bpf: Add verifier support for 16-byte returns in R0: R2

LLVM 23 added support for returning a value in two registers for an
__int128, or a struct/union whose size is greater than 8 but not more than
16 bytes. See LLVM patches [1] and [2].

Before LLVM 23 the BPF backend could not return these values at all. A
by-value struct or union return (of any size) was rejected at compile time
with:

  error: aggregate returns are not supported

and an __int128 return failed later in the backend with:

  fatal error: error in backend: unable to allocate function return #1

Both are resolved in LLVM 23, which lowers such returns into the R0:R2
register pair.

Model that pair at calls to global and static BPF subprograms and at kfunc
calls: R2 is marked alongside R0 at the call, propagated out of a callee
at its exit, and held to the same scalar-only and no-stack-pointer rules
that already apply to R0. A struct returned by a kfunc must be composed of
scalars, since its bytes reach the program as raw register contents and a
pointer field would otherwise be laundered into a scalar.

An extension program is the one caller of the convention that cannot take
part in it: only R0 is checked at its exit, since that is the program exit
code, so it has no way to hand back an upper half. Replacing a function
whose return value is larger than 8 bytes is therefore rejected in
btf_check_func_type_match(). That function compares btf_type->info, which
carries no size for an int, so an 8 byte long and a 16 byte __int128
compared equal; sizes up to 8 bytes stay interchangeable, as they always
have been, so this only rejects what the R0:R2 convention newly makes
incompatible.

  [1] https://github.com/llvm/llvm-project/pull/190894
  [2] https://github.com/llvm/llvm-project/pull/206876

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20260819055309.3297684-1-yonghong.song@linux.dev
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

0 files changed, 0 insertions, 0 deletions