bisecting fixing commit since 14cfdbd39e316efd91ae6e403ef8211f0b022603
building syzkaller on 78267cec1aaa5e066d66e6a6c76fea1753e51b46
testing commit 14cfdbd39e316efd91ae6e403ef8211f0b022603 with gcc (GCC) 8.1.0
kernel signature: 242d2daad4bb163aad4568257a316d2ece01bc7fe0812483b1094cf7544238bc
all runs: crashed: BUG: unable to handle kernel NULL pointer dereference in generic_perform_write
testing current HEAD 1bab61d3e8cd96f2badf515dcb06e4e1029bc017
testing commit 1bab61d3e8cd96f2badf515dcb06e4e1029bc017 with gcc (GCC) 8.1.0
kernel signature: 57c50b68784fd8395687888ca963f16d1e99000b819e260ad77cb29ed4966bc8
all runs: crashed: BUG: unable to handle kernel NULL pointer dereference in generic_perform_write
revisions tested: 2, total time: 25m59.277519488s (build: 18m36.325492156s, test: 6m59.36223891s)
the crash still happens on HEAD
commit msg: Linux 4.19.124
crash: BUG: unable to handle kernel NULL pointer dereference in generic_perform_write
EXT4-fs: 137 callbacks suppressed
EXT4-fs (sda1): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
EXT4-fs (sda1): warning: refusing change of dax flag with busy inodes while remounting
EXT4-fs (sda1): re-mounted. Opts: dax,
BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
PGD a9118067 P4D a9118067 PUD 8db58067 PMD 0 
Oops: 0010 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 11670 Comm: syz-executor.5 Not tainted 4.19.124-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:          (null)
Code: Bad RIP value.
RSP: 0018:ffff88806fe679c8 EFLAGS: 00010246
RAX: ffffffff8738cea0 RBX: 0000000000000001 RCX: 0000000000000001
RDX: 0000000000000000 RSI: ffff888073232478 RDI: ffff8880a72b8280
RBP: ffff88806fe67b00 R08: 0000000000000000 R09: ffff88806fe67a58
R10: ffffed100e646489 R11: ffff88807323244f R12: 0000000000000000
R13: ffff88806fe67ad8 R14: ffff888073232478 R15: ffff888093f5c4c0
FS:  00007fe6516d2700(0000) GS:ffff8880aea00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 0000000095f6a000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 generic_perform_write+0x289/0x480 mm/filemap.c:3162
 __generic_file_write_iter+0x201/0x580 mm/filemap.c:3287
 ext4_file_write_iter+0x282/0xe60 fs/ext4/file.c:272
 call_write_iter include/linux/fs.h:1821 [inline]
 new_sync_write fs/read_write.c:474 [inline]
 __vfs_write+0x443/0x890 fs/read_write.c:487
 vfs_write+0x150/0x4d0 fs/read_write.c:549
 ksys_write+0x103/0x260 fs/read_write.c:599
 __do_sys_write fs/read_write.c:611 [inline]
 __se_sys_write fs/read_write.c:608 [inline]
 __x64_sys_write+0x6e/0xb0 fs/read_write.c:608
 do_syscall_64+0xd0/0x4e0 arch/x86/entry/common.c:293
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45c849
Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fe6516d1c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fe6516d26d4 RCX: 000000000045c849
RDX: 0000000000000001 RSI: 0000000020000080 RDI: 0000000000000003
RBP: 000000000076bfa0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 0000000000000cdc R14: 00000000004cf01a R15: 000000000076bfac
Modules linked in:
CR2: 0000000000000000
---[ end trace 5e23e9ce2405f1e2 ]---
RIP: 0010:          (null)
Code: Bad RIP value.
RSP: 0018:ffff88806fe679c8 EFLAGS: 00010246
RAX: ffffffff8738cea0 RBX: 0000000000000001 RCX: 0000000000000001
RDX: 0000000000000000 RSI: ffff888073232478 RDI: ffff8880a72b8280
RBP: ffff88806fe67b00 R08: 0000000000000000 R09: ffff88806fe67a58
R10: ffffed100e646489 R11: ffff88807323244f R12: 0000000000000000
R13: ffff88806fe67ad8 R14: ffff888073232478 R15: ffff888093f5c4c0
FS:  00007fe6516d2700(0000) GS:ffff8880aeb00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f1ffaa81000 CR3: 0000000095f6a000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400