[PATCH v2 3/9] initrd: add a generic mechanism to add fs detectors

[Julian Stecklina via B4 Relay]

From: Julian Stecklina <julian.stecklina@xxxxxxxxxxxxxxxxxxxxx>

So far all filesystems that are supported as initrd have their
filesystem detection code implemented in init/do_mounts_rd.c. A better
approach is to let filesystem implementations register a detection
hook. This allows the filesystem detection code to live with the rest
of the filesystem implementation.

We could have done a more flexible mechanism than passing a block of
data, but this simple solution works for all filesystems and keeps the
boilerplate low.

The following patches will convert each of the filesystems.

Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Julian Stecklina <julian.stecklina@xxxxxxxxxxxxxxxxxxxxx>
---
 include/asm-generic/vmlinux.lds.h |  6 ++++++
 include/linux/initrd.h            | 37 +++++++++++++++++++++++++++++++++++++
 init/do_mounts_rd.c               | 28 +++++++++++++++++++++++++++-
 3 files changed, 70 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 0d5b186abee86d27f3b02a49299155453a8c8e9e..d0816e6c41a9bbedf8f5a68c33b6f3e18014019a 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -968,8 +968,13 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
 	KEEP(*(.init.ramfs))						\
 	. = ALIGN(8);							\
 	KEEP(*(.init.ramfs.info))
+
+#define INITRD_FS_DETECT()						\
+	. = ALIGN(16);							\
+	BOUNDED_SECTION(_initrd_fs_detect)
 #else
 #define INIT_RAM_FS
+#define INITRD_FS_DETECT()
 #endif
 
 /*
@@ -1170,6 +1175,7 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
 		INIT_CALLS						\
 		CON_INITCALL						\
 		INIT_RAM_FS						\
+		INITRD_FS_DETECT()					\
 	}
 
 #define BSS_SECTION(sbss_align, bss_align, stop_align)			\
diff --git a/include/linux/initrd.h b/include/linux/initrd.h
index f1a1f4c92ded3921bf56d53bee3e20b549d851fb..25463ce9c26ad4e4e9d3b333aa9f5596585c1762 100644
--- a/include/linux/initrd.h
+++ b/include/linux/initrd.h
@@ -3,6 +3,9 @@
 #ifndef __LINUX_INITRD_H
 #define __LINUX_INITRD_H
 
+#include <linux/init.h>
+#include <linux/types.h>
+
 #define INITRD_MINOR 250 /* shouldn't collide with /dev/ram* too soon ... */
 
 /* starting block # of image */
@@ -18,12 +21,46 @@ extern int initrd_below_start_ok;
 extern unsigned long initrd_start, initrd_end;
 extern void free_initrd_mem(unsigned long, unsigned long);
 
+struct file;
+
 #ifdef CONFIG_BLK_DEV_INITRD
 extern void __init reserve_initrd_mem(void);
 extern void wait_for_initramfs(void);
+
+/*
+ * Detect a filesystem on the initrd. You get 1 KiB (BLOCK_SIZE) of
+ * data to work with. The offset of the block is specified in
+ * initrd_fs_detect().
+ *
+ * @block_data: A pointer to BLOCK_SIZE of data
+ *
+ * Returns the size of the filesystem in bytes or 0, if the filesystem
+ * was not detected.
+ */
+typedef size_t initrd_fs_detect_fn(void * const block_data);
+
+struct initrd_detect_fs {
+	initrd_fs_detect_fn *detect_fn;
+	loff_t detect_byte_offset;
+};
+
+extern struct initrd_detect_fs __start_initrd_fs_detect[];
+extern struct initrd_detect_fs __stop_initrd_fs_detect[];
+
+/*
+ * Add a filesystem detector for initrds. See the documentation of
+ * initrd_fs_detect_fn above.
+ */
+#define initrd_fs_detect(fn, byte_offset)					\
+	static const struct initrd_detect_fs __initrd_fs_detect_ ## fn		\
+	__used __section("_initrd_fs_detect") =					\
+		{ .detect_fn = fn, .detect_byte_offset = byte_offset}
+
 #else
 static inline void __init reserve_initrd_mem(void) {}
 static inline void wait_for_initramfs(void) {}
+
+#define initrd_fs_detect(detectfn)
 #endif
 
 extern phys_addr_t phys_initrd_start;
diff --git a/init/do_mounts_rd.c b/init/do_mounts_rd.c
index d026df401afa0b7458ab1f266b21830aab974b92..56c1fa935c7ee780870142923046a3d2fd2d6d96 100644
--- a/init/do_mounts_rd.c
+++ b/init/do_mounts_rd.c
@@ -58,7 +58,7 @@ static int __init
 identify_ramdisk_image(struct file *file, loff_t pos,
 		decompress_fn *decompressor)
 {
-	const int size = 512;
+	const int size = BLOCK_SIZE;
 	struct minix_super_block *minixsb;
 	struct romfs_super_block *romfsb;
 	struct cramfs_super *cramfsb;
@@ -68,6 +68,7 @@ identify_ramdisk_image(struct file *file, loff_t pos,
 	const char *compress_name;
 	unsigned long n;
 	int start_block = rd_image_start;
+	struct initrd_detect_fs *detect_fs;
 
 	buf = kmalloc(size, GFP_KERNEL);
 	if (!buf)
@@ -165,6 +166,31 @@ identify_ramdisk_image(struct file *file, loff_t pos,
 		goto done;
 	}
 
+	/* Try to find a filesystem in the initrd */
+	for (detect_fs = __start_initrd_fs_detect;
+	     detect_fs < __stop_initrd_fs_detect;
+	     detect_fs++
+	     ) {
+		size_t fs_size;
+
+		pos = (start_block * BLOCK_SIZE) + detect_fs->detect_byte_offset;
+		kernel_read(file, buf, size, &pos);
+
+		fs_size = detect_fs->detect_fn(buf);
+
+		if (fs_size == 0)
+			continue;
+
+		nblocks = (fs_size + BLOCK_SIZE + 1)
+			>> BLOCK_SIZE_BITS;
+
+		printk(KERN_NOTICE
+		       "RAMDISK: filesystem found (%d blocks)\n",
+		       nblocks);
+
+		goto done;
+	}
+
 	printk(KERN_NOTICE
 	       "RAMDISK: Couldn't find valid RAM disk image starting at %d.\n",
 	       start_block);

-- 
2.47.0