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will trigger > following panic: > > BUG: kernel NULL pointer dereference, address: 0000000000000010 > Oops: Oops: 0000 [#1] SMP NOPTI > CPU: 27 UID: 0 PID: 920 Comm: bash Not tainted 6.15.0-02023-gadbdb95c8696-dirty #1238 PREEMPT(undef) > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.1-2.fc37 04/01/2014 > RIP: 0010:__bitmap_or+0x48/0x70 > Call Trace: > > __group_cpus_evenly+0x822/0x8c0 > group_cpus_evenly+0x2d9/0x490 > blk_mq_map_queues+0x1e/0x110 > null_map_queues+0xc9/0x170 [null_blk] > blk_mq_update_queue_map+0xdb/0x160 > blk_mq_update_nr_hw_queues+0x22b/0x560 > nullb_update_nr_hw_queues+0x71/0xf0 [null_blk] > nullb_device_poll_queues_store+0xa4/0x130 [null_blk] > configfs_write_iter+0x109/0x1d0 > vfs_write+0x26e/0x6f0 > ksys_write+0x79/0x180 > __x64_sys_write+0x1d/0x30 > x64_sys_call+0x45c4/0x45f0 > do_syscall_64+0xa5/0x240 > entry_SYSCALL_64_after_hwframe+0x76/0x7e > > Root cause is that numgrps is set to 0, and ZERO_SIZE_PTR is returned > from kcalloc(), then __group_cpus_evenly() will deference the > ZERO_SIZE_PTR. > > Fix the problem by checking numgrps first in group_cpus_evenly(), and > return NULL directly if numgrps is zero. > > Fixes: 6a6dcae8f486 ("blk-mq: Build default queue map via group_cpus_evenly()") > Signed-off-by: Yu Kuai > --- > Changes in v2: > - check numgrps directly. > > lib/group_cpus.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/lib/group_cpus.c b/lib/group_cpus.c > index ee272c4cefcc..d68407e3601e 100644 > --- a/lib/group_cpus.c > +++ b/lib/group_cpus.c > @@ -352,6 +352,9 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps) > int ret = -ENOMEM; > struct cpumask *masks = NULL; > > + if (numgrps == 0) > + return NULL; > + > if (!zalloc_cpumask_var(&nmsk, GFP_KERNEL)) > return NULL; > There's a non-SMP version of group_cpus_evenly() at the bottom of lib/group_cpus.c It also does a kcalloc(numgrps, ...) Does that need to be fixed as well?