Re: possible deadlock in shmem_file_llseek

[Dmitry Vyukov <dvyukov@google.com>]

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On Wed, Jan 24, 2018 at 7:42 PM, syzbot
<syzbot+8ec30bb7bf1a981a2012@syzkaller.appspotmail.com> wrote:
> Hello,
>
> syzbot tried to test the proposed patch but build/boot failed:
>
> patch is already applied
>
>
> Tested on https://github.com/joelagnel/linux.git/test-ashmem commit
> 32f813bb0d06c1e189ac336f8c3c7377f85c71f0 (Wed Jan 24 01:45:04 2018 +0000)
> ashmem: Fix lockdep issue during llseek
>
> compiler: gcc (GCC) 7.1.1 20170620
> Patch is attached.

Right. We probably want:

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
master

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diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index 0f695df14c9d..248983cf2db1 100644
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -343,7 +343,9 @@ static loff_t ashmem_llseek(struct file *file, loff_t offset, int origin)
 		goto out;
 	}
 
+	mutex_unlock(&ashmem_mutex);
 	ret = vfs_llseek(asma->file, offset, origin);
+	mutex_lock(&ashmem_mutex);
 	if (ret < 0)
 		goto out;