On Sat, Dec 22, 2018 at 08:10:07PM -0800, Matthew Wilcox wrote: > Pretty much every file format has the ability to put arbitrary blocks > of information into a file somewhere the tools which don't know about > it will skip it. For example, ZIP "includes an extra field facility > within file headers, which can be used to store extra data not defined > by existing ZIP specifications, and which allow compliant archivers that > do not recognize the fields to safely skip them. Header IDs 0–31 are > reserved for use by PKWARE. The remaining IDs can be used by third-party > vendors for proprietary usage. " (Wikipedia) > > ELF, PNG, PDF and many other formats have the ability to put data > _somewhere_. It might not be at the tail of the file, but there's > somewhere to do it. > > (I appreciate this isn't what Linus is asking for, but I'm pointing out > that this is by no means as intractable as you make it sound.) That design would require the fs-verity code to know the type of eacho file, and where to find the in-band Merkle tree for each file type that we wanted to support. And if you wanted to use fs-verity to protect a sudoers text configuration file (for example), we'd have to teach sudo how to ignore the userspace visible Merkle tree. So I agree with you that it's *possible*. But it's ***ugly***. *Way* uglier than putting the Merkle tree at the end of the file data and then making it invisible to userspace. Cheers, - Ted
