vanishing ioctl handler debugging

We've had several reoprts of the CPU jumping to 0x00000000 is do_ioctl(). I
assume that there's a race and someone is zeroing out the ioctl handler while
this CPU waits for the lock_kernel().

The patch adds code to detect this, then emits stuff which will hopefuly lead
us to the culprit.

Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/ioctl.c

···
 #include <linux/fs.h>
 #include <linux/security.h>
 #include <linux/module.h>
+#include <linux/kallsyms.h>
 
 #include <asm/uaccess.h>
 #include <asm/ioctls.h>
···
 		unsigned long arg)
 {
 	int error = -ENOTTY;
+	void *f;
 
 	if (!filp->f_op)
 		goto out;
···
 		if (error == -ENOIOCTLCMD)
 			error = -EINVAL;
 		goto out;
-	} else if (filp->f_op->ioctl) {
+	} else if ((f = filp->f_op->ioctl)) {
 		lock_kernel();
-		error = filp->f_op->ioctl(filp->f_path.dentry->d_inode,
-					  filp, cmd, arg);
+		if (!filp->f_op->ioctl) {
+			printk("%s: ioctl %p disappeared\n", __FUNCTION__, f);
+			print_symbol("symbol: %s\n", (unsigned long)f);
+			dump_stack();
+		} else {
+			error = filp->f_op->ioctl(filp->f_path.dentry->d_inode,
+						  filp, cmd, arg);
+		}
 		unlock_kernel();
 	}