• By allowing Open(at)InRoot to opt-out of the extra work done by MkdirAll to do the necessary "partial lookups", Open(at)InRoot now does less work for both implementations (resulting in a many-fold decrease in the number of operations for openat2, and a modest improvement for non-openat2) and is far more guaranteed to match the correct openat2(RESOLVE_IN_ROOT) behaviour.

• We now use readlinkat(fd, "") where possible. For Open(at)InRoot this effectively just means that we no longer risk getting spurious errors during rename races. However, for our hardened procfs handler, this in theory should prevent mount attacks from tricking us when doing magic-link readlinks (even when using the unsafe host /proc handle). Unfortunately Reopen is still potentially vulnerable to those kinds of somewhat-esoteric attacks.

  Technically this will only work on post-2.6.39 kernels but it seems incredibly unlikely anyone is using filepath-securejoin on a pre-2011 kernel.

• Several improvements were made to the errors returned by Open(at)InRoot and MkdirAll when dealing with invalid paths under the emulated (ie. non-openat2) implementation. Previously, some paths would return the wrong error (ENOENT when the last component was a non-directory), and other paths would be returned as though they were acceptable (trailing-slash components after a non-directory would be ignored by Open(at)InRoot).

  These changes were done to match openat2's behaviour and purely is a consistency fix (most users are going to be using openat2 anyway).

Signed-off-by: Aleksa Sarai cyphar@cyphar.com