1 SQUASHFS 4.1 - A squashed read-only filesystem for Linux 2 3 Copyright 2002-2010 Phillip Lougher <phillip (a] lougher.demon.co.uk> 4 5 Released under the GPL licence (version 2 or later). 6 7 Welcome to Squashfs 4.1. This is a tools only release, support for Squashfs 8 file systems is in mainline (2.6.29 and later). 9 10 New features in Squashfs-tools 4.1 11 ---------------------------------- 12 13 1. Support for extended attributes 14 2. Support for LZMA and LZO compression 15 3. New pseudo file features 16 17 Compatiblity 18 ------------ 19 20 Mksquashfs 4.1 generates 4.0 filesystems. These filesystems are fully 21 compatible/interchangable with filesystems generated by Mksquashfs 4.0 and are 22 mountable on 2.6.29 and later kernels. 23 24 Extended attributes (xattrs) 25 ---------------------------- 26 27 Squashfs file systems now have extended attribute support. The 28 extended attribute implementation has the following features: 29 30 1. Layout can store up to 2^48 bytes of compressed xattr data. 31 2. Number of xattrs per inode unlimited. 32 3. Total size of xattr data per inode 2^48 bytes of compressed data. 33 4. Up to 4 Gbytes of data per xattr value. 34 5. Inline and out-of-line xattr values supported for higher performance 35 in xattr scanning (listxattr & getxattr), and to allow xattr value 36 de-duplication. 37 6. Both whole inode xattr duplicate detection and individual xattr value 38 duplicate detection supported. These can obviously nest, file C's 39 xattrs can be a complete duplicate of file B, and file B's xattrs 40 can be a partial duplicate of file A. 41 7. Xattr name prefix types stored, allowing the redundant "user.", "trusted." 42 etc. characters to be eliminated and more concisely stored. 43 8. Support for files, directories, symbolic links, device nodes, fifos 44 and sockets. 45 46 Extended attribute support is in 2.6.35 and later kernels. File systems 47 with extended attributes can be mounted on 2.6.29 and later kernels, the 48 extended attributes will be ignored with a warning. 49 50 LZMA and LZO compression 51 ------------------------ 52 53 Squashfs now supports LZMA and LZO compression. 54 55 LZO support is in 2.6.36 and newer kernels. LZMA is not yet in mainline. 56 57 New Mksquashfs options 58 ---------------------- 59 60 -comp <comp> 61 62 Select <comp> compression. 63 64 The compression algorithms supported by the build of Mksquashfs can be 65 found by typing mksquashfs without any arguments. The compressors available 66 are displayed at the end of the help message, e.g. 67 68 Compressors available: 69 gzip (default) 70 lzma 71 lzo 72 73 The default compression used when -comp isn't specified on the command line 74 is indicated by "(default)". 75 76 -no-xattrs 77 Don't store extended attributes 78 79 -xattrs 80 Store extended attributes 81 82 The default behaviour of Mksquashfs with respect to extended attribute 83 storage is build time selectable. The Mksquashfs help message indicates 84 whether extended attributes are stored or not, e.g. 85 86 -no-xattrs don't store extended attributes 87 -xattrs store extended attributes (default) 88 89 shows that extended attributes are stored by default, and can be disabled 90 by the -no-xattrs option. 91 92 -no-xattrs don't store extended attributes (default) 93 -xattrs store extended attributes 94 95 shows that extended attributes are not stored by default, storage can be 96 enabled by the -xattrs option. 97 98 99 -noX 100 -noXattrCompression 101 Don't compress extended attributes 102 103 104 New Unsquashfs options 105 ---------------------- 106 107 -n[o-xattrs] 108 Don't extract xattrs in filesystem 109 110 -x[attrs] 111 Extract xattrs in filesystem 112 113 The default behaviour of Unsquashfs with respect to extended attributes 114 is build time selectable. The Unsquashfs help message indicates whether 115 extended attributes are stored or not, e.g. 116 117 -no[-xattrs] don't extract xattrs in file system 118 -x[attrs] extract xattrs in file system (default) 119 120 shows that xattrs are extracted by default. 121 122 -no[-xattrs] don't extract xattrs in file system (default) 123 -x[attrs] extract xattrs in file system 124 125 shows that xattrs are not extracted by default. 126 127 128 New pseudo file support 129 ----------------------- 130 131 Mksquashfs supports pseudo files, these allow fake files, directories, character 132 and block devices to be specified and added to the Squashfs filesystem being 133 built, rather than requiring them to be present in the source directories. 