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