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

[Jiri Olsa <olsajiri@gmail.com>]

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;
+}
+
 int module_kallsyms_on_each_symbol(const char *modname,
 				   int (*fn)(void *, const char *,
 					     struct module *, unsigned long),