linux-user: add openat2 support in linux-user

linux-user: add openat2 support in linux-user

This commit adds support for the `openat2()` syscall in the
`linux-user` userspace emulator.

It is implemented by extracting a new helper `maybe_do_fake_open()`
out of the exiting `do_guest_openat()` and share that with the
new `do_guest_openat2()`. Unfortunately we cannot just make
do_guest_openat2() a superset of do_guest_openat() because the
openat2() syscall is stricter with the argument checking and
will return an error for invalid flags or mode combinations (which
open()/openat() will ignore).

The implementation is similar to SYSCALL_DEFINE(openat2), i.e.
a new `copy_struct_from_user()` is used that works the same
as the kernels version to support backwards-compatibility
for struct syscall argument.

Instead of including openat2.h we create a copy of `open_how`
as `open_how_ver0` to ensure that if the structure grows we
can log a LOG_UNIMP warning.

Note that in this commit using openat2() for a "faked" file in
/proc will honor the "resolve" flags for
RESOLVE_NO_{MAGIC,SYM}LINKS for path based access to /proc/self/exe
(which is the only magic link we support for faked files).
Note it will not catch special access via e.g. dirfd. This is not
great but it seems similar to the exiting behavior when openat()
is called with a dirfd to "/proc". Here too the fake file lookup
may not catch the special file because no dirfd is used to
determine if the path is in /proc.

Signed-off-by: Michael Vogt <mvogt@redhat.com>
Buglink: https://github.com/osbuild/bootc-image-builder/issues/619
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-ID: <1c2c8c9db3731ed4c6fd9b10c63637c3e4caf8f5.1727795334.git.mvogt@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>