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common: Drop functions to disable/enable ext[234] read-only hack

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The last user of this code switched to disk_util in
b3120441722365e74200d14644b5ebbb597cf372.
This commit is contained in:
Benjamin Gilbert 2017-04-21 13:59:31 -07:00
parent a77edcddf0
commit 97b09dc943
1 changed files with 0 additions and 41 deletions
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common.sh
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@ -660,47 +660,6 @@ get_git_id() {
git var GIT_COMMITTER_IDENT | sed -e 's/^.*<\(\S\+\)>.*$/\1/'
}
# These two helpers clobber the ro compat value in our root filesystem.
#
# When the system is built with --enable_rootfs_verification, bit-precise
# integrity checking is performed. That precision poses a usability issue on
# systems that automount partitions with recognizable filesystems, such as
# ext2/3/4. When the filesystem is mounted 'rw', ext2 metadata will be
# automatically updated even if no other writes are performed to the
# filesystem. In addition, ext2+ does not support a "read-only" flag for a
# given filesystem. That said, forward and backward compatibility of
# filesystem features are supported by tracking if a new feature breaks r/w or
# just write compatibility. We abuse the read-only compatibility flag[1] in
# the filesystem header by setting the high order byte (le) to FF. This tells
# the kernel that features R24-R31 are all enabled. Since those features are
# undefined on all ext-based filesystem, all standard kernels will refuse to
# mount the filesystem as read-write -- only read-only[2].
#
# [1] 32-bit flag we are modifying:
# http://git.chromium.org/cgi-bin/gitweb.cgi?p=kernel.git;a=blob;f=include/linux/ext2_fs.h#l417
# [2] Mount behavior is enforced here:
# http://git.chromium.org/cgi-bin/gitweb.cgi?p=kernel.git;a=blob;f=fs/ext2/super.c#l857
#
# N.B., if the high order feature bits are used in the future, we will need to
# revisit this technique.
disable_rw_mount() {
local rootfs=$1
local offset="${2-0}" # in bytes
local ro_compat_offset=$((0x464 + 3)) # Set 'highest' byte
printf '\377' |
sudo dd of="${rootfs}" seek=$((offset + ro_compat_offset)) \
conv=notrunc count=1 bs=1
}
enable_rw_mount() {
local rootfs=$1
local offset="${2-0}"
local ro_compat_offset=$((0x464 + 3)) # Set 'highest' byte
printf '\000' |
sudo dd of="${rootfs}" seek=$((offset + ro_compat_offset)) \
conv=notrunc count=1 bs=1
}
# Generate a DIGESTS file, as normally used by Gentoo.
# This is an alternative to shash which doesn't know how to report errors.
# Usage: make_digests -d file.DIGESTS file1 [file2...]