[PATCH 1/2] chrdev: emit a warning when we go below dynamic major range

[Linus Walleij]

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@xxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Alan Cox <alan@xxxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
---
Alan: is your list at LANANA even maintained? It seems wildly
out of sync with what's in the kernel. Maybe time to patch out
some of the text in devices.txt directing people over there?
---
 Documentation/devices.txt | 6 +++---
 fs/char_dev.c             | 4 ++++
 include/linux/fs.h        | 2 ++
 3 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/Documentation/devices.txt b/Documentation/devices.txt
index 87b4c5e82d39..0a3588a9798d 100644
--- a/Documentation/devices.txt
+++ b/Documentation/devices.txt
@@ -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
diff --git a/fs/char_dev.c b/fs/char_dev.c
index 24b142569ca9..687471dc04a0 100644
--- a/fs/char_dev.c
+++ b/fs/char_dev.c
@@ -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;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 1a2046275cdf..6301ac091e54 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2384,6 +2384,8 @@ 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,
-- 
2.4.3