[RFC bpf-next v1 3/8] bpf, x86: no_caller_saved_registers for bpf_get_smp_processor_id()

[Eduard Zingerman <eddyz87@gmail.com>]

On x86 verifier replaces call to bpf_get_smp_processor_id() with a
sequence of instructions that modify only R0.
This satisfies 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>
---
 kernel/bpf/helpers.c  |  1 +
 kernel/bpf/verifier.c | 11 +++++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 229396172026..1df01f454590 100644
--- a/kernel/bpf/helpers.c
+++ b/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,
+	.nocsr		= true,
 };
 
 BPF_CALL_0(bpf_get_numa_node_id)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1340d3e60d30..d68ed70186c8 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -16030,7 +16030,14 @@ static u8 get_helper_reg_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;
+	}
 }
 
 /* If 'insn' is a call that follows no_caller_saved_registers contract
@@ -20666,7 +20673,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 #ifdef CONFIG_X86_64
 		/* 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
-- 
2.45.2