bpf, x86: Place trampoline arguments by the x86-64 calling convention

save_args() numbers the on-stack argument slots consecutively, and the
area built for the original function is sized the same way, while the
x86-64 ABI rounds the stack slot up to an even one for an argument
aligned to 16 bytes: an __int128, or an aggregate holding one. When such
an argument lands on an odd stack slot, it and every argument after it
are read from the wrong place, so a bpf program attached to the function
reads a neighbouring eightbyte, and the arguments handed on to the
original function are shifted. For example:

	/* s holds an __int128 */
	void foo(u64 a, u64 b, u64 c, u64 d, u64 e, u64 f, u64 g,
		 struct s h, u64 i)

x86-64 puts g in the first stack slot, leaves the second empty and puts
h in the third and fourth, while the trampoline reads h from the second
and third.

Take the position of each argument slot from bpf_jit_place_args(), which
knows the x86-64 rules, backfill included. save_args() then reads each
slot from its register or incoming stack slot, restore_regs() puts back
only the register slots, and the area built for the original function
mirrors the incoming one, holes included.

A trampoline on a BPF function is not changed by this: since the
previous patch, the function's model has one u64 per argument slot,
which bpf_jit_place_args() places in slot order, so the trampoline still
takes the arguments where BPF code put them.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261016183105.1134967-1-yonghong.song@linux.dev

0 files changed, 0 insertions, 0 deletions