Re: [PATCH v3] lockdep: Allow tuning tracing capacity constants.

[Dmitry Vyukov <dvyukov@google.com>]

On Thu, Nov 19, 2020 at 1:43 PM Dmitry Vyukov <dvyukov@google.com> wrote:
> > On Wed, Nov 18, 2020 at 4:32 PM Tetsuo Handa
> > <penguin-kernel@i-love.sakura.ne.jp> wrote:
> > >
> > > On 2020/11/19 0:10, Peter Zijlstra wrote:
> > > > On Wed, Nov 18, 2020 at 11:30:05PM +0900, Tetsuo Handa wrote:
> > > >> The problem is that we can't know what exactly is consuming these resources.
> > > >> My question is do you have a plan to make it possible to know what exactly is
> > > >> consuming these resources.
> > > >
> > > > I'm pretty sure it's in /proc/lockdep* somewhere.
> > >
> > > OK. Then...
> > >
> > > Dmitry, can you update syzkaller to dump /proc/lockdep* before terminating as
> > > a crash as soon as encountering one of
> > >
> > >   BUG: MAX_LOCKDEP_ENTRIES too low!
> > >   BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!
> > >   BUG: MAX_LOCKDEP_CHAINS too low!
> > >   BUG: MAX_LOCKDEP_KEYS too low!
> > >   WARNING in print_bfs_bug
> > >
> > > messages?
> > >
> > > On 2020/09/16 21:14, Dmitry Vyukov wrote:
> > > > On Wed, Sep 16, 2020 at 1:51 PM <peterz@infradead.org> wrote:
> > > >>
> > > >> On Wed, Sep 16, 2020 at 01:28:19PM +0200, Dmitry Vyukov wrote:
> > > >>> On Fri, Sep 4, 2020 at 6:05 PM Tetsuo Handa
> > > >>> <penguin-kernel@i-love.sakura.ne.jp> wrote:
> > > >>>>
> > > >>>> Hello. Can we apply this patch?
> > > >>>>
> > > >>>> This patch addresses top crashers for syzbot, and applying this patch
> > > >>>> will help utilizing syzbot's resource for finding other bugs.
> > > >>>
> > > >>> Acked-by: Dmitry Vyukov <dvyukov@google.com>
> > > >>>
> > > >>> Peter, do you still have concerns with this?
> > > >>
> > > >> Yeah, I still hate it with a passion; it discourages thinking. A bad
> > > >> annotation that blows up the lockdep storage, no worries, we'll just
> > > >> increase this :/
> > > >>
> > > >> IIRC the issue with syzbot is that the current sysfs annotation is
> > > >> pretty terrible and generates a gazillion classes, and syzbot likes
> > > >> poking at /sys a lot and thus floods the system.
> > > >>
> > > >> I don't know enough about sysfs to suggest an alternative, and haven't
> > > >> exactly had spare time to look into it either :/
> > > >>
> > > >> Examples of bad annotations is getting every CPU a separate class, that
> > > >> leads to nr_cpus! chains if CPUs arbitrarily nest (nr_cpus^2 if there's
> > > >> only a single nesting level).
> > > >
> > > > Maybe on "BUG: MAX_LOCKDEP_CHAINS too low!" we should then aggregate,
> > > > sort and show existing chains so that it's possible to identify if
> > > > there are any worst offenders and who they are.
> > > >
> > > > Currently we only have a hypothesis that there are some worst
> > > > offenders vs lots of normal load. And we can't point fingers which
> > > > means that, say, sysfs, or other maintainers won't be too inclined to
> > > > fix anything.
> > > >
> > > > If we would know for sure that lock class X is guilty. That would make
> > > > the situation much more actionable.
> >
> > I am trying to reproduce this locally first. syzbot caims it can
> > reproduce it with a number of very simpler reproducers (like spawn
> > process, unshare, create socket):
> > https://syzkaller.appspot.com/bug?id=8a18efe79140782a88dcd098808d6ab20ed740cc
> >
> > I see a very slow drift, but it's very slow, so get only to:
> >  direct dependencies:                 22072 [max: 32768]
> >
> > But that's running a very uniform workload.
> >
> > However when I tried to cat /proc/lockdep to see if there is anything
> > fishy already,
> > I got this (on c2e7554e1b85935d962127efa3c2a76483b0b3b6).
> >
> > Some missing locks?
> >
> > ==================================================================
> > BUG: KASAN: use-after-free in string_nocheck lib/vsprintf.c:611 [inline]
> > BUG: KASAN: use-after-free in string+0x39c/0x3d0 lib/vsprintf.c:693
> > Read of size 1 at addr ffff888295833740 by task less/1855
> >
> > CPU: 0 PID: 1855 Comm: less Tainted: G        W         5.10.0-rc4+ #68
> > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
> > rel-1.13.0-44-g88ab0c15525c-prebuilt.qemu.org 04/01/2014
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0x107/0x163 lib/dump_stack.c:118
> >  print_address_description.constprop.0.cold+0xae/0x4c8 mm/kasan/report.c:385
> >  __kasan_report mm/kasan/report.c:545 [inline]
> >  kasan_report.cold+0x1f/0x37 mm/kasan/report.c:562
> >  string_nocheck lib/vsprintf.c:611 [inline]
