Merge branch 'bpf: Support struct argument for trampoline base progs'

Yonghong Song says:

====================
Currently struct arguments are not supported for trampoline based progs.
One of major reason is that struct argument may pass by value which may
use more than one registers. This breaks trampoline progs where
each argument is assumed to take one register. bcc community reported the
issue ([1]) where struct argument is not supported for fentry program.
  typedef struct {
        uid_t val;
  } kuid_t;
  typedef struct {
        gid_t val;
  } kgid_t;
  int security_path_chown(struct path *path, kuid_t uid, kgid_t gid);
Inside Meta, we also have a use case to attach to tcp_setsockopt()
  typedef struct {
        union {
                void            *kernel;
                void __user     *user;
        };
        bool            is_kernel : 1;
  } sockptr_t;
  int tcp_setsockopt(struct sock *sk, int level, int optname,
                     sockptr_t optval, unsigned int optlen);

This patch added struct value support for bpf tracing programs which
uses trampoline. Only <= 16 byte struct size is supported for now
which covers use cases in the above. For x86/arm64/bpf, <= 16
struct value will be passed in registers instead of by reference.
Only x86_64 is supported in this patch. arm64 support can be
added later.

 [1] https://github.com/iovisor/bcc/issues/3657

Changelog:
  v3 -> v4:
   - fix a test failure where no casting for
     bpf_get_func_arg() value as the value type is 'int'.
   - add tracing_struct test in DENYLIST.s390x
   - simplify macro BPF_REG_CNT for BPF_PROG2.
  v2 -> v3:
   - previously struct arguments (<= 16 bytes) are passed
     by reference for bpf programs. Suggested by Alexei,
     it is passed by value now.
   - in order to support passing <= 16 struct value, a
     new macro BPF_PROG2 is invented.
  rfc v1 -> v2:
   - changed bpf_func_model struct info fields to
     arg_flags[] to make it easy to iterate arguments
     in arch specific {save|restore}_regs() functions.
   - added fexit tests to test return values with
     struct arguments.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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