On 2020/2/2 上午10:16, Andreas Dilger wrote: > On Feb 1, 2020, at 7:02 PM, Qu Wenruo <quwenruo.btrfs@xxxxxxx> wrote: >> >> Hi ext4 guys, >> >> Recently I found an image from android (vendor.img) has this RO feature >> set, but kernel doesn't support it, thus no easy way to modify it. >> (Although I can just modify the underlying block for my purpose, it's >> just one line change, I still want a more elegant way). >> >> Thus it can only be mounted RO. So far so good, as from its name, it's >> kinda of deduped (BTW, both XFS and Btrfs supports RW mount for >> reflinked/deduped fs). >> >> But the problem is, how to create such image? >> >> Man page of mke2fs has no mention of such thing at all, and obviously >> for whoever comes up with such "brilliant" way to block users from >> modifying things, the "-E unshare_blocks" will just make the image too >> large for the device. >> >> Or we must go the Android rabbit hole to find an exotic tool to modify >> even one line of a config file? > > I believe that this feature was only implemented inside Google. Well, "Don't be evil" is a joke now, right? > > However, if you want to make changes to some files in this filesystem > there should be a number of ways to do it: > - use "dd" to dump file block(s) from image, edit them, then write back. > use "debugfs -c -R 'stat /path/to/file' vendor.img" for block addresses Exactly what I'm going to try. (Thankfully EXT4 hasn't implemented data checksum) Thanks for the info, Qu > - use debugfs to clear the flag, mount the filesystem normally, then > overwrite the file *in place* (using "dd" or similar) so that the > blocks for the shared file are not reallocated due to unlink, write > - make a simple patch for the kernel to "support" this feature, then > mount it and modify the file in a similar manner > > Cheers, Andreas > > > > >
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