On musl, O_ACCMODE is defined as (03|O_SEARCH), unlike glibc which
defines it as (O_RDONLY|O_WRONLY|O_RDWR). Additionally, O_SEARCH is
simply defined as O_PATH.
This causes problems for systemd on musl, as it changes the
behaviour of open_mkdir_at_full() to return -EINVAL if O_PATH is
included in flags due to the fact that O_ACCMODE includes O_SEARCH
(i.e. O_PATH). Consequently, this makes the test-fs-util test fail.
Upstream musl seems content with this behaviour and doesn't seem
interested in matching glibc's behaviour due to that defining it this
way allows for O_SEARCH to match POSIX better by allowing it to open
directories where read permission is missing. Apparently musl does some
emulation in other places to make this work more consistently as well.
Initially I took the approach of working around this by redefining
O_SEARCH as O_RDONLY if O_SEARCH == O_PATH. This fixes the test and is
the approach taken by both XZ[1] and Gzip[2][3], but was not taken as
redefining system headers potentially could be problematic.
Instead, introduce O_ACCMODE_STRICT which just is a copy of glibc's
O_ACCMODE and use it everywhere. This way we don't have to deal with
unusual definitions of O_ACCMODE from C standard libraries other than
glibc.
[1]: https://git.tukaani.org/?p=xz.git;a=blob;f=src/xz/file_io.c;h=8c83269b13fa31284f7ea5f3627a1dfbce7d6e14;hb=HEAD#l72
[2]: https://git.savannah.gnu.org/cgit/gnulib.git/tree/lib/fcntl.in.h
(lines 380 and 396, commit d7f551b30f3f2a0fa57c1b10c12f4eea41a9b89e)
[3]: https://lists.gnu.org/archive/html/bug-gzip/2025-01/msg00000.html
Catch up with the nice little toys the kernel fs developers have added
for us. Preferably, let's make use of the new F_DUPFD_QUERY fcntl() call
that checks whether two fds are just duplicates of each other
(duplicates as in dup(), not as in open() of the same inode, i.e.
whether they share a single file offset and so on).
This API is much nicer, since it is a core kernel feature, unlike the
kcmp() call we so far used, which is part of the (optional)
checkpoint/restore stuff.
F_DUPFD_QUERY is available since kernel 6.10.
Newer kernels support a new flag for name_to_handle_at(): AT_HANDLE_FID.
This flag is supposed to return an identifier for an inode that we can
use for checking inode identity. It's supposed to be a replacement for
checking .st_ino which doesn't work anymore today because inode numbers
are no longer unique on file systems (not on overlayfs, and not on btrfs
for example). Hence, be a good citizen and add infrastructure to support
AT_HANDLE_FID. Unfortunately that doesn't work for old kernels, hence
add a fallback logic: if we can use the flag, use it. If we cannot use
name_to_handle_at() without it, which might give us a good ID too. But
of course tha tcan fail as well, which callers have to check.
Daan De Meyer
Stefan Hansson
Yu Watanabe
Lennart Poettering
Adrian Vovk