bpf, x86, riscv, arm: no_caller_saved_registers for bpf_get_smp_proce…

…ssor_id()

The function bpf_get_smp_processor_id() is processed in a different
way, depending on the arch:
- on x86 verifier replaces call to bpf_get_smp_processor_id() with a
  sequence of instructions that modify only r0;
- on riscv64 jit replaces call to bpf_get_smp_processor_id() with a
  sequence of instructions that modify only r0;
- on arm64 jit replaces call to bpf_get_smp_processor_id() with a
  sequence of instructions that modify only r0 and tmp registers.

These rewrites satisfy attribute no_caller_saved_registers contract.
Allow rewrite of no_caller_saved_registers patterns for
bpf_get_smp_processor_id() in order to use this function as a canary
for no_caller_saved_registers tests.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240722233844.1406874-4-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

kernel/bpf/helpers.c

@@ -158,6 +158,7 @@ const struct bpf_func_proto bpf_get_smp_processor_id_proto = {
 	.func		= bpf_get_smp_processor_id,
 	.gpl_only	= false,
 	.ret_type	= RET_INTEGER,
+	.allow_nocsr	= true,
 };
 
 BPF_CALL_0(bpf_get_numa_node_id)

kernel/bpf/verifier.c

@@ -16130,7 +16130,14 @@ static u32 helper_nocsr_clobber_mask(const struct bpf_func_proto *fn)
  */
 static bool verifier_inlines_helper_call(struct bpf_verifier_env *env, s32 imm)
 {
-	return false;
+	switch (imm) {
+#ifdef CONFIG_X86_64
+	case BPF_FUNC_get_smp_processor_id:
+		return env->prog->jit_requested && bpf_jit_supports_percpu_insn();
+#endif
+	default:
+		return false;
+	}
 }
 
 /* GCC and LLVM define a no_caller_saved_registers function attribute.
@@ -20834,7 +20841,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 #if defined(CONFIG_X86_64) && !defined(CONFIG_UML)
 		/* Implement bpf_get_smp_processor_id() inline. */
 		if (insn->imm == BPF_FUNC_get_smp_processor_id &&
-		    prog->jit_requested && bpf_jit_supports_percpu_insn()) {
+		    verifier_inlines_helper_call(env, insn->imm)) {
 			/* BPF_FUNC_get_smp_processor_id inlining is an
 			 * optimization, so if pcpu_hot.cpu_number is ever
 			 * changed in some incompatible and hard to support