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authorKees Cook <kees+treewide@kernel.org>2026-09-02 15:31:14 -0700
committerKees Cook <kees@kernel.org>2026-09-04 21:37:00 -0700
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treec65086f9bdcd48c6360fb7cb4598bca084da1f32 /Documentation/filesystems/cramfs.rst
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treewide: refresh kmalloc_obj() conversionsgrafted
This is another run of the Coccinelle script for converting kmalloc() family of allocations to kmalloc_obj() via the existing rules in scripts/coccinelle/api/kmalloc_objs.cocci This catches both the set of kmalloc() uses added since the first kmalloc_obj() conversions in v7.0 and adds a large group missed in the first pass due to Coccinelle not interacting well with the cleanup.h scoped_...() family of macros[1]. I worked around this with spatch's "--macro-file" argument to a file with all the scoped_...() macros mapped to Coccinelle's YACFE_ITERATOR[2] as that was the closest viable control flow indicator I could find. Build tested allmodconfig on x86, arm64, arm, loongarch, mips, powerpc, riscv, and s390 with no new warnings. Link: https://lore.kernel.org/lkml/202609021314.8A9C0B8@keescook/ [1] Link: https://github.com/coccinelle/coccinelle/blob/master/standard.h [2] Signed-off-by: Kees Cook <kees+treewide@kernel.org>
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+.. SPDX-License-Identifier: GPL-2.0
+
+===========================================
+Cramfs - cram a filesystem onto a small ROM
+===========================================
+
+cramfs is designed to be simple and small, and to compress things well.
+
+It uses the zlib routines to compress a file one page at a time, and
+allows random page access. The meta-data is not compressed, but is
+expressed in a very terse representation to make it use much less
+diskspace than traditional filesystems.
+
+You can't write to a cramfs filesystem (making it compressible and
+compact also makes it _very_ hard to update on-the-fly), so you have to
+create the disk image with the "mkcramfs" utility.
+
+
+Usage Notes
+-----------
+
+File sizes are limited to less than 16MB.
+
+Maximum filesystem size is a little over 256MB. (The last file on the
+filesystem is allowed to extend past 256MB.)
+
+Only the low 8 bits of gid are stored. The current version of
+mkcramfs simply truncates to 8 bits, which is a potential security
+issue.
+
+Hard links are not preserved. mkcramfs deduplicates files with
+identical content, but two names for the same on-disk inode in the
+source tree become two separate (content-shared) entries in the
+image, and cramfs always reports a link count of 1.
+
+Cramfs directories have no ``.`` or ``..`` entries. Directories (like
+every other file on cramfs) always have a link count of 1. (There's
+no need to use -noleaf in ``find``, btw.)
+
+No timestamps are stored in a cramfs, so these default to the epoch
+(1970 GMT). Recently-accessed files may have updated timestamps, but
+the update lasts only as long as the inode is cached in memory, after
+which the timestamp reverts to 1970, i.e. moves backwards in time.
+
+The on-disk layout is host-endian: the kernel does not swab, and
+refuses to mount an image whose endianness does not match the CPU.
+For cross-builds, mkcramfs -B / -L forces the output endianness so
+that a host of one endianness can produce an image for a target of
+the other.
+
+The on-disk block size is fixed at 4096 bytes. On systems with a
+larger PAGE_SIZE you can change the #define in mkcramfs.c, with the
+caveat that the resulting image will only be readable on kernels
+configured for the same PAGE_SIZE.
+
+
+Memory Mapped cramfs image
+--------------------------
+
+The CRAMFS_MTD Kconfig option adds support for loading data directly from
+a physical linear memory range (usually non volatile memory like Flash)
+instead of going through the block device layer. This saves some memory
+since no intermediate buffering is necessary to hold the data before
+decompressing.
+
+And when data blocks are kept uncompressed and properly aligned, they will
+automatically be mapped directly into user space whenever possible providing
+eXecute-In-Place (XIP) from ROM of read-only segments. Data segments mapped
+read-write (hence they have to be copied to RAM) may still be compressed in
+the cramfs image in the same file along with non compressed read-only
+segments. Both MMU and no-MMU systems are supported. This is particularly
+handy for tiny embedded systems with very tight memory constraints.
+
+The location of the cramfs image in memory is system dependent. You must
+know the proper physical address where the cramfs image is located and
+configure an MTD device for it. Also, that MTD device must be supported
+by a map driver that implements the "point" method. Examples of such
+MTD drivers are cfi_cmdset_0001 (Intel/Sharp CFI flash) or physmap
+(Flash device in physical memory map). MTD partitions based on such devices
+are fine too. Then that device should be specified with the "mtd:" prefix
+as the mount device argument. For example, to mount the MTD device named
+"fs_partition" on the /mnt directory::
+
+ $ mount -t cramfs mtd:fs_partition /mnt
+
+To boot a kernel with this as root filesystem, suffice to specify
+something like "root=mtd:fs_partition" on the kernel command line.
+
+
+Tools
+-----
+
+A version of mkcramfs that can take advantage of the latest capabilities
+described above can be found here:
+
+https://github.com/npitre/cramfs-tools
+
+
+For /usr/share/magic
+--------------------
+
+===== ======================= =======================
+0 ulelong 0x28cd3d45 Linux cramfs offset 0
+>4 ulelong x size %d
+>8 ulelong x flags 0x%x
+>12 ulelong x future 0x%x
+>16 string >\0 signature "%.16s"
+>32 ulelong x fsid.crc 0x%x
+>36 ulelong x fsid.edition %d
+>40 ulelong x fsid.blocks %d
+>44 ulelong x fsid.files %d
+>48 string >\0 name "%.16s"
+512 ulelong 0x28cd3d45 Linux cramfs offset 512
+>516 ulelong x size %d
+>520 ulelong x flags 0x%x
+>524 ulelong x future 0x%x
+>528 string >\0 signature "%.16s"
+>544 ulelong x fsid.crc 0x%x
+>548 ulelong x fsid.edition %d
+>552 ulelong x fsid.blocks %d
+>556 ulelong x fsid.files %d
+>560 string >\0 name "%.16s"
+===== ======================= =======================
+
+
+Hacker Notes
+------------
+
+See fs/cramfs/README for filesystem layout and implementation notes.