[RFC PATCH bpf-next v2 11/16] bpf: allow tailcalls in subprograms for MIPS64/MIPS32

[Tony Ambardar <tony.ambardar@gmail.com>]

The BPF core/verifier is hard-coded to permit mixing bpf2bpf and tail
calls for only x86-64. Change the logic to instead rely on a new weak
function 'bool bpf_jit_supports_subprog_tailcalls(void)', which a capable
JIT backend can override.

Update the x86-64 eBPF JIT to reflect this, and also enable the feature
for the MIPS64/MIPS32 JIT.

Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com>
---
 arch/mips/net/ebpf_jit.c    | 6 ++++++
 arch/x86/net/bpf_jit_comp.c | 6 ++++++
 include/linux/filter.h      | 1 +
 kernel/bpf/core.c           | 6 ++++++
 kernel/bpf/verifier.c       | 3 ++-
 5 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/mips/net/ebpf_jit.c b/arch/mips/net/ebpf_jit.c
index 7d8ed8bb19ab..501c1d532be6 100644
--- a/arch/mips/net/ebpf_jit.c
+++ b/arch/mips/net/ebpf_jit.c
@@ -2416,3 +2416,9 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 		bpf_jit_binary_free(header);
 	goto out_ctx;
 }
+
+/* Indicate the JIT backend supports mixing bpf2bpf and tailcalls. */
+bool bpf_jit_supports_subprog_tailcalls(void)
+{
+	return true;
+}
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 3ae729bb2475..2d78588fb5b5 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -2367,3 +2367,9 @@ bool bpf_jit_supports_kfunc_call(void)
 {
 	return true;
 }
+
+/* Indicate the JIT backend supports mixing bpf2bpf and tailcalls. */
+bool bpf_jit_supports_subprog_tailcalls(void)
+{
+	return true;
+}
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 16e5cebea82c..50b50fb271b5 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -933,6 +933,7 @@ u64 __bpf_call_base(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5);
 struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog);
 void bpf_jit_compile(struct bpf_prog *prog);
 bool bpf_jit_needs_zext(void);
+bool bpf_jit_supports_subprog_tailcalls(void);
 bool bpf_jit_supports_kfunc_call(void);
 bool bpf_helper_changes_pkt_data(void *func);
 
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 189934d2a3f2..c82b48ed0005 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -2366,6 +2366,12 @@ bool __weak bpf_jit_needs_zext(void)
 	return false;
 }
 
+/* Return TRUE if the JIT backend supports mixing bpf2bpf and tailcalls. */
+bool __weak bpf_jit_supports_subprog_tailcalls(void)
+{
+	return false;
+}
+
 bool __weak bpf_jit_supports_kfunc_call(void)
 {
 	return false;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6e2ebcb0d66f..267de5428fad 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5144,7 +5144,8 @@ static bool may_update_sockmap(struct bpf_verifier_env *env, int func_id)
 
 static bool allow_tail_call_in_subprogs(struct bpf_verifier_env *env)
 {
-	return env->prog->jit_requested && IS_ENABLED(CONFIG_X86_64);
+	return env->prog->jit_requested &&
+	       bpf_jit_supports_subprog_tailcalls();
 }
 
 static int check_map_func_compatibility(struct bpf_verifier_env *env,
-- 
2.25.1