Commit 49db08c3 authored Feb 19, 2016 by Linus Walleij Committed by Greg Kroah-Hartman Mar 29, 2016

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 <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: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg 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,

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