[PATCH v4 1/2] powerpc64/bpf: Implement JIT support for private stack
Venkat
venkat88 at linux.ibm.com
Thu Feb 26 17:49:05 AEDT 2026
> On 26 Feb 2026, at 8:43 AM, adubey at linux.ibm.com wrote:
>
> From: Abhishek Dubey <adubey at linux.ibm.com>
>
> Provision the private stack as a per-CPU allocation during
> bpf_int_jit_compile(). Align the stack to 16 bytes and place guard
> regions at both ends to detect runtime stack overflow and underflow.
>
> Round the private stack size up to the nearest 16-byte boundary.
> Make each guard region 16 bytes to preserve the required overall
> 16-byte alignment. When private stack is set, skip bpf stack size
> accounting in kernel stack.
>
> There is no stack pointer in powerpc. Stack referencing during JIT
> is done using frame pointer. Frame pointer calculation goes like:
>
> BPF frame pointer = Priv stack allocation start address +
> Overflow guard +
> Actual stack size defined by verifier
>
> Memory layout:
>
> High Addr +--------------------------------------------------+
> | |
> | 16 bytes Underflow guard (0xEB9F12345678eb9fULL) |
> | |
> BPF FP -> +--------------------------------------------------+
> | |
> | Private stack - determined by verifier |
> | 16-bytes aligned |
> | |
> +--------------------------------------------------+
> | |
> Lower Addr | 16 byte Overflow guard (0xEB9F12345678eb9fULL) |
> | |
> Priv stack alloc ->+--------------------------------------------------+
> start
>
> Update BPF_REG_FP to point to the calculated offset within the
> allocated private stack buffer. Now, BPF stack usage reference
> in the allocated private stack.
>
> The patch is rebase over fixes by Hari:
> https://lore.kernel.org/bpf/20260220063933.196141-1-hbathini@linux.ibm.com/
>
> v1->v2:
> Fix ci-bot warning for percpu pointer casting
> Minor refactoring
> v2->v3:
> Fix ci-bot bug targeting clobbered NVRs on stack rollback
> v3->v4:
> Added new field to fix priv_stack allocation
>
> [v3]: https://lore.kernel.org/bpf/20260226005440.9570-1-adubey@linux.ibm.com
> [v2]: https://lore.kernel.org/bpf/20260225153950.15331-1-adubey@linux.ibm.com
> [v1]: https://lore.kernel.org/bpf/20260216152234.36632-1-adubey@linux.ibm.com
>
> Signed-off-by: Abhishek Dubey <adubey at linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88 at linux.ibm.com>
> ---
> arch/powerpc/net/bpf_jit.h | 6 ++
> arch/powerpc/net/bpf_jit_comp.c | 97 +++++++++++++++++++++++++++++--
> arch/powerpc/net/bpf_jit_comp64.c | 29 ++++++++-
> 3 files changed, 124 insertions(+), 8 deletions(-)
>
> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
> index 7354e1d72f79..a232f3fb73be 100644
> --- a/arch/powerpc/net/bpf_jit.h
> +++ b/arch/powerpc/net/bpf_jit.h
> @@ -178,8 +178,14 @@ struct codegen_context {
> bool is_subprog;
> bool exception_boundary;
> bool exception_cb;
> + void __percpu *priv_sp;
> + unsigned int priv_stack_size;
> };
>
> +/* Memory size & magic-value to detect private stack overflow/underflow */
> +#define PRIV_STACK_GUARD_SZ 16
> +#define PRIV_STACK_GUARD_VAL 0xEB9F12345678eb9fULL
> +
> #define bpf_to_ppc(r) (ctx->b2p[r])
>
> #ifdef CONFIG_PPC32
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index 278e09b57560..ad70c3e1294e 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -129,25 +129,60 @@ bool bpf_jit_needs_zext(void)
