[PATCH v2 bpf-next 1/3] riscv: bpf: Add BPF private stack support
Chen Pei
cp0613 at linux.alibaba.com
Wed Nov 4 02:04:09 PST 2026
A program on a private stack keeps its BPF frames in a per-CPU area of
its own rather than on the kernel stack of whichever context entered it.
bpf_enable_priv_stack() grants that mode only once the JIT reports
bpf_jit_supports_private_stack(), which riscv64 did not: programs there
always ran on the kernel stack, and a struct_ops program that requires
one was rejected with -EACCES.
Allocate a per-CPU area of the verifier calculated depth plus a 16 byte
guard region at each end, and point BPF_REG_FP (S5) at the top of it
while leaving SP on the kernel stack. Every BPF stack access already
goes through S5, and S5 is callee-saved with each subprog deriving its
own, so nothing else in the JIT has to know about the private stack.
The outgoing stack argument area stays in the kernel frame: callers
write it relative to SP, callees relative to RV_REG_FP, which the
prologue sets to the caller's SP.
The per-CPU lookup is emitted after the tail call entry point: a program
using a private stack can still be entered as a tail call target, and
RV_TAILCALL_OFFSET is a constant added to the target's bpf_func, so what
is in front of it must be the same size in every program.
The base is materialized with absolute moves, as on arm64, x86-64 and
powerpc64: the vmalloc'd per-CPU area is beyond an auipc pair from the
JIT region, and riscv64 has no register holding the per-CPU offset, so
the address of __per_cpu_offset is loaded too. emit_imm() is a choice
there, not a requirement; emit_addr() as used by
insn_is_mov_percpu_addr() would need an extra_pass argument and an error
return in bpf_jit_build_prologue(), touching bpf_jit_comp32.c too. Both
constants are fixed before the offset convergence loop, so the prologue
length is stable across passes.
The guard value follows arm64, x86-64 and powerpc64, and the 16 byte
two-copy region at each end follows arm64 and powerpc64, since x86-64
uses an 8 byte guard with a single copy. A jited program has them
checked when it is freed, reporting the first corrupted CPU only;
failure paths release the area unchecked since the image never ran,
which also clears priv_stack_ptr for the later bpf_jit_free().
RV32 does not declare support and keeps the existing kernel stack
behaviour.
Signed-off-by: Chen Pei <cp0613 at linux.alibaba.com>
---
arch/riscv/net/bpf_jit.h | 1 +
arch/riscv/net/bpf_jit_comp64.c | 42 +++++++++++++++--
arch/riscv/net/bpf_jit_core.c | 84 +++++++++++++++++++++++++++++----
3 files changed, 115 insertions(+), 12 deletions(-)
diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
index 21a190a6a6d9..efab2b6803e4 100644
--- a/arch/riscv/net/bpf_jit.h
+++ b/arch/riscv/net/bpf_jit.h
@@ -14,6 +14,7 @@
#define RV_MAX_REG_ARGS 8
#define RV_EXTRA_STK_ARGS (RV_MAX_REG_ARGS - MAX_BPF_FUNC_REG_ARGS)
+#define PRIV_STACK_GUARD_SZ 16
/* verify runtime detection extension status */
#define rv_ext_enabled(ext) \
diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index 371405598faa..a7a7d2f9da85 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -2134,12 +2134,34 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
return 0;
}
+static void emit_priv_stack_frame(struct rv_jit_context *ctx)
+{
+ u64 fp = (__force u64)ctx->prog->aux->priv_stack_ptr + PRIV_STACK_GUARD_SZ;
+
+ fp += round_up(ctx->prog->aux->stack_depth, STACK_ALIGN);
+ emit_imm(RV_REG_S5, fp, ctx);
+#ifdef CONFIG_SMP
+ emit_lw(RV_REG_T1, offsetof(struct thread_info, cpu), RV_REG_TP, ctx);
+ emit_imm(RV_REG_T2, (u64)__per_cpu_offset, ctx);
+ emit_sh3add(RV_REG_T1, RV_REG_T1, RV_REG_T2, ctx);
+ emit_ld(RV_REG_T1, 0, RV_REG_T1, ctx);
+ emit_add(RV_REG_S5, RV_REG_S5, RV_REG_T1, ctx);
+#endif
+}
+
void bpf_jit_build_prologue(struct rv_jit_context *ctx, bool is_subprog)
{
+ bool priv_stack = ctx->prog->aux->priv_stack_ptr;
int i, stack_adjust = 0, store_offset, bpf_stack_adjust = ctx->stack_arg_sz;
- bpf_stack_adjust += round_up(ctx->prog->aux->stack_depth, STACK_ALIGN);
- if (bpf_stack_adjust)
+ /*
+ * With a private stack only the outgoing stack argument area is left in
+ * this frame. It has to stay: the caller writes it relative to SP while
+ * the callee reads it relative to RV_REG_FP, the caller's SP set below.
