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SubjectChange in register_blkdev() behavior
Date

Hi,

Before commit 133d55cdb2f "block: order /proc/devices by major number",
if register_blkdev() was called with major = [1..UINT_MAX], it used to
succeed (provided the requested major number was actually free).

However, while fixing the ordering in /proc/devices, commit 133d55cdb2f
also added this change:

@@ -309,6 +309,14 @@ int register_blkdev(unsigned int major, const char *name)
ret = major;
}

+ if (major >= BLKDEV_MAJOR_MAX) {
+ pr_err("register_blkdev: major requested (%d) is greater than the maximum (%d) for %s\n",
+ major, BLKDEV_MAJOR_MAX, name);
+
+ ret = -EINVAL;
+ goto out;
+ }
+
p = kmalloc(sizeof(struct blk_major_name), GFP_KERNEL);
if (p == NULL) {
ret = -ENOMEM;

So, after this commit, calls to register_blkdev() fail if the requested
major number is >= 512 (BLKDEV_MAJOR_MAX). I'm wondering if this was an
intentional change or not, as it wasn't explicitly called out in the
changelog (and the comment on top of register_blkdev() describing its
inputs seems quite out-of-date). This also breaks LTP testcase
block_dev/tc05, which tests for edge-cases and expects register_blkdev()
to succeed with major=UINT_MAX.

If the restriction on the major number was intentional, perhaps we
should get the LTP testcase modified for kernel versions >= 4.14.
Otherwise, we should fix register_blkdev to preserve the old behavior.
(I guess the same thing applies to commit 8a932f73e5b "char_dev: order
/proc/devices by major number" as well).

Thoughts?

Regards,
Srivatsa

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