[PATCH bpf] bpf, riscv: Make arena support depend on ZACAS

[Chen Pei <cp0613@linux.alibaba.com>]

The arena range tree allocates its nodes with kmalloc_nolock() since
commit f8c67d8550ee ("bpf: Use kmalloc_nolock() in range tree").
kmalloc_nolock() requires slab caches with cmpxchg128 support
(__CMPXCHG_DOUBLE); on riscv cmpxchg128 is provided by the ZACAS
extension. On systems without ZACAS every arena map creation fails
with a misleading -ENOMEM.

Report the missing support instead: make bpf_jit_supports_arena()
return system_has_cmpxchg128(), so arena map creation fails with
-EOPNOTSUPP on systems without ZACAS. This matches how arena
BPF_CMPXCHG instructions are already gated on ZACAS in
bpf_jit_supports_insn().

Fixes: f8c67d8550ee ("bpf: Use kmalloc_nolock() in range tree")
Cc: stable@vger.kernel.org
Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
---
This issue was reported by sashiko-bot:
https://sashiko.dev/#/patchset/20260901120013.16104-1-cp0613@linux.alibaba.com?part=1

Question for reviewers: should the arena selftests gate on ZACAS,
e.g. probing it via riscv_hwprobe() (RISCV_ISA_EXT_ZACAS) and
SKIPping cleanly on systems without the extension?

 arch/riscv/net/bpf_jit_comp64.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index 74efe4b138d2..0061c1d4ed98 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -2128,7 +2128,11 @@ bool bpf_jit_supports_ptr_xchg(void)
 
 bool bpf_jit_supports_arena(void)
 {
-	return true;
+	/*
+	 * The arena range tree uses kmalloc_nolock(), which needs
+	 * cmpxchg128, provided by ZACAS on riscv.
+	 */
+	return system_has_cmpxchg128();
 }
 
 bool bpf_jit_supports_insn(struct bpf_insn *insn, bool in_arena)
-- 
2.50.1