[PATCH bpf-next v5 06/11] bpf: Add verifier support for 16-byte returns in R0:R2

[Yonghong Song <yonghong.song@linux.dev>]

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.

This patch models 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 R0 already is. 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: its own return value is the program exit code, read out of R0
alone, 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 with
-EOPNOTSUPP rather than supported.

  [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>
---
 kernel/bpf/verifier.c | 75 +++++++++++++++++++++++++++++++++++++++----
 1 file changed, 68 insertions(+), 7 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 161d77791bc6..0c68ab3bd6ce 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -412,6 +412,9 @@ static u32 ret_regs_cnt(u32 size)
 	return size > 8 && size <= 16 ? 2 : 1;
 }
 
+/* Registers holding a function return value, in order. See ret_regs_cnt(). */
+static const int ret_regs[] = { BPF_REG_0, BPF_REG_2 };
+
 static void bpf_compute_subprog_ret_regs(struct bpf_verifier_env *env)
 {
 	const struct btf *btf = env->prog->aux->btf;
@@ -9456,6 +9459,7 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 	u16 callee_incoming, stack_arg_cnt;
 	struct bpf_func_state *caller;
 	int err, subprog, target_insn;
+	u32 i, nregs;
 
 	target_insn = *insn_idx + insn->imm + 1;
 	subprog = bpf_find_subprog(env, target_insn);
@@ -9498,9 +9502,14 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		clear_caller_saved_regs(env, caller->regs);
 		invalidate_outgoing_stack_args(env, cur_func(env));
 
-		/* All non-void global functions return a 64-bit SCALAR_VALUE. */
+		/*
+		 * A non-void global function returns a 64-bit SCALAR_VALUE in
+		 * R0, or a >8 byte SCALAR_VALUE in the R0:R2 register pair.
+		 */
 		if (!subprog_returns_void(env, subprog)) {
-			mark_reg_unknown(env, caller->regs, BPF_REG_0);
+			nregs = bpf_ret_reg_pair(env, subprog) ? 2 : 1;
+			for (i = 0; i < nregs; i++)
+				mark_reg_unknown(env, caller->regs, ret_regs[i]);
 		}
 
 		if (env->subprog_info[subprog].might_throw) {
@@ -9858,11 +9867,15 @@ static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
 	struct bpf_func_state *caller, *callee;
 	struct bpf_reg_state *r0;
 	bool in_callback_fn;
+	u32 i, nregs;
 	int err;
 
 	callee = state->frame[state->curframe];
 	r0 = &callee->regs[BPF_REG_0];
-	if (r0->type == PTR_TO_STACK) {
+	nregs = bpf_ret_reg_pair(env, callee->subprogno) ? 2 : 1;
+	for (i = 0; i < nregs; i++) {
+		if (callee->regs[ret_regs[i]].type != PTR_TO_STACK)
+			continue;
 		/* technically it's ok to return caller's stack pointer
 		 * (or caller's caller's pointer) back to the caller,
 		 * since these pointers are valid. Only current stack
@@ -9897,8 +9910,12 @@ static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
 			return -EFAULT;
 		}
 	} else {
-		/* return to the caller whatever r0 had in the callee */
-		caller->regs[BPF_REG_0] = *r0;
+		/*
+		 * return to the caller whatever the callee had in the
+		 * return register(s)
+		 */
+		for (i = 0; i < nregs; i++)
+			caller->regs[ret_regs[i]] = callee->regs[ret_regs[i]];
 	}
 
 	/* for callbacks like bpf_loop or bpf_for_each_map_elem go back to callsite,
@@ -10796,6 +10813,19 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 	return 0;
 }
 
+/*
+ * Mark the register(s) holding a @size byte kfunc return value as unknown
+ * scalars. Both halves of a register pair are treated the same way.
+ */
+static void mark_kfunc_ret_regs(struct bpf_verifier_env *env,
+				struct bpf_reg_state *regs, u32 size)
+{
+	u32 i, nregs = ret_regs_cnt(size);
+
+	for (i = 0; i < nregs; i++)
+		mark_reg_unknown(env, regs, ret_regs[i]);
+}
+
 static bool is_kfunc_acquire(struct bpf_call_arg_meta *meta)
 {
 	return meta->kfunc_flags & KF_ACQUIRE;
@@ -13265,10 +13295,25 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 	}
 
 	if (btf_type_is_scalar(t)) {
-		mark_reg_unknown(env, regs, BPF_REG_0);
+		mark_kfunc_ret_regs(env, regs, t->size);
 		if (meta.btf == btf_vmlinux && (meta.func_id == special_kfunc_list[KF_bpf_res_spin_lock] ||
 		    meta.func_id == special_kfunc_list[KF_bpf_res_spin_lock_irqsave]))
 			__mark_reg_const_zero(env, &regs[BPF_REG_0]);
+	} else if (btf_type_is_struct(t)) {
+		/*
+		 * The returned struct comes back as raw register bits modeled
+		 * as an unknown scalar, so it must contain only scalars:
+		 * otherwise a pointer field would be laundered into a scalar
+		 * and escape provenance and reference tracking.
+		 */
+		if (!__btf_type_is_scalar_struct(env, desc_btf, t, 0)) {
+			verbose(env,
+				"kernel function %s returns %s %s that is not composed of scalars\n",
+				func_name, btf_type_str(t),
+				btf_name_by_offset(desc_btf, t->name_off));
+			return -EINVAL;
+		}
+		mark_kfunc_ret_regs(env, regs, t->size);
 	} else if (btf_type_is_ptr(t)) {
 		ptr_type = btf_type_skip_modifiers(desc_btf, t->type, &ptr_type_id);
 		err = check_special_kfunc(env, &meta, regs, insn_aux, ptr_type, desc_btf);
@@ -16746,11 +16791,20 @@ static int check_global_subprog_return_code(struct bpf_verifier_env *env)
 {
 	struct bpf_func_state *cur_frame = cur_func(env);
 	u32 subprog = cur_frame->subprogno;
+	u32 i, nregs;
+	int err;
 
 	if (subprog_returns_void(env, subprog))
 		return 0;
 
-	return check_global_ret_scalar_reg(env, BPF_REG_0);
+	nregs = bpf_ret_reg_pair(env, subprog) ? 2 : 1;
+	for (i = 0; i < nregs; i++) {
+		err = check_global_ret_scalar_reg(env, ret_regs[i]);
+		if (err)
+			return err;
+	}
+
+	return 0;
 }
 
 /* Bitmask with 1s for all caller saved registers */
@@ -19404,6 +19458,13 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 			return -EOPNOTSUPP;
 		}
 
+		if (prog_extension && tgt_info->fmodel.ret_size > 8) {
+			bpf_log(log,
+				"Cannot replace function %s with a >8 byte return value\n",
+				tname);
+			return -EOPNOTSUPP;
+		}
+
 		/*
 		 * *.multi programs don't need an address during program
 		 * verification, we just take the module ref if needed.
-- 
2.53.0-Meta