[PATCH bpf-next v3 5/7] bpf: Clean up redundant stack arg checks for non-JITed programs

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

Remove a redundant stack_arg_cnt check in __bpf_prog_select_runtime()
and start the stack arg loop from index 0 in bpf_fixup_call_args().
Both changes are no-ops that simplify the code:

In __bpf_prog_select_runtime(), the subprog_info[0].stack_arg_cnt
check is unreachable:
  - when there is only a main program (no bpf-to-bpf calls),
    subprog_info[0].stack_arg_cnt is always 0 because the main
    program's arg_cnt is forced to 1
  - when bpf-to-bpf calls use stack args and JIT succeeds,
    fp->bpf_func is set and this code is skipped
  - when JIT fails, bpf_fixup_call_args() rejects the program
    before we get to __bpf_prog_select_runtime().

In bpf_fixup_call_args(), starting the loop at i=1 skipped subprog 0,
which is safe since the main program always has arg_cnt=1 and thus
bpf_in_stack_arg_cnt() returns 0. Starting at i=0 removes the need
to reason about this invariant.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 kernel/bpf/core.c   | 2 +-
 kernel/bpf/fixups.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 427a6d828e01..cdbe9fdf474f 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -2609,7 +2609,7 @@ struct bpf_prog *__bpf_prog_select_runtime(struct bpf_verifier_env *env, struct
 		goto finalize;
 
 	if (IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) ||
-	    bpf_prog_has_kfunc_call(fp) || (env && env->subprog_info[0].stack_arg_cnt))
+	    bpf_prog_has_kfunc_call(fp))
 		jit_needed = true;
 
 	if (!bpf_prog_select_interpreter(fp))
diff --git a/kernel/bpf/fixups.c b/kernel/bpf/fixups.c
index 19056016eed8..2cec4e8cd4a0 100644
--- a/kernel/bpf/fixups.c
+++ b/kernel/bpf/fixups.c
@@ -1407,7 +1407,7 @@ int bpf_fixup_call_args(struct bpf_verifier_env *env)
 		verbose(env, "calling kernel functions are not allowed in non-JITed programs\n");
 		return -EINVAL;
 	}
-	for (i = 1; i < env->subprog_cnt; i++) {
+	for (i = 0; i < env->subprog_cnt; i++) {
 		if (bpf_in_stack_arg_cnt(&env->subprog_info[i])) {
 			verbose(env, "stack args are not supported in non-JITed programs\n");
 			return -EINVAL;
-- 
2.53.0-Meta