s390/bpf: Do not write tail call counter into helper and kfunc frames

Only BPF functions make use of the tail call counter; helpers and
kfuncs ignore and most likely also clobber it. Writing it into these
functions' frames is pointless and misleading, so do not do it.

Fixes: dd691e8 ("s390/bpf: Implement bpf_jit_supports_subprog_tailcalls()")
Signed-off-by: Ilya Leoshkevich <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: iii-i

arch/s390/net/bpf_jit_comp.c

@@ -1790,6 +1790,7 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 
 		REG_SET_SEEN(BPF_REG_5);
 		jit->seen |= SEEN_FUNC;
+
 		/*
 		 * Copy the tail call counter to where the callee expects it.
 		 *
@@ -1800,10 +1801,17 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		 * Note 2: We assume that the verifier does not let us call the
 		 * main program, which clears the tail call counter on entry.
 		 */
-		/* mvc tail_call_cnt(4,%r15),frame_off+tail_call_cnt(%r15) */
-		_EMIT6(0xd203f000 | offsetof(struct prog_frame, tail_call_cnt),
-		       0xf000 | (jit->frame_off +
-				 offsetof(struct prog_frame, tail_call_cnt)));
+
+		if (insn->src_reg == BPF_PSEUDO_CALL)
+			/*
+			 * mvc tail_call_cnt(4,%r15),
+			 *     frame_off+tail_call_cnt(%r15)
+			 */
+			_EMIT6(0xd203f000 | offsetof(struct prog_frame,
+						     tail_call_cnt),
+			       0xf000 | (jit->frame_off +
+					 offsetof(struct prog_frame,
+						  tail_call_cnt)));
 
 		/* Sign-extend the kfunc arguments. */
 		if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL) {