[PATCH] block: deprecate autoloading based on dev_t

[PATCH] block: deprecate autoloading based on dev_t

[Christoph Hellwig]

Make the legacy dev_t based autoloading optional and add a deprecation
warning.  This kind of autoloading has ceased to be useful about 20 years
ago.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 block/Kconfig | 11 +++++++++++
 block/bdev.c  |  9 ++++++---
 block/genhd.c |  6 ++++++
 3 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/block/Kconfig b/block/Kconfig
index d5d4197b7ed2d..9cd8706faa7be 100644
--- a/block/Kconfig
+++ b/block/Kconfig
@@ -26,6 +26,17 @@ menuconfig BLOCK
 
 if BLOCK
 
+config BLOCK_LEGACY_AUTOLOAD
+	bool "Legacy autoloading support"
+	help
+	  Enable loading modules and creating block device instances based on
+	  accesses through their device special file.  This is a historic Linux
+	  feature and makes no sense in a udev world where device files are
+	  created on demand.
+
+	  Say N here unless your boot broke without it, in which case you should
+	  send a report to linux-block@vger.kernel.org and your distribution.
+
 config BLK_RQ_ALLOC_TIME
 	bool
 
diff --git a/block/bdev.c b/block/bdev.c
index 8bf93a19041b7..dd6fd5b807b30 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -738,12 +738,15 @@ struct block_device *blkdev_get_no_open(dev_t dev)
 	struct inode *inode;
 
 	inode = ilookup(blockdev_superblock, dev);
-	if (!inode) {
+	if (!inode && IS_ENABLED(CONFIG_BLOCK_LEGACY_AUTOLOAD)) {
 		blk_request_module(dev);
 		inode = ilookup(blockdev_superblock, dev);
-		if (!inode)
-			return NULL;
+		if (inode)
+			pr_warn_ratelimited(
+"block device autoloading is deprecated in will be removed in Linux 5.19\n");
 	}
+	if (!inode)
+		return NULL;
 
 	/* switch from the inode reference to a device mode one: */
 	bdev = &BDEV_I(inode)->bdev;
diff --git a/block/genhd.c b/block/genhd.c
index 626c8406f21a6..6ae990ff02660 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -185,7 +185,9 @@ static struct blk_major_name {
 	struct blk_major_name *next;
 	int major;
 	char name[16];
+#ifdef CONFIG_BLOCK_LEGACY_AUTOLOAD
 	void (*probe)(dev_t devt);
+#endif
 } *major_names[BLKDEV_MAJOR_HASH_SIZE];
 static DEFINE_MUTEX(major_names_lock);
 static DEFINE_SPINLOCK(major_names_spinlock);
@@ -275,7 +277,9 @@ int __register_blkdev(unsigned int major, const char *name,
 	}
 
 	p->major = major;
+#ifdef CONFIG_BLOCK_LEGACY_AUTOLOAD
 	p->probe = probe;
+#endif
 	strlcpy(p->name, name, sizeof(p->name));
 	p->next = NULL;
 	index = major_to_index(major);
@@ -679,6 +683,7 @@ static ssize_t disk_badblocks_store(struct device *dev,
 	return badblocks_store(disk->bb, page, len, 0);
 }
 
+#ifdef CONFIG_BLOCK_LEGACY_AUTOLOAD
 void blk_request_module(dev_t devt)
 {
 	unsigned int major = MAJOR(devt);
@@ -698,6 +703,7 @@ void blk_request_module(dev_t devt)
 		/* Make old-style 2.4 aliases work */
 		request_module("block-major-%d", MAJOR(devt));
 }
+#endif /* CONFIG_BLOCK_LEGACY_AUTOLOAD */
 
 /*
  * print a full list of all partitions - intended for places where the root
-- 
2.30.2

Re: [PATCH] block: deprecate autoloading based on dev_t

[Tetsuo Handa]

On 2022/01/04 4:03, Christoph Hellwig wrote:
> Make the legacy dev_t based autoloading optional and add a deprecation
> warning.  This kind of autoloading has ceased to be useful about 20 years
> ago.

This is good for locking dependency simplification. But

> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  block/Kconfig | 11 +++++++++++
>  block/bdev.c  |  9 ++++++---
>  block/genhd.c |  6 ++++++
>  3 files changed, 23 insertions(+), 3 deletions(-)
> 
> diff --git a/block/Kconfig b/block/Kconfig
> index d5d4197b7ed2d..9cd8706faa7be 100644
> --- a/block/Kconfig
> +++ b/block/Kconfig
> @@ -26,6 +26,17 @@ menuconfig BLOCK
>  
>  if BLOCK
>  
> +config BLOCK_LEGACY_AUTOLOAD
> +	bool "Legacy autoloading support"
> +	help
> +	  Enable loading modules and creating block device instances based on
> +	  accesses through their device special file.  This is a historic Linux
> +	  feature and makes no sense in a udev world where device files are
> +	  created on demand.
> +
> +	  Say N here unless your boot broke without it, in which case you should
> +	  send a report to linux-block@vger.kernel.org and your distribution.

I don't think the affected scope is limited to the boot process.
This change affects userspace programs which do not run in the boot process.
A program which do not use ioctl(LOOP_CTL_GET_FREE) would be possible.

Current:
----------
root@fuzz:~# ls -l /dev/loop*
crw-rw---- 1 root disk 10, 237 Jan  4 09:29 /dev/loop-control
root@fuzz:~# mknod /dev/loop0 b 7 0
root@fuzz:~# cat /dev/loop0
root@fuzz:~# echo $?
0
----------

With CONFIG_BLOCK_LEGACY_AUTOLOAD=n:
----------
root@fuzz:~# ls -l /dev/loop*
crw-rw---- 1 root disk 10, 237 Jan  4 09:33 /dev/loop-control
root@fuzz:~# mknod /dev/loop0 b 7 0
root@fuzz:~# cat /dev/loop0
cat: /dev/loop0: No such device or address
root@fuzz:~# echo $?
1
root@fuzz:~# losetup /dev/loop0 testfile
losetup: /dev/loop0: failed to set up loop device: No such device or address
----------

> +
>  config BLK_RQ_ALLOC_TIME
>  	bool
>  
> diff --git a/block/bdev.c b/block/bdev.c
> index 8bf93a19041b7..dd6fd5b807b30 100644
> --- a/block/bdev.c
> +++ b/block/bdev.c
> @@ -738,12 +738,15 @@ struct block_device *blkdev_get_no_open(dev_t dev)
>  	struct inode *inode;
>  
>  	inode = ilookup(blockdev_superblock, dev);
> -	if (!inode) {
> +	if (!inode && IS_ENABLED(CONFIG_BLOCK_LEGACY_AUTOLOAD)) {
>  		blk_request_module(dev);
>  		inode = ilookup(blockdev_superblock, dev);
> -		if (!inode)
> -			return NULL;
> +		if (inode)
> +			pr_warn_ratelimited(
> +"block device autoloading is deprecated in will be removed in Linux 5.19\n");

s/deprecated in/deprecated; it/

>  	}
> +	if (!inode)
> +		return NULL;
>  
>  	/* switch from the inode reference to a device mode one: */
>  	bdev = &BDEV_I(inode)->bdev;