[PATCH bpf v2] 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() where it is defined, so arena map
creation fails with -EOPNOTSUPP on systems without ZACAS. The macro
is only defined when both CONFIG_RISCV_ISA_ZACAS and
CONFIG_TOOLCHAIN_HAS_ZACAS are enabled, so guard it with #ifdef the
same way mm/slab.h consumes it, and reject arena otherwise. 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>
---
Changes since v1:
- Guard system_has_cmpxchg128() with #ifdef instead of calling it
  unconditionally: the macro is only defined when both
  CONFIG_RISCV_ISA_ZACAS and CONFIG_TOOLCHAIN_HAS_ZACAS are enabled
  (as reported by sashiko-bot), so v1 broke the build when either
  was disabled. This mirrors how mm/slab.h consumes the macro.

Why #ifdef rather than rv_ext_enabled(ZACAS)? The predicates differ
exactly in the configurations that matter:

  scenario (ISA_ZACAS/TOOLCHAIN/hw)  v1           rv_ext_enabled  #ifdef
  ISA=n or TOOLCHAIN=n               build fails  rejects         rejects
  ISA=y TOOLCHAIN=n hw has ZACAS     build fails  accepts, then   rejects
                                                  -ENOMEM again
  ISA=y TOOLCHAIN=y hw has ZACAS     exact        exact           exact

rv_ext_enabled(ZACAS) does not check CONFIG_TOOLCHAIN_HAS_ZACAS, but
slab's cmpxchg128 - and thus kmalloc_nolock() - does require it, so
on an old toolchain with ZACAS hardware it would accept arena maps
and bring back the very -ENOMEM failure this patch fixes. The #ifdef
form builds in every configuration and matches exactly the
kmalloc_nolock() availability gate in mm/slab.h.

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 | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index 74efe4b138d2..151031e97a24 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -2128,7 +2128,15 @@ 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.
+	 */
+#ifdef system_has_cmpxchg128
+	return system_has_cmpxchg128();
+#else
+	return false;
+#endif
 }
 
 bool bpf_jit_supports_insn(struct bpf_insn *insn, bool in_arena)
-- 
2.50.1