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

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

[Chen Pei]

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

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

[Chen Pei]

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


_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv

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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] The unconditional use of `system_has_cmpxchg128()` in `bpf_jit_supports_arena()` causes a build failure when `CONFIG_RISCV_ISA_ZACAS` or `CONFIG_TOOLCHAIN_HAS_ZACAS` is disabled.
--

commit c73d679b84188e98d5cff0171b751fa3f4ce5627
Author: Chen Pei <cp0613@linux.alibaba.com>

bpf, riscv: Make arena support depend on ZACAS

This patch updates bpf_jit_supports_arena() to return the result of
system_has_cmpxchg128(). This ensures that BPF arena map creation fails
with -EOPNOTSUPP on RISC-V systems that do not have the ZACAS extension.

> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
> index e7378be171a9f..a5870d386f983 100644
> --- a/arch/riscv/net/bpf_jit_comp64.c
> +++ b/arch/riscv/net/bpf_jit_comp64.c
> @@ -2141,7 +2141,11 @@ 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();

[Severity: Low]
Does this code cause a build regression when compiling without ZACAS support?

Looking at arch/riscv/include/asm/cmpxchg.h, system_has_cmpxchg128() is
conditionally defined without a fallback:

#if defined(CONFIG_64BIT) && defined(CONFIG_RISCV_ISA_ZACAS) && defined(CONFIG_TOOLCHAIN_HAS_ZACAS)
#define system_has_cmpxchg128()        riscv_has_extension_unlikely(...)
...
#endif

If a kernel is built with CONFIG_BPF_JIT=y but CONFIG_RISCV_ISA_ZACAS=n
(or CONFIG_TOOLCHAIN_HAS_ZACAS=n), will the unconditional use of
system_has_cmpxchg128() in bpf_jit_supports_arena() result in an implicit
declaration error?

>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260901131132.16241-1-cp0613@linux.alibaba.com?part=1