+ */
+ if (!priv_stack)
+ bpf_stack_adjust += round_up(ctx->prog->aux->stack_depth, STACK_ALIGN);
+ if (bpf_stack_adjust || priv_stack)
mark_fp(ctx);
if (seen_reg(RV_REG_RA, ctx))
@@ -2217,7 +2239,16 @@ void bpf_jit_build_prologue(struct rv_jit_context *ctx, bool is_subprog)
emit_addi(RV_REG_FP, RV_REG_SP, stack_adjust, ctx);
- if (bpf_stack_adjust)
+ /*
+ * Point BPF_REG_FP at the top of this CPU's private stack. A program
+ * with one can still be entered as a tail call target, at
+ * RV_TAILCALL_OFFSET, so this has to come after that entry point; the
+ * offset is a constant added to the target's bpf_func, so everything in
+ * front of it must also be the same size in every program.
+ */
+ if (priv_stack)
+ emit_priv_stack_frame(ctx);
+ else if (bpf_stack_adjust)
emit_addi(RV_REG_S5, RV_REG_SP, bpf_stack_adjust, ctx);
ctx->stack_size = stack_adjust;
@@ -2320,3 +2351,8 @@ bool bpf_jit_supports_hmul(void)
{
return true;
}
+
+bool bpf_jit_supports_private_stack(void)
+{
+ return true;
+}
diff --git a/arch/riscv/net/bpf_jit_core.c b/arch/riscv/net/bpf_jit_core.c
index f2497ec7d378..f2021088e447 100644
--- a/arch/riscv/net/bpf_jit_core.c
+++ b/arch/riscv/net/bpf_jit_core.c
@@ -9,6 +9,7 @@
#include <linux/bpf.h>
#include <linux/filter.h>
#include <linux/memory.h>
+#include <linux/percpu.h>
#include <asm/text-patching.h>
#include <asm/cfi.h>
#include "bpf_jit.h"
@@ -44,6 +45,62 @@ bool bpf_jit_needs_zext(void)
return true;
}
+#define PRIV_STACK_GUARD_VAL 0xEB9F12345678eb9fULL
+
+static u32 priv_stack_size(struct bpf_prog *prog)
+{
+ return round_up(prog->aux->stack_depth, STACK_ALIGN) +
+ 2 * PRIV_STACK_GUARD_SZ;
+}
+
+static int alloc_priv_stack(struct bpf_prog *prog)
+{
+ u32 size = priv_stack_size(prog);
+ u32 end = (size - PRIV_STACK_GUARD_SZ) / sizeof(u64);
+ void __percpu *ptr;
+ int cpu, i;
+
+ ptr = __alloc_percpu_gfp(size, STACK_ALIGN, GFP_KERNEL);
+ if (!ptr)
+ return -ENOMEM;
+
+ for_each_possible_cpu(cpu) {
+ u64 *stack = per_cpu_ptr(ptr, cpu);
+
+ for (i = 0; i < PRIV_STACK_GUARD_SZ / sizeof(u64); i++) {
+ stack[i] = PRIV_STACK_GUARD_VAL;
+ stack[end + i] = PRIV_STACK_GUARD_VAL;
+ }
+ }
+ prog->aux->priv_stack_ptr = ptr;
+ return 0;
+}
+
+static void priv_stack_check_guard(struct bpf_prog *prog)
+{
+ u32 end = (priv_stack_size(prog) - PRIV_STACK_GUARD_SZ) / sizeof(u64);
+ int cpu;
+
+ for_each_possible_cpu(cpu) {
+ u64 *stack = per_cpu_ptr(prog->aux->priv_stack_ptr, cpu);
+
+ if (stack[0] != PRIV_STACK_GUARD_VAL ||
+ stack[1] != PRIV_STACK_GUARD_VAL ||
+ stack[end] != PRIV_STACK_GUARD_VAL ||
+ stack[end + 1] != PRIV_STACK_GUARD_VAL) {
+ pr_err("bpf-jit: private stack overflow/underflow guard of prog %s corrupted on CPU %d\n",
