Add a new variant of bpf_d_path() named bpf_path_d_path() which takes
the form of a BPF kfunc and enforces KF_TRUSTED_ARGS semantics onto
its arguments.

This new d_path() based BPF kfunc variant is intended to address the
legacy bpf_d_path() BPF helper's susceptability to memory corruption
issues [0, 1, 2] by ensuring to only operate on supplied arguments
which are deemed trusted by the BPF verifier. Typically, this means
that only pointers to a struct path which have been referenced counted
may be supplied.

In addition to the new bpf_path_d_path() BPF kfunc, we also add a
KF_ACQUIRE based BPF kfunc bpf_get_task_exe_file() and KF_RELEASE
counterpart BPF kfunc bpf_put_file(). This is so that the new
bpf_path_d_path() BPF kfunc can be used more flexibily from within the
context of a BPF LSM program. It's rather common to ascertain the
backing executable file for the calling process by performing the
following walk current->mm->exe_file while instrumenting a given
operation from the context of the BPF LSM program. However, walking
current->mm->exe_file directly is never deemed to be OK, and doing so
from both inside and outside of BPF LSM program context should be
considered as a bug. Using bpf_get_task_exe_file() and in turn
bpf_put_file() will allow BPF LSM programs to reliably get and put
references to current->mm->exe_file.

As of now, all the newly introduced BPF kfuncs within this patch are
limited to BPF LSM program types. These can be either sleepable or
non-sleepable variants of BPF LSM program types.

[0] https://lore.kernel.org/bpf/CAG48ez0ppjcT=QxU-jtCUfb5xQb3mLr=5FcwddF_VKfEBPs_Dg@mail.gmail.com/
[1] https://lore.kernel.org/bpf/20230606181714.532998-1-jolsa@kernel.org/
[2] https://lore.kernel.org/bpf/20220219113744.1852259-1-memxor@gmail.com/Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240731110833.1834742-2-mattbobrowski@google.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>