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@xxxxxxxxx> 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
> >>>>
> >>>>
> >>>> hum, for some reason I can't reproduce, but looks like we need to disable
> >>>> preemption for find_kallsyms_symbol_value.. could you please check with
> >>>> patch below?
> >>>>
> >>>> also could you please share your .config? not sure why I can't reproduce
> >>>>
> >>>> thanks,
> >>>> jirka
> >>>>
> >>>>
> >>>> ---
> >>>> diff --git a/kernel/module/kallsyms.c b/kernel/module/kallsyms.c
> >>>> index ab2376a1be88..bdc911dbcde5 100644
> >>>> --- a/kernel/module/kallsyms.c
> >>>> +++ b/kernel/module/kallsyms.c
> >>>> @@ -442,7 +442,7 @@ int module_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
> >>>> }
> >>>>
> >>>> /* Given a module and name of symbol, find and return the symbol's value */
> >>>> -unsigned long find_kallsyms_symbol_value(struct module *mod, const char *name)
> >>>> +static unsigned long __find_kallsyms_symbol_value(struct module *mod, const char *name)
> >>>> {
> >>>> unsigned int i;
> >>>> struct mod_kallsyms *kallsyms = rcu_dereference_sched(mod->kallsyms);
> >>>> @@ -466,7 +466,7 @@ static unsigned long __module_kallsyms_lookup_name(const char *name)
> >>>> if (colon) {
> >>>> mod = find_module_all(name, colon - name, false);
> >>>> if (mod)
> >>>> - return find_kallsyms_symbol_value(mod, colon + 1);
> >>>> + return __find_kallsyms_symbol_value(mod, colon + 1);
> >>>> return 0;
> >>>> }
> >>>>
> >>>> @@ -475,7 +475,7 @@ static unsigned long __module_kallsyms_lookup_name(const char *name)
> >>>>
> >>>> if (mod->state == MODULE_STATE_UNFORMED)
> >>>> continue;
> >>>> - ret = find_kallsyms_symbol_value(mod, name);
> >>>> + ret = __find_kallsyms_symbol_value(mod, name);
> >>>> if (ret)
> >>>> return ret;
> >>>> }
> >>>> @@ -494,6 +494,16 @@ unsigned long module_kallsyms_lookup_name(const char *name)
> >>>> return ret;
> >>>> }
> >>>>
> >>>> +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
> >>
> >> Petr, Zhen, any idea/insight?
>
> 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
>
> Petr, Zhen, any idea/insight?
thanks,
jirka
