s390/bpf: Store backchain even for leaf progs

Currently a crash in a leaf prog (caused by a bug) produces the
following call trace:

     [<000003ff600ebf00>] bpf_prog_6df0139e1fbf2789_fentry+0x20/0x78
     [<0000000000000000>] 0x0

This is because leaf progs do not store backchain. Fix by making all
progs do it. This is what GCC and Clang-generated code does as well.
Now the call trace looks like this:

     [<000003ff600eb0f2>] bpf_prog_6df0139e1fbf2789_fentry+0x2a/0x80
     [<000003ff600ed096>] bpf_trampoline_201863462940+0x96/0xf4
     [<000003ff600e3a40>] bpf_prog_05f379658fdd72f2_classifier_0+0x58/0xc0
     [<000003ffe0aef070>] bpf_test_run+0x210/0x390
     [<000003ffe0af0dc2>] bpf_prog_test_run_skb+0x25a/0x668
     [<000003ffe038a90e>] __sys_bpf+0xa46/0xdb0
     [<000003ffe038ad0c>] __s390x_sys_bpf+0x44/0x50
     [<000003ffe0defea8>] __do_syscall+0x150/0x280
     [<000003ffe0e01d5c>] system_call+0x74/0x98

Fixes: 054623105728 ("s390/bpf: Add s390x eBPF JIT compiler backend")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20250512122717.54878-1-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index 0776dfd..945106b 100644 (file)
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -605,17 +605,15 @@ static void bpf_jit_prologue(struct bpf_jit *jit, struct bpf_prog *fp,
        }
        /* Setup stack and backchain */
        if (is_first_pass(jit) || (jit->seen & SEEN_STACK)) {
-               if (is_first_pass(jit) || (jit->seen & SEEN_FUNC))
-                       /* lgr %w1,%r15 (backchain) */
-                       EMIT4(0xb9040000, REG_W1, REG_15);
+               /* lgr %w1,%r15 (backchain) */
+               EMIT4(0xb9040000, REG_W1, REG_15);
                /* la %bfp,STK_160_UNUSED(%r15) (BPF frame pointer) */
                EMIT4_DISP(0x41000000, BPF_REG_FP, REG_15, STK_160_UNUSED);
                /* aghi %r15,-STK_OFF */
                EMIT4_IMM(0xa70b0000, REG_15, -(STK_OFF + stack_depth));
-               if (is_first_pass(jit) || (jit->seen & SEEN_FUNC))
-                       /* stg %w1,152(%r15) (backchain) */
-                       EMIT6_DISP_LH(0xe3000000, 0x0024, REG_W1, REG_0,
-                                     REG_15, 152);
+               /* stg %w1,152(%r15) (backchain) */
+               EMIT6_DISP_LH(0xe3000000, 0x0024, REG_W1, REG_0,
+                             REG_15, 152);
        }
 }