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

[Jiri Olsa <olsajiri@gmail.com>]

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
> > >
> > >
> > > 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?
> 
> thanks,
> jirka