Re: KMSAN broken with lockdep again?

Alexander Potapenko
2022-11-18T18:20:13Z

On Fri, Nov 18, 2022 at 2:39 PM Alexander Potapenko <glider@google.com> wrote:
>
> > > As far as I can tell, removing `KMSAN_SANITIZE_lockdep.o := n` does
> > > not actually break anything now (although the kernel becomes quite
> > > slow with both lockdep and KMSAN). Let me experiment a bit and send a
> > > patch.
>
> Hm, no, lockdep isn't particularly happy with the nested
> lockdep->KMSAN->lockdep calls:
>
> ------------[ cut here ]------------
> DEBUG_LOCKS_WARN_ON(lockdep_hardirqs_enabled())
> WARNING: CPU: 0 PID: 0 at kernel/locking/lockdep.c:5508 check_flags+0x63/0x180
> ...
> <TASK>
> lock_acquire+0x196/0x640 kernel/locking/lockdep.c:5665
> __raw_spin_lock_irqsave ./include/linux/spinlock_api_smp.h:110
> _raw_spin_lock_irqsave+0xb3/0x110 kernel/locking/spinlock.c:162
> __stack_depot_save+0x1b1/0x4b0 lib/stackdepot.c:479
> stack_depot_save+0x13/0x20 lib/stackdepot.c:533
> __msan_poison_alloca+0x100/0x1a0 mm/kmsan/instrumentation.c:263
> native_save_fl ./include/linux/spinlock_api_smp.h:?
> arch_local_save_flags ./arch/x86/include/asm/irqflags.h:70
> arch_irqs_disabled ./arch/x86/include/asm/irqflags.h:130
> __raw_spin_unlock_irqrestore ./include/linux/spinlock_api_smp.h:151
> _raw_spin_unlock_irqrestore+0x60/0x100 kernel/locking/spinlock.c:194
> tty_register_ldisc+0xcb/0x120 drivers/tty/tty_ldisc.c:68
> n_tty_init+0x1f/0x21 drivers/tty/n_tty.c:2521
> console_init+0x1f/0x7ee kernel/printk/printk.c:3287
> start_kernel+0x577/0xaff init/main.c:1073
> x86_64_start_reservations+0x2a/0x2c arch/x86/kernel/head64.c:556
> x86_64_start_kernel+0x114/0x119 arch/x86/kernel/head64.c:537
> secondary_startup_64_no_verify+0xcf/0xdb arch/x86/kernel/head_64.S:358
> </TASK>
> ---[ end trace 0000000000000000 ]---

In fact, this message is printed in both cases: with and without KMSAN
instrumenting kernel/locking/lockdep.c
I wonder if this is a sign of a real problem in KMSAN, or just an
unavoidable consequence of instrumented code calling lockdep when
taking the stackdepot lock...

> > > If this won't work out, we'll need an explicit call to
> > > kmsan_unpoison_memory() somewhere in lockdep_init_map_type() to
> > > suppress these reports.
>
> I'll go for this option.
>
> > Thanks.
> >
> > I tried just disabling CONFIG_PROVE_LOCKING, but now KMSAN warnings are being
> > spammed from check_stack_object() in mm/usercopy.c.
> >
> > Commenting out the call to arch_within_stack_frames() makes it go away.
>
> Yeah, arch_within_stack_frames() performs stack frame walking, which
> confuses KMSAN.
> We'll need to apply __no_kmsan_checks to it, like we did for other
> stack unwinding functions.

Sent the patch.