> >  string+0x39c/0x3d0 lib/vsprintf.c:693
> >  vsnprintf+0x71b/0x14f0 lib/vsprintf.c:2618
> >  seq_vprintf fs/seq_file.c:398 [inline]
> >  seq_printf+0x195/0x240 fs/seq_file.c:413
> >  print_name+0x98/0x1d0 kernel/locking/lockdep_proc.c:50
> >  l_show+0x111/0x2c0 kernel/locking/lockdep_proc.c:82
> >  seq_read_iter+0xae4/0x10c0 fs/seq_file.c:268
> >  proc_reg_read_iter+0x1fb/0x2d0 fs/proc/inode.c:310
> >  call_read_iter include/linux/fs.h:1897 [inline]
> >  new_sync_read+0x41e/0x6e0 fs/read_write.c:415
> >  vfs_read+0x35c/0x570 fs/read_write.c:496
> >  ksys_read+0x12d/0x250 fs/read_write.c:634
> >  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
> >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > RIP: 0033:0x7f1d48906310
> > Code: 73 01 c3 48 8b 0d 28 4b 2b 00 31 d2 48 29 c2 64 89 11 48 83 c8
> > ff eb ea 90 90 83 3d e5 a2 2b 00 00 75 10 b8 00 00 00 00 0f 05 <48> 3d
> > 01 f04
> > RSP: 002b:00007fff8ad3f048 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> > RAX: ffffffffffffffda RBX: 0000000000ae0ec0 RCX: 00007f1d48906310
> > RDX: 0000000000002000 RSI: 0000000000ae0eec RDI: 0000000000000004
> > RBP: 0000000000072000 R08: 0000000000000038 R09: 0000000001000000
> > R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000a6d7f0
> > R13: 0000000000000000 R14: 0000000000000039 R15: 0000000000ae0ec0
> >
> > Allocated by task 2828:
> >  kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
> >  kasan_set_track mm/kasan/common.c:56 [inline]
> >  __kasan_kmalloc.constprop.0+0xc2/0xd0 mm/kasan/common.c:461
> >  kmalloc include/linux/slab.h:557 [inline]
> >  kzalloc include/linux/slab.h:664 [inline]
> >  tomoyo_encode2.part.0+0xe9/0x3a0 security/tomoyo/realpath.c:45
> >  tomoyo_encode2 security/tomoyo/realpath.c:31 [inline]
> >  tomoyo_encode+0x28/0x50 security/tomoyo/realpath.c:80
> >  tomoyo_path_perm+0x368/0x400 security/tomoyo/file.c:831
> >  tomoyo_path_symlink+0x94/0xe0 security/tomoyo/tomoyo.c:200
> >  security_path_symlink+0xdf/0x150 security/security.c:1110
> >  do_symlinkat+0x123/0x2c0 fs/namei.c:3985
> >  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
> >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> >
> > Freed by task 2828:
> >  kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
> >  kasan_set_track+0x1c/0x30 mm/kasan/common.c:56
> >  kasan_set_free_info+0x1b/0x30 mm/kasan/generic.c:355
> >  __kasan_slab_free+0x102/0x140 mm/kasan/common.c:422
> >  slab_free_hook mm/slub.c:1544 [inline]
> >  slab_free_freelist_hook+0x5d/0x150 mm/slub.c:1577
> >  slab_free mm/slub.c:3142 [inline]
> >  kfree+0xdb/0x360 mm/slub.c:4124
> >  tomoyo_path_perm+0x3b0/0x400 security/tomoyo/file.c:840
> >  tomoyo_path_symlink+0x94/0xe0 security/tomoyo/tomoyo.c:200
> >  security_path_symlink+0xdf/0x150 security/security.c:1110
> >  do_symlinkat+0x123/0x2c0 fs/namei.c:3985
> >  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
> >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> >
> > The buggy address belongs to the object at ffff888295833740
> >  which belongs to the cache kmalloc-32 of size 32
> > The buggy address is located 0 bytes inside of
> >  32-byte region [ffff888295833740, ffff888295833760)
> > The buggy address belongs to the page:
> > page:00000000706b2f94 refcount:1 mapcount:0 mapping:0000000000000000
> > index:0x0 pfn:0x295833
> > flags: 0x57ffe0000000200(slab)
> > raw: 057ffe0000000200 ffffea0004adca40 0000000200000002 ffff888100041a00
> > raw: 0000000000000000 0000000000400040 00000001ffffffff 0000000000000000
> > page dumped because: kasan: bad access detected
> >
> > Memory state around the buggy address:
> >  ffff888295833600: fa fb fb fb fc fc fc fc fa fb fb fb fc fc fc fc
> >  ffff888295833680: fa fb fb fb fc fc fc fc fa fb fb fb fc fc fc fc
> > >ffff888295833700: fa fb fb fb fc fc fc fc fa fb fb fb fc fc fc fc
> >                                            ^
> >  ffff888295833780: 00 00 00 fc fc fc fc fc 00 00 00 fc fc fc fc fc
> >  ffff888295833800: fa fb fb fb fc fc fc fc fa fb fb fb fc fc fc fc
> > ==================================================================
>
>
> Trying again I wasn't able to reproduce the drift.
> Then I realized that running that simple workload I am getting a mix of:
> [ 1459.589213][ T3142] WARNING: CPU: 1 PID: 3142 at
> drivers/net/bonding/bond_main.c:4310
> bond_update_slave_arr+0xcaf/0x10c0
> [  222.027968][ T8662] BUG: sleeping function called from invalid
> context at net/mac80211/sta_info.c:1962
> which torns off lockdep completely.
> Perhaps it would be useful to add ON/OFF status to lockdep_stats.

