commit bc3905a71f02511607d3ccf732360580209cac4c upstream.
The tailcall_bpf2bpf_hierarchy_fentry test hangs on s390. Its call graph is as follows:
entry() subprog_tail() trampoline() fentry() the rest of subprog_tail() # via BPF_TRAMP_F_CALL_ORIG return to entry()
The problem is that the rest of subprog_tail() increments the tail call counter, but the trampoline discards the incremented value. This results in an astronomically large number of tail calls.
Fix by making the trampoline write the incremented tail call counter back.
Fixes: 528eb2cb87bc ("s390/bpf: Implement arch_prepare_bpf_trampoline()") Signed-off-by: Ilya Leoshkevich iii@linux.ibm.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Link: https://lore.kernel.org/bpf/20250813121016.163375-4-iii@linux.ibm.com --- arch/s390/net/bpf_jit_comp.c | 3 +++ 1 file changed, 3 insertions(+)
diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c index 2526a3d53fadb..f305cb42070df 100644 --- a/arch/s390/net/bpf_jit_comp.c +++ b/arch/s390/net/bpf_jit_comp.c @@ -2828,6 +2828,9 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, /* stg %r2,retval_off(%r15) */ EMIT6_DISP_LH(0xe3000000, 0x0024, REG_2, REG_0, REG_15, tjit->retval_off); + /* mvc tccnt_off(%r15),tail_call_cnt(4,%r15) */ + _EMIT6(0xd203f000 | tjit->tccnt_off, + 0xf000 | offsetof(struct prog_frame, tail_call_cnt));
im->ip_after_call = jit->prg_buf + jit->prg;