bpf: Describe a BPF function to its trampoline as one u64 per slot

A trampoline on a BPF function, fentry, fexit or fsession attached
through attach_prog_fd, takes the function's arguments from where BPF
code left them. A call from one BPF function to another passes slot N
where the kernel convention passes its Nth u64 argument, in a register
or a stack slot, whatever the types of the arguments: the x86-64, arm64
and riscv JITs all place a stack argument that way. The model of the
function, distilled from its BTF, still carries those types, so each
arch places the arguments by its own rules for them. For a 16-byte
argument at an odd slot, such as s in

	u64 f(u64 a, struct { __int128 v; } s, u64 b)

the arm64 trampoline reads s from x2 and x3, where the caller left it in
x1 and x2, and b from x4. And an x86-64 trampoline puts a 16-byte
argument at slot 6 wholly on the stack, while x86-64 BPF code splits it
between r9 and the stack. The s390 trampoline, which takes a 16-byte
argument by reference, loads s through its first half as if that were a
pointer, and saves and restores only three of the four registers the
caller passed.

Rewrite the model of a target program's function in
bpf_check_attach_target() as one u64 argument per argument slot.
Every convention places N u64 arguments in slot order, so all of the
above take them from where BPF code put them, and no arch has to know
that the function is a BPF one.

The model no longer gives the size and flags of each argument, so an
arch that sign-extends a narrow signed argument, or clears the unused
bytes of a narrow stack argument, for the program does not do that for a
BPF function any more: the program gets the eightbyte BPF code passed,
as it does on x86-64, and fexit hands it back unchanged.

Fixes: afdbb4cdd3d2 ("bpf, arm64: Place trampoline arguments by the arm64 calling convention")
Fixes: 4dd676ca5b7a ("bpf: Support by-value struct arguments up to 16 bytes")
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261016183100.1134528-1-yonghong.song@linux.dev

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