_disable();
> >> 	>>> +       ret = __find_kallsyms_symbol_value(mod, name);
> >> 	>>> +       preempt_enable();
> >> 	>>> +       return ret;
> >> 	>>> +}
> >> 	>>
> >> 	>> That doesn't look right.
> >> 	>> I think the issue is misuse of rcu_dereference_sched in
> >> 	>> find_kallsyms_symbol_value.
> >> 	>
> >> 	> it seems to be using rcu pointer to keep symbols for module init time and
> >> 	> then core symbols for after init.. and switch between them when module is
> >> 	> loaded, hence the strange rcu usage I think
> 
> load_module
> 	post_relocation
> 		add_kallsyms
> 			mod->kallsyms = (void __rcu *)mod->init_layout.base + info->mod_kallsyms_init_off;   (1)
> 	do_init_module
> 		freeinit->module_init = mod->init_layout.base;
> 		rcu_assign_pointer(mod->kallsyms, &mod->core_kallsyms);                                      (2)
> 		if (llist_add(&freeinit->node, &init_free_list))
> 			schedule_work(&init_free_wq);
> 
> do_free_init
> 	synchronize_rcu();
> 	module_memfree(initfree->module_init);
> 
> IIUC, the RCU can help synchronize_rcu() in do_free_init() to make sure that no one
> is still using the first mod->kallsyms (1). If find_kallsyms_symbol_value() is executed
> between (1) and (2).

Yes, this seems to be another scenario where the RCU synchronization/access
is needed.

Best Regards,
Petr