[syzbot] [trace?] [block?] BUG: unable to handle kernel NULL pointer dereference in print_one_line

[syzbot <syzbot+2fca70610179be6c24a4@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    2637bdec0535 Add linux-next specific files for 20261102
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
console output: https://syzkaller.appspot.com/x/log.txt?x=15de1eea580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=82467d52d846ff27
dashboard link: https://syzkaller.appspot.com/bug?extid=2fca70610179be6c24a4
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1276162d580000

AI-generated root cause summary:
When blktrace events are recorded with version 1 (configured via the
BLKTRACESETUP ioctl) while the blk ftrace tracer is enabled, __blk_add_trace()
writes events using the v1 struct blk_io_trace layout instead of the v2 struct
blk_io_trace2 layout expected by ftrace. In print_one_line(), the entry is
unconditionally cast to struct blk_io_trace2, interpreting the process PID (at
offset 32 in v1) as the action field (at offset 32 in v2). If this value
corresponds to 14 (__BLK_TA_BOUNCE), the bounds check against
ARRAY_SIZE(what2act) passes, but what2act[14].print is NULL because
__BLK_TA_BOUNCE is an uninitialized entry in what2act[], leading to an
instruction fetch at address 0x0 and a kernel panic.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=2fca70610179be6c24a4

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2fca70610179be6c24a4@syzkaller.appspotmail.com

BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor instruction fetch in kernel mode
#PF: error_code(0x0010) - not-present page
PGD 0 P4D 0 
Oops: Oops: 0010 [#1] SMP KASAN PTI
CPU: 1 UID: 0 PID: 6058 Comm: syz-executor155 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/15/2026
RIP: 0010:0x0
Code: Unable to access opcode bytes at 0xffffffffffffffd6.
RSP: 0018:ffffc900039cf9f8 EFLAGS: 00010206
RAX: 1ffff1100f6dc81e RBX: 0000000000000000 RCX: ffff888027a8ddc0
RDX: 0000000000000001 RSI: ffff88807b428004 RDI: ffff88807b6e20f0
RBP: ffffffff8c334eb0 R08: ffff88807b6e215d R09: 1ffff1100f6dc42b
R10: dffffc0000000000 R11: 0000000000000000 R12: ffff88807b6e0000
R13: ffffffff81d21760 R14: ffff88807b6e20f0 R15: ffff88807b6e40f0
FS:  000055555b9db400(0000) GS:ffff8881234ff000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 000000005d394000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 print_one_line+0x29d/0x300 kernel/trace/blktrace.c:1757
 s_show+0x142/0x320 kernel/trace/trace.c:3332
 seq_read_iter+0x8d5/0xca0 fs/seq_file.c:273
 seq_read+0x367/0x480 fs/seq_file.c:163
 vfs_read+0x213/0xa80 fs/read_write.c:572
 ksys_read+0x150/0x270 fs/read_write.c:716
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fc3268e9437
Code: 48 89 fa 4c 89 df e8 98 1e 00 00 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 1a 5b c3 0f 1f 84 00 00 00 00 00 48 8b 44 24 10 0f 05 <5b> c3 0f 1f 80 00 00 00 00 83 e2 39 83 fa 08 75 de e8 23 ff ff ff
RSP: 002b:00007ffe64d7f280 EFLAGS: 00000202 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 000055555b9db400 RCX: 00007fc3268e9437
RDX: 0000000000001000 RSI: 00007ffe64d7f530 RDI: 0000000000000005
RBP: 0000000000000005 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000202 R12: 00007fc326926928
R13: 0000000000000004 R14: 00007ffe64d7f430 R15: 00007ffe64d7f2fc
 </TASK>
Modules linked in:
CR2: 0000000000000000
---[ end trace 0000000000000000 ]---
RIP: 0010:0x0
Code: Unable to access opcode bytes at 0xffffffffffffffd6.
RSP: 0018:ffffc900039cf9f8 EFLAGS: 00010206
RAX: 1ffff1100f6dc81e RBX: 0000000000000000 RCX: ffff888027a8ddc0
RDX: 0000000000000001 RSI: ffff88807b428004 RDI: ffff88807b6e20f0
RBP: ffffffff8c334eb0 R08: ffff88807b6e215d R09: 1ffff1100f6dc42b
R10: dffffc0000000000 R11: 0000000000000000 R12: ffff88807b6e0000
R13: ffffffff81d21760 R14: ffff88807b6e20f0 R15: ffff88807b6e40f0
FS:  000055555b9db400(0000) GS:ffff8881234ff000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 000000005d394000 CR4: 00000000003526f0


---
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