+ bpf_jit_get_prog_name(prog), cpu);
+ return;
+ }
+ }
+}
+
+static void free_priv_stack(struct bpf_prog *prog)
+{
+ free_percpu(prog->aux->priv_stack_ptr);
+ prog->aux->priv_stack_ptr = NULL;
+}
+
struct bpf_prog *bpf_int_jit_compile(struct bpf_verifier_env *env, struct bpf_prog *prog)
{
unsigned int prog_size = 0, extable_size = 0;
@@ -84,12 +141,15 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_verifier_env *env, struct bpf_pr
round_up((stack_arg_cnt - RV_EXTRA_STK_ARGS) * 8, STACK_ALIGN) : 0;
ctx->prog = prog;
+ if (prog->aux->jits_use_priv_stack && alloc_priv_stack(prog))
+ goto out_free_hdr;
+
ctx->offset = kvzalloc_objs(int, prog->len);
if (!ctx->offset)
- goto out_offset;
+ goto out_free_hdr;
if (build_body(ctx, extra_pass, NULL))
- goto out_offset;
+ goto out_free_hdr;
for (i = 0; i < prog->len; i++) {
prev_ninsns += 32;
@@ -104,7 +164,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_verifier_env *env, struct bpf_pr
ctx->prologue_len = ctx->ninsns;
if (build_body(ctx, extra_pass, ctx->offset))
- goto out_offset;
+ goto out_free_hdr;
ctx->epilogue_offset = ctx->ninsns;
bpf_jit_build_epilogue(ctx);
@@ -124,7 +184,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_verifier_env *env, struct bpf_pr
bpf_fill_ill_insns,
bpf_prog_was_classic(prog));
if (!jit_data->ro_header)
- goto out_offset;
+ goto out_free_hdr;
/*
* Use the image(RW) for writing the JITed instructions. But also save
@@ -190,10 +250,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_verifier_env *env, struct bpf_pr
ctx->offset[i] = ctx->offset[i - 1];
ctx->offset[0] = ninsns_rvoff(ctx->prologue_len);
bpf_prog_update_insn_ptrs(prog, ctx->offset, jit_data->ro_image);
-out_offset:
- kvfree(ctx->offset);
- kfree(jit_data);
- prog->aux->jit_data = NULL;
+ goto out_free_ctx;
}
return prog;
@@ -209,7 +266,12 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_verifier_env *env, struct bpf_pr
sizeof(jit_data->header->size));
bpf_jit_binary_pack_free(jit_data->ro_header, jit_data->header);
}
- goto out_offset;
+ free_priv_stack(prog);
+out_free_ctx:
+ kvfree(ctx->offset);
+ kfree(jit_data);
+ prog->aux->jit_data = NULL;
+ return prog;
}
u64 bpf_jit_alloc_exec_limit(void)
@@ -261,6 +323,10 @@ void bpf_jit_free(struct bpf_prog *prog)
prog->bpf_func = (void *)prog->bpf_func - cfi_get_offset();
hdr = bpf_jit_binary_pack_hdr(prog);
bpf_jit_binary_pack_free(hdr, NULL);
+ if (prog->aux->priv_stack_ptr) {
+ priv_stack_check_guard(prog);
+ free_priv_stack(prog);
+ }
WARN_ON_ONCE(!bpf_prog_kallsyms_verify_off(prog));
}
--
2.50.1
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