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;
>>
>> thanks,
>> jirka
> .
>
--
Regards,
Zhen Lei
