Hi, These patches are follow-up to this conversation on fsdevel which provides a good break-down of the core problem: https://www.spinics.net/lists/linux-fsdevel/msg128003.html That e-mail was in turn a follow-up to the following patch series: https://lwn.net/Articles/753917/ which tried to solve the dev portion of this problem with a bit of structure re-routing but was never accepted. We have an inconsistency in how the kernel is exporting inode number / device pairs for user space. I'm not talking about stat(2) or statx(2) here but everywhere else where an ino/dev is exported into a buffer for user space. We typically do this by simply dumping the potentially 32 bit inode->i_ino and single device from super->s_dev. Sometimes these values differ from what is returned via stat(2) or statx(2). Some filesystems might even show duplicate (but internally different!) pairs when the raw i_ino/s_dev is used. Aside from breaking software which expects these pairs to be unique, this can put the user in a situation where they might not be able to find an inode referenced from some kernel log (be it /proc/, printk buffer, etc). What's even worse - depending on how ino is exported, they might even find an existing but /wrong/ file. The following patches fix these inconsistencies by introducing a VFS helper function which calls the underlying filesystem ->getattr to get our real inode number / device pair. The returned values can then be used at those places in the kernel where we are incorrectly reporting our ino/dev pair. We then update fs/proc/ and fs/locks.c to use this helper when writing to /proc/PID/maps and /proc/locks respectively. For anyone who is watching the evolution of these patches, this would be one implementation of the 'callback' method which I discussed in my last e-mail. This approach is very straight forward and easy to understand but has some drawbacks: - Not all of the call sites can tolerate errors. Instead, we fallback to inode->i_ino and super->s_dev in case of getattr error. That behavior is no worse than what we have today but what we have today is pretty bad so I don't particularly feel good about that. It's better than nothing though. - There are places in the kernel where we could never call into ->getattr. There are tons of tracepoints for example that are printing the the wrong ino/dev pair. - The helper function has to fake a path struct because getting a vfsmount from inside these call sites is impossible. This isn't actually a big deal as nobody except NFS cares and the NFS patch is very trivial. It's ugly though, so if we had consensus on this approach I would happily rework our ->getattr function signature, perhaps pulling the vfsmount and dentry arguments back out. - The dentry argument to ->getattr is potentially problematic as we have some places which _only_ have an inode. Even if we had a dentry I'm not sure what would keep it from going negative while in the middle of our ->getattr call. Comments and review would be greatly appreciated. Thanks, --Mark -- To unsubscribe from this list: send the line "unsubscribe linux-unionfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