> return true;
> }
>
> +static void priv_stack_init_guard(void __percpu *priv_stack_ptr, int alloc_size)
> +{
> + int cpu, underflow_idx = (alloc_size - PRIV_STACK_GUARD_SZ) >> 3;
> + u64 *stack_ptr;
> +
> + for_each_possible_cpu(cpu) {
> + stack_ptr = per_cpu_ptr(priv_stack_ptr, cpu);
> + stack_ptr[0] = PRIV_STACK_GUARD_VAL;
> + stack_ptr[1] = PRIV_STACK_GUARD_VAL;
> + stack_ptr[underflow_idx] = PRIV_STACK_GUARD_VAL;
> + stack_ptr[underflow_idx + 1] = PRIV_STACK_GUARD_VAL;
> + }
> +}
> +
> +static void priv_stack_check_guard(void __percpu *priv_stack_ptr, int alloc_size,
> + struct bpf_prog *fp)
> +{
> + int cpu, underflow_idx = (alloc_size - PRIV_STACK_GUARD_SZ) >> 3;
> + u64 *stack_ptr;
> +
> + for_each_possible_cpu(cpu) {
> + stack_ptr = per_cpu_ptr(priv_stack_ptr, cpu);
> + if (stack_ptr[0] != PRIV_STACK_GUARD_VAL ||
> + stack_ptr[1] != PRIV_STACK_GUARD_VAL ||
> + stack_ptr[underflow_idx] != PRIV_STACK_GUARD_VAL ||
> + stack_ptr[underflow_idx + 1] != PRIV_STACK_GUARD_VAL) {
> + pr_err("BPF private stack overflow/underflow detected for prog %s\n",
> + bpf_jit_get_prog_name(fp));
> + break;
> + }
> + }
> +}
> +
> struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> {
> u32 proglen;
> u32 alloclen;
> u8 *image = NULL;
> - u32 *code_base;
> - u32 *addrs;
> - struct powerpc_jit_data *jit_data;
> + u32 *code_base = NULL;
> + u32 *addrs = NULL;
> + struct powerpc_jit_data *jit_data = NULL;
> struct codegen_context cgctx;
> int pass;
> int flen;
> + int priv_stack_alloc_size;
> + void __percpu *priv_stack_ptr = NULL;
> struct bpf_binary_header *fhdr = NULL;
> struct bpf_binary_header *hdr = NULL;
> struct bpf_prog *org_fp = fp;
> - struct bpf_prog *tmp_fp;
> + struct bpf_prog *tmp_fp = NULL;
> bool bpf_blinded = false;
> bool extra_pass = false;
> u8 *fimage = NULL;
> - u32 *fcode_base;
> + u32 *fcode_base = NULL;
> u32 extable_len;
> u32 fixup_len;
>
> @@ -173,6 +208,26 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> fp->aux->jit_data = jit_data;
> }
>
> + priv_stack_ptr = fp->aux->priv_stack_ptr;
> + if (!priv_stack_ptr && fp->aux->jits_use_priv_stack) {
> + /*
> + * Allocate private stack of size equivalent to
> + * verifier-calculated stack size plus two memory
> + * guard regions to detect private stack overflow
> + * and underflow.
> + */
> + priv_stack_alloc_size = round_up(fp->aux->stack_depth, 16) +
> + 2 * PRIV_STACK_GUARD_SZ;
> + priv_stack_ptr = __alloc_percpu_gfp(priv_stack_alloc_size, 16, GFP_KERNEL);
> + if (!priv_stack_ptr) {
> + fp = org_fp;
> + goto out_priv_stack;
> + }
> +
> + priv_stack_init_guard(priv_stack_ptr, priv_stack_alloc_size);
> + fp->aux->priv_stack_ptr = priv_stack_ptr;
> + }
> +
> flen = fp->len;
> addrs = jit_data->addrs;
> if (addrs) {
> @@ -209,6 +264,19 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> cgctx.is_subprog = bpf_is_subprog(fp);
> cgctx.exception_boundary = fp->aux->exception_boundary;
> cgctx.exception_cb = fp->aux->exception_cb;
> + cgctx.priv_sp = priv_stack_ptr;
> + cgctx.priv_stack_size = 0;
> + if (priv_stack_ptr) {
> + /*
> + * priv_stack_size required for setting bpf FP inside
> + * percpu allocation.
