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Commit 49db08c3 authored by Linus Walleij's avatar Linus Walleij Committed by Greg Kroah-Hartman
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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: default avatarYing Huang <ying.huang@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: default avatarLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 00411b7b

Documentation/devices.txt

+3 −3
Original line number Diff line number Diff line
@@ -3099,9 +3099,9 @@ Your cooperation is appreciated. 
		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

+4 −0
Original line number Diff line number Diff line
@@ -91,6 +91,10 @@ __register_chrdev_region(unsigned int major, unsigned int baseminor, 
				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

+2 −0
Original line number Diff line number Diff line
@@ -2385,6 +2385,8 @@ static inline void bd_unlink_disk_holder(struct block_device *bdev, 


/* 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,