syzbot

loop8: detected capacity change from 0 to 256
==================================================================
BUG: KCSAN: data-race in data_alloc / prb_reserve

write to 0xffffffff86c6c548 of 8 bytes by task 3294 on cpu 0:
 data_alloc+0x31c/0x390 kernel/printk/printk_ringbuffer.c:1130
 prb_reserve+0x8d6/0xad0 kernel/printk/printk_ringbuffer.c:1727
 vprintk_store+0x54a/0x910 kernel/printk/printk.c:2354
 vprintk_emit+0x1a4/0x600 kernel/printk/printk.c:2455
 vprintk_default+0x26/0x30 kernel/printk/printk.c:2494
 vprintk+0x1d/0x30 kernel/printk/printk_safe.c:82
 _printk+0x79/0xa0 kernel/printk/printk.c:2504
 blk_add_partition block/partitions/core.c:542 [inline]
 blk_add_partitions block/partitions/core.c:635 [inline]
 bdev_disk_changed+0x883/0xe00 block/partitions/core.c:695
 loop_reread_partitions drivers/block/loop.c:447 [inline]
 loop_set_status+0x5db/0x6a0 drivers/block/loop.c:1276
 loop_set_status64 drivers/block/loop.c:1372 [inline]
 lo_ioctl+0x671/0x13a0 drivers/block/loop.c:1558
 blkdev_ioctl+0x387/0x460 block/ioctl.c:797
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xce/0x140 fs/ioctl.c:583
 __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:583
 x64_sys_call+0x1563/0x3020 arch/x86/include/generated/asm/syscalls_64.h:17
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffffffff86c6c548 of 8 bytes by task 3313 on cpu 1:
 desc_read kernel/printk/printk_ringbuffer.c:496 [inline]
 desc_push_tail kernel/printk/printk_ringbuffer.c:792 [inline]
 desc_reserve kernel/printk/printk_ringbuffer.c:938 [inline]
 prb_reserve+0x216/0xad0 kernel/printk/printk_ringbuffer.c:1677
 vprintk_store+0x54a/0x910 kernel/printk/printk.c:2354
 vprintk_emit+0x1a4/0x600 kernel/printk/printk.c:2455
 vprintk_default+0x26/0x30 kernel/printk/printk.c:2494
 vprintk+0x1d/0x30 kernel/printk/printk_safe.c:82
 _printk+0x79/0xa0 kernel/printk/printk.c:2504
 _fat_msg+0xb0/0xe0 fs/fat/misc.c:62
 fat__get_entry fs/fat/dir.c:104 [inline]
 fat_get_entry+0x52b/0x5d0 fs/fat/dir.c:130
 fat_get_short_entry fs/fat/dir.c:888 [inline]
 fat_subdirs+0xf6/0x190 fs/fat/dir.c:954
 fat_read_root+0x32e/0x370 fs/fat/inode.c:1351
 fat_fill_super+0x14c5/0x1c10 fs/fat/inode.c:1824
 vfat_fill_super+0x24/0x30 fs/fat/namei_vfat.c:1202
 get_tree_bdev_flags+0x291/0x300 fs/super.c:1694
 get_tree_bdev+0x1f/0x30 fs/super.c:1717
 vfat_get_tree+0x1c/0x30 fs/fat/namei_vfat.c:1207
 vfs_get_tree+0x57/0x1d0 fs/super.c:1754
 fc_mount fs/namespace.c:1193 [inline]
 do_new_mount_fc fs/namespace.c:3758 [inline]
 do_new_mount+0x288/0x8d0 fs/namespace.c:3834
 path_mount+0x4d0/0xbc0 fs/namespace.c:4154
 do_mount fs/namespace.c:4167 [inline]
 __do_sys_mount fs/namespace.c:4383 [inline]
 __se_sys_mount+0x28c/0x2e0 fs/namespace.c:4360
 __x64_sys_mount+0x67/0x80 fs/namespace.c:4360
 x64_sys_call+0x2d61/0x3020 arch/x86/include/generated/asm/syscalls_64.h:166
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xfffffffffffd6630 -> 0x000000000009cde0

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 3313 Comm: syz.8.13405 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
==================================================================
FAT-fs (loop8): Directory bread(block 64) failed
FAT-fs (loop8): Directory bread(block 65) failed
FAT-fs (loop8): Directory bread(block 66) failed
FAT-fs (loop8): Directory bread(block 67) failed
FAT-fs (loop8): Directory bread(block 68) failed
FAT-fs (loop8): Directory bread(block 69) failed
FAT-fs (loop8): Directory bread(block 70) failed
FAT-fs (loop8): Directory bread(block 71) failed
FAT-fs (loop8): Directory bread(block 72) failed
FAT-fs (loop8): Directory bread(block 73) failed
bio_check_eod: 8826 callbacks suppressed
syz.8.13405: attempt to access beyond end of device
loop8: rw=0, sector=1192, nr_sectors = 4 limit=256