chrdev: emit a warning when we go below dynamic major range

Currently a dynamically allocated character device major is taken
from 254 and downward. This mechanism is used for RTC, IIO and a
few other subsystems.

The kernel currently has no check prevening these dynamic
allocations from eating into the assigned numbers at 233 and
downward.

In a recent test it was reported that so many dynamic device
majors were used on a test server, that the major number for
infiniband (231) was stolen. This occurred when allocating a new
major number for GPIO chips. The error messages from the kernel
were not helpful. (See: https://lkml.org/lkml/2016/2/14/124)

This patch adds a defined lower limit of the dynamic major
allocation region will henceforth emit a warning if we start to
eat into the assigned numbers. It does not do any semantic
changes and will not change the kernels behaviour: numbers will
still continue to be stolen, but we will know from dmesg what
is going on.

This also updates the Documentation/devices.txt to clearly
reflect that we are using this range of major numbers for dynamic
allocation.

Reported-by: Ying Huang <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Alan Cox <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devices.txt

···
 		129 = /dev/ipath_sma    Device used by Subnet Management Agent
 		130 = /dev/ipath_diag   Device used by diagnostics programs
 
-234-239		UNASSIGNED
-
-240-254 char	LOCAL/EXPERIMENTAL USE
+234-254	char	RESERVED FOR DYNAMIC ASSIGNMENT
+		Character devices that request a dynamic allocation of major number will
+		take numbers starting from 254 and downward.
 
 240-254 block	LOCAL/EXPERIMENTAL USE
 		Allocated for local/experimental use.  For devices not

fs/char_dev.c

···
 				break;
 		}
 
+		if (i < CHRDEV_MAJOR_DYN_END)
+			pr_warn("CHRDEV \"%s\" major number %d goes below the dynamic allocation range",
+				name, i);
+
 		if (i == 0) {
 			ret = -EBUSY;
 			goto out;

include/linux/fs.h

···
 
 /* fs/char_dev.c */
 #define CHRDEV_MAJOR_HASH_SIZE	255
+/* Marks the bottom of the first segment of free char majors */
+#define CHRDEV_MAJOR_DYN_END 234
 extern int alloc_chrdev_region(dev_t *, unsigned, unsigned, const char *);
 extern int register_chrdev_region(dev_t, unsigned, const char *);
 extern int __register_chrdev(unsigned int major, unsigned int baseminor,