UBSAN: shift-out-of-bounds in chkSuper

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    14240d4c Add linux-next specific files for 20201210
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=13457c5b500000
kernel config:  https://syzkaller.appspot.com/x/.config?x=6dbe20fdaa5aaebe
dashboard link: https://syzkaller.appspot.com/bug?extid=36315852ece4132ec193
compiler:       gcc (GCC) 10.1.0-syz 20200507
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1172cef3500000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15b1845b500000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+36315852ece4132ec193@xxxxxxxxxxxxxxxxxxxxxxxxx

loop0: detected capacity change from 128 to 0
================================================================================
UBSAN: shift-out-of-bounds in fs/jfs/jfs_mount.c:373:25
shift exponent -9716 is negative
CPU: 0 PID: 8511 Comm: syz-executor446 Not tainted 5.10.0-rc7-next-20201210-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:79 [inline]
 dump_stack+0x107/0x163 lib/dump_stack.c:120
 ubsan_epilogue+0xb/0x5a lib/ubsan.c:148
 __ubsan_handle_shift_out_of_bounds.cold+0xb1/0x181 lib/ubsan.c:395
 chkSuper.cold+0x1e/0x8b fs/jfs/jfs_mount.c:373
 jfs_mount+0x47/0x3d0 fs/jfs/jfs_mount.c:82
 jfs_fill_super+0x5b1/0xbc0 fs/jfs/super.c:562
 mount_bdev+0x34d/0x410 fs/super.c:1366
 legacy_get_tree+0x105/0x220 fs/fs_context.c:592
 vfs_get_tree+0x89/0x2f0 fs/super.c:1496
 do_new_mount fs/namespace.c:2896 [inline]
 path_mount+0x12ae/0x1e70 fs/namespace.c:3227
 do_mount fs/namespace.c:3240 [inline]
 __do_sys_mount fs/namespace.c:3448 [inline]
 __se_sys_mount fs/namespace.c:3425 [inline]
 __x64_sys_mount+0x27f/0x300 fs/namespace.c:3425
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x446d5a
Code: b8 08 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 fd ad fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 0f 83 da ad fb ff c3 66 0f 1f 84 00 00 00 00 00
RSP: 002b:00007fff49981e98 EFLAGS: 00000283 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007fff49981ee0 RCX: 0000000000446d5a
RDX: 0000000020000000 RSI: 0000000020000080 RDI: 00007fff49981ea0
RBP: 00007fff49981ea0 R08: 00007fff49981ee0 R09: 6f6f6c2f7665642f
R10: 0000000000000001 R11: 0000000000000283 R12: 0000000000000004
R13: 0000000000000003 R14: 0000000000000003 R15: 0000000000000001
================================================================================


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