Re: [BUG] kernel: kcov: a possible sleep-in-atomic-context bug in kcov_ioctl()

[Al Viro]

On Thu, Jun 21, 2018 at 11:20:59AM +0800, Jia-Ju Bai wrote:
> The kernel may sleep with holding a spinlock.
> The function call path (from bottom to top) in Linux-4.16.7 is:
> 
> [FUNC] vfree --> can sleep
> kernel/kcov.c, 237: vfree in kcov_put
> kernel/kcov.c, 396: kcov_put in kcov_ioctl_locked
> kernel/kcov.c, 410: kcov_ioctl_locked in kcov_ioctl
> kernel/kcov.c, 409: spin_lock in kcov_ioctl
> 
> This bug is found by my static analysis tool (DSAC-2) and checked by my
> code review.
> 
> I do not know how to correctly fix this bug, so I just report them.

Assuming it's a bug in the first place, that is.  Note that
	* we never modify task->kcov for task != current
	* task->kcov contributes to refcount
	* opened file contributes to refcount
	* that kcov_put() of yours happens from ->ioctl() and removes
current->kcov reference; it *can't* be the last reference - the one
held by struct file used to call ->ioctl() is also there.

IOW, it's a false positive.