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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: 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>
This commit is contained in:
Linus Walleij 2016-02-19 15:36:07 +01:00 committed by Greg Kroah-Hartman
parent 00411b7b1e
commit 49db08c358
3 changed files with 9 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
Documentation/devices.txt
View File

@ -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

4
fs/char_dev.c
View File

@ -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;

2
include/linux/fs.h
View File

@ -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,