[syzbot] general protection fault in do_blk_trace_setup

Hello,

syzbot found the following issue on:

HEAD commit:    372b2891c15a Add linux-next specific files for 20210824
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=15dc8389300000
kernel config:  https://syzkaller.appspot.com/x/.config?x=18ce42745c8b0dd6
dashboard link: https://syzkaller.appspot.com/bug?extid=f74aa89114a236643919
compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.1
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10979275300000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10fe84a9300000

The issue was bisected to:

commit 45938335d0a9773d65a82a7ca722bb76e4b997a8
Author: Christoph Hellwig [off-list ref]
Date:   Mon Aug 16 13:19:03 2021 +0000

    st: do not allocate a gendisk

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=14682399300000
final oops:     https://syzkaller.appspot.com/x/report.txt?x=16682399300000
console output: https://syzkaller.appspot.com/x/log.txt?x=12682399300000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: <redacted>
Fixes: 45938335d0a9 ("st: do not allocate a gendisk")

general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 1 PID: 6540 Comm: syz-executor323 Not tainted 5.14.0-rc7-next-20210824-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:strncpy+0x32/0xb0 lib/string.c:130
Code: 54 55 53 48 83 ec 08 48 85 d2 74 5c 4c 8d 2c 17 48 89 fb 49 bc 00 00 00 00 00 fc ff df 48 89 f0 48 89 f2 48 c1 e8 03 83 e2 07 <42> 0f b6 04 20 38 d0 7f 04 84 c0 75 41 48 89 d8 48 89 da 0f b6 2e
RSP: 0018:ffffc90002d2fb98 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffffc90002d2fc70 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffc90002d2fc70
RBP: 0000000000000000 R08: 0000000000000000 R09: ffffc90002d2fcb7
R10: ffffffff817c66f5 R11: 0000000000000000 R12: dffffc0000000000

79 more lines

On Tue, Aug 24, 2021 at 11:19:21AM -0700, syzbot wrote:

Author: Christoph Hellwig [off-list ref]
Date:   Mon Aug 16 13:19:03 2021 +0000

    st: do not allocate a gendisk

That's very obviously a bisection one-off, the culprit was the sg
conversion.  I've send a fix.