For now, the BPF program of type BPF_PROG_TYPE_TRACING can only be used
on the kernel functions whose arguments count less than or equal to 6, if
not considering '> 8 bytes' struct argument. This is not friendly at all,
as too many functions have arguments count more than 6.

According to the current kernel version, below is a statistics of the
function arguments count:

argument count | function count
7              | 704
8              | 270
9              | 84
10             | 47
11             | 47
12             | 27
13             | 22
14             | 5
15             | 0
16             | 1

Therefore, let's enhance it by increasing the function arguments count
allowed in arch_prepare_bpf_trampoline(), for now, only x86_64.

For the case that we don't need to call origin function, which means
without BPF_TRAMP_F_CALL_ORIG, we need only copy the function arguments
that stored in the frame of the caller to current frame. The 7th and later
arguments are stored in "$rbp + 0x18", and they will be copied to the
stack area following where register values are saved.

For the case with BPF_TRAMP_F_CALL_ORIG, we need prepare the arguments
in stack before call origin function, which means we need alloc extra
"8 * (arg_count - 6)" memory in the top of the stack. Note, there should
not be any data be pushed to the stack before calling the origin function.
So 'rbx' value will be stored on a stack position higher than where stack
arguments are stored for BPF_TRAMP_F_CALL_ORIG.

According to the research of Yonghong, struct members should be all in
register or all on the stack. Meanwhile, the compiler will pass the
argument on regs if the remaining regs can hold the argument. Therefore,
we need save the arguments in order. Otherwise, disorder of the args can
happen. For example:

  struct foo_struct {
      long a;
      int b;
  };
  int foo(char, char, char, char, char, struct foo_struct,
          char);

the arg1-5,arg7 will be passed by regs, and arg6 will by stack. Therefore,
we should save/restore the arguments in the same order with the
declaration of foo(). And the args used as ctx in stack will be like this:

  reg_arg6   -- copy from regs
  stack_arg2 -- copy from stack
  stack_arg1
  reg_arg5   -- copy from regs
  reg_arg4
  reg_arg3
  reg_arg2
  reg_arg1

We use EMIT3_off32() or EMIT4() for "lea" and "sub". The range of the
imm in "lea" and "sub" is [-128, 127] if EMIT4() is used. Therefore,
we use EMIT3_off32() instead if the imm out of the range.

It works well for the FENTRY/FEXIT/MODIFY_RETURN.
Signed-off-by: Menglong Dong <imagedong@tencent.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20230713040738.1789742-3-imagedong@tencent.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>