134 This, for example, allows device nodes to be added to the filesystem without 135 requiring root access. 136 137 Mksquashfs 4.1 adds support for "dynamic pseudo files" and a modify operation. 138 Dynamic pseudo files allow files to be dynamically created when Mksquashfs 139 is run, their contents being the result of running a command or piece of 140 shell script. The modifiy operation allows the mode/uid/gid of an existing 141 file in the source filesystem to be modified. 142 143 Two Mksquashfs options are supported, -p allows one pseudo file to be specified 144 on the command line, and -pf allows a pseudo file to be specified containing a 145 list of pseduo definitions, one per line. 146 147 Pseudo operations 148 ----------------- 149 150 1. Creating a dynamic file 151 -------------------------- 152 153 Pseudo definition 154 155 Filename f mode uid gid command 156 157 mode is the octal mode specifier, similar to that expected by chmod. 158 159 uid and gid can be either specified as a decimal number, or by name. 160 161 command can be an executable or a piece of shell script, and it is executed 162 by running "/bin/sh -c command". The stdout becomes the contents of 163 "Filename". 164 165 Examples: 166 167 Running a basic command 168 ----------------------- 169 170 /somedir/dmesg f 444 root root dmesg 171 172 creates a file "/somedir/dmesg" containing the output from dmesg. 173 174 Executing shell script 175 ---------------------- 176 177 RELEASE f 444 root root \ 178 if [ ! -e /tmp/ver ]; then \ 179 echo 0 > /tmp/ver; \ 180 fi; \ 181 ver=`cat /tmp/ver`; \ 182 ver=$((ver +1)); \ 183 echo $ver > /tmp/ver; \ 184 echo -n `cat /tmp/release`; \ 185 echo "-dev #"$ver `date` "Build host" `hostname` 186 187 Creates a file RELEASE containing the release name, date, build host, and 188 an incrementing version number. The incrementing version is a side-effect 189 of executing the shell script, and ensures every time Mksquashfs is run a 190 new version number is used without requiring any other shell scripting. 191 192 The above example also shows that commands can be split across multiple lines 193 using "\". Obviously as the script will be presented to the shell as a single 194 line, a semicolon is need to separate individual shell commands within the 195 shell script. 196 197 Reading from a device (or fifo/named socket) 198 -------------------------------------------- 199 200 input f 444 root root dd if=/dev/sda1 bs=1024 count=10 201 202 Copies 10K from the device /dev/sda1 into the file input. Ordinarily Mksquashfs 203 given a device, fifo, or named socket will place that special file within the 204 Squashfs filesystem, the above allows input from these special files to be 205 captured and placed in the Squashfs filesystem. 206 207 2. Creating a block or character device 208 --------------------------------------- 209 210 Pseudo definition 211 212 Filename type mode uid gid major minor 213 214 Where type is either 215 b - for block devices, and 216 c - for character devices 217 218 mode is the octal mode specifier, similar to that expected by chmod. 219 220 uid and gid can be either specified as a decimal number, or by name. 221 222 For example: 223 224 /dev/chr_dev c 666 root root 100 1 225 /dev/blk_dev b 666 0 0 200 200 226 227 creates a character device "/dev/chr_dev" with major:minor 100:1 and 228 a block device "/dev/blk_dev" with major:minor 200:200, both with root 229 uid/gid and a mode of rw-rw-rw. 230 231 3. Creating a directory 232 ----------------------- 233 234 Pseudo definition 235 236 Filename d mode uid gid 237 238 mode is the octal mode specifier, similar to that expected by chmod. 239 240 uid and gid can be either specified as a decimal number, or by name. 241 242 For example: 243 244 /pseudo_dir d 666 root root 245 246 creates a directory "/pseudo_dir" with root uid/gid and mode of rw-rw-rw. 247 248 4. Modifying attributes of an existing file 249 ------------------------------------------- 250 251 Pseudo definition 252 253 Filename m mode uid gid 254 255 mode is the octal mode specifier, similar to that expected by chmod. 256 257 uid and gid can be either specified as a decimal number, or by name. 258 259 For example: 260 261 dmesg m 666 root root 262 263 Changes the attributes of the file "dmesg" in the filesystem to have 264 root uid/gid and a mode of rw-rw-rw, overriding the attributes obtained 265 from the source filesystem. 266
