[PATCH] s390/bpf: Zero-extend bpf prog return values and kfunc arguments

[Ilya Leoshkevich <iii@linux.ibm.com>]

s390x ABI requires callers to zero-extend unsigned arguments and
sign-extend signed arguments, and callees to zero-extend unsigned
return values and sign-extend signed return values.

s390 BPF JIT currently implements only sign extension. Fix this
omission and implement zero extension too.

Fixes: 528eb2cb87bc ("s390/bpf: Implement arch_prepare_bpf_trampoline()")
Reported-by: Hari Bathini <hbathini@linux.ibm.com>
Closes: https://lore.kernel.org/bpf/20260312080113.843408-1-hbathini@linux.ibm.com/
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 arch/s390/net/bpf_jit_comp.c | 39 ++++++++++++++++++++++--------------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index 1f9a6b728beb..d08d159b6319 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -830,25 +830,34 @@ static int bpf_jit_probe_post(struct bpf_jit *jit, struct bpf_prog *fp,
 }
 
 /*
- * Sign-extend the register if necessary
+ * Sign- or zero-extend the register if necessary
  */
-static int sign_extend(struct bpf_jit *jit, int r, u8 size, u8 flags)
+static int sign_zero_extend(struct bpf_jit *jit, int r, u8 size, u8 flags)
 {
-	if (!(flags & BTF_FMODEL_SIGNED_ARG))
-		return 0;
-
 	switch (size) {
 	case 1:
-		/* lgbr %r,%r */
-		EMIT4(0xb9060000, r, r);
+		if (flags & BTF_FMODEL_SIGNED_ARG)
+			/* lgbr %r,%r */
+			EMIT4(0xb9060000, r, r);
+		else
+			/* llgcr %r,%r */
+			EMIT4(0xb9840000, r, r);
 		return 0;
 	case 2:
-		/* lghr %r,%r */
-		EMIT4(0xb9070000, r, r);
+		if (flags & BTF_FMODEL_SIGNED_ARG)
+			/* lghr %r,%r */
+			EMIT4(0xb9070000, r, r);
+		else
+			/* llghr %r,%r */
+			EMIT4(0xb9850000, r, r);
 		return 0;
 	case 4:
-		/* lgfr %r,%r */
-		EMIT4(0xb9140000, r, r);
+		if (flags & BTF_FMODEL_SIGNED_ARG)
+			/* lgfr %r,%r */
+			EMIT4(0xb9140000, r, r);
+		else
+			/* llgfr %r,%r */
+			EMIT4(0xb9160000, r, r);
 		return 0;
 	case 8:
 		return 0;
@@ -1798,9 +1807,9 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 				return -1;
 
 			for (j = 0; j < m->nr_args; j++) {
-				if (sign_extend(jit, BPF_REG_1 + j,
-						m->arg_size[j],
-						m->arg_flags[j]))
+				if (sign_zero_extend(jit, BPF_REG_1 + j,
+						     m->arg_size[j],
+						     m->arg_flags[j]))
 					return -1;
 			}
 		}
@@ -2566,7 +2575,7 @@ static int invoke_bpf_prog(struct bpf_tramp_jit *tjit,
 	EMIT6_PCREL_RILB_PTR(0xc0050000, REG_14, p->bpf_func);
 	/* stg %r2,retval_off(%r15) */
 	if (save_ret) {
-		if (sign_extend(jit, REG_2, m->ret_size, m->ret_flags))
+		if (sign_zero_extend(jit, REG_2, m->ret_size, m->ret_flags))
 			return -1;
 		EMIT6_DISP_LH(0xe3000000, 0x0024, REG_2, REG_0, REG_15,
 			      tjit->retval_off);
-- 
2.53.0