I can reproduce slow drift by running just:

#define _GNU_SOURCE
#include <unistd.h>
#include <sched.h>
int main() {
    for (int i = 0; i < 16; i++) {
        if (fork() == 0) {
            for (;;) {
                unshare(CLONE_NEWNET);
            }
        }
    }
    sleep(1<<20);
}

 lock-classes:                         1765 [max: 8192]
 direct dependencies:                 12882 [max: 32768]
 dependency chains:                   16841 [max: 65536]
 dependency chain hlocks used:        65720 [max: 327680]
 stack-trace entries:                141213 [max: 524288]
...
 lock-classes:                         1768 [max: 8192]
 direct dependencies:                 13048 [max: 32768]
 dependency chains:                   17144 [max: 65536]
 dependency chain hlocks used:        67194 [max: 327680]
 stack-trace entries:                143043 [max: 524288]
...
 lock-classes:                         1768 [max: 8192]
 direct dependencies:                 13056 [max: 32768]
 dependency chains:                   17173 [max: 65536]
 dependency chain hlocks used:        67353 [max: 327680]
 stack-trace entries:                143138 [max: 524288]
...
 lock-classes:                         1770 [max: 8192]
 direct dependencies:                 13072 [max: 32768]
 dependency chains:                   17232 [max: 65536]
 dependency chain hlocks used:        67604 [max: 327680]
 stack-trace entries:                143390 [max: 524288]