> + * stack_size is marked 0 to prevent allocation on
> + * general stack and offset calculation don't go for
> + * a toss in bpf_jit_stack_offsetof() & bpf_jit_stack_local()
> + */
> + cgctx.priv_stack_size = cgctx.stack_size;
> + cgctx.stack_size = 0;
> + }
>
> /* Scouting faux-generate pass 0 */
> if (bpf_jit_build_body(fp, NULL, NULL, &cgctx, addrs, 0, false)) {
> @@ -306,6 +374,11 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> }
> bpf_prog_fill_jited_linfo(fp, addrs);
> out_addrs:
> + if (!image && priv_stack_ptr) {
> + fp->aux->priv_stack_ptr = NULL;
> + free_percpu(priv_stack_ptr);
> + }
> +out_priv_stack:
> kfree(addrs);
> kfree(jit_data);
> fp->aux->jit_data = NULL;
> @@ -419,6 +492,8 @@ void bpf_jit_free(struct bpf_prog *fp)
> if (fp->jited) {
> struct powerpc_jit_data *jit_data = fp->aux->jit_data;
> struct bpf_binary_header *hdr;
> + void __percpu *priv_stack_ptr;
> + int priv_stack_alloc_size;
>
> /*
> * If we fail the final pass of JIT (from jit_subprogs),
> @@ -432,6 +507,13 @@ void bpf_jit_free(struct bpf_prog *fp)
> }
> hdr = bpf_jit_binary_pack_hdr(fp);
> bpf_jit_binary_pack_free(hdr, NULL);
> + priv_stack_ptr = fp->aux->priv_stack_ptr;
> + if (priv_stack_ptr) {
> + priv_stack_alloc_size = round_up(fp->aux->stack_depth, 16) +
> + 2 * PRIV_STACK_GUARD_SZ;
> + priv_stack_check_guard(priv_stack_ptr, priv_stack_alloc_size, fp);
> + free_percpu(priv_stack_ptr);
> + }
> WARN_ON_ONCE(!bpf_prog_kallsyms_verify_off(fp));
> }
>
> @@ -453,6 +535,11 @@ bool bpf_jit_supports_kfunc_call(void)
> return true;
> }
>
> +bool bpf_jit_supports_private_stack(void)
> +{
> + return IS_ENABLED(CONFIG_PPC64);
> +}
> +
> bool bpf_jit_supports_arena(void)
> {
> return IS_ENABLED(CONFIG_PPC64);
> diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
> index 640b84409687..4d997da242fe 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c
> @@ -183,6 +183,22 @@ void bpf_jit_realloc_regs(struct codegen_context *ctx)
> {
> }
>
> +static void emit_fp_priv_stack(u32 *image, struct codegen_context *ctx)
> +{
> + /* Load percpu data offset */
> + EMIT(PPC_RAW_LD(bpf_to_ppc(TMP_REG_1), _R13,
> + offsetof(struct paca_struct, data_offset)));
> + PPC_LI64(bpf_to_ppc(BPF_REG_FP), (__force long)ctx->priv_sp);
> + /*
> + * Load base percpu pointer of private stack allocation.
> + * Runtime per-cpu address = (base + data_offset) + (guard + stack_size)
> + */
> + EMIT(PPC_RAW_ADD(bpf_to_ppc(BPF_REG_FP),
> + bpf_to_ppc(TMP_REG_1), bpf_to_ppc(BPF_REG_FP)));
> + EMIT(PPC_RAW_ADDI(bpf_to_ppc(BPF_REG_FP), bpf_to_ppc(BPF_REG_FP),
> + PRIV_STACK_GUARD_SZ + round_up(ctx->priv_stack_size, 16)));
> +}
> +
> /*
> * For exception boundary & exception_cb progs:
> * return increased size to accommodate additional NVRs.
> @@ -307,9 +323,16 @@ void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
> * Exception_cb not restricted from using stack area or arena.
> * Setup frame pointer to point to the bpf stack area
> */
> - if (bpf_is_seen_register(ctx, bpf_to_ppc(BPF_REG_FP)))
> - EMIT(PPC_RAW_ADDI(bpf_to_ppc(BPF_REG_FP), _R1,
> - STACK_FRAME_MIN_SIZE + ctx->stack_size));
> + if (bpf_is_seen_register(ctx, bpf_to_ppc(BPF_REG_FP))) {
> + if (ctx->priv_sp) {
> + /* Set up fp in private stack */
> + emit_fp_priv_stack(image, ctx);
> + } else {
> + /* Setup frame pointer to point to the bpf stack area */
> + EMIT(PPC_RAW_ADDI(bpf_to_ppc(BPF_REG_FP), _R1,
> + STACK_FRAME_MIN_SIZE + ctx->stack_size));
> + }
> + }
>
> if (ctx->arena_vm_start)
> PPC_LI64(bpf_to_ppc(ARENA_VM_START), ctx->arena_vm_start);
> --
> 2.52.0
>
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