Subject: Re: [PATCH 1/2] powerpc64/bpf: Implement JIT support for private stack

From: "Christophe Leroy (CS GROUP)" <[email-masked]>
Date: Mon, 23 Feb 2026 12:10:44 +0100

Le 16/02/2026 à 16:22, [email-masked] a écrit :
> From: Abhishek Dubey <[email-masked]>
> 
> 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/[email-masked]/T/#mf02cad9096fa4ad1f05610b1f464da1cddf7445a
> 
> Signed-off-by: Abhishek Dubey <[email-masked]>
> ---
>   arch/powerpc/net/bpf_jit.h        |  5 +++
>   arch/powerpc/net/bpf_jit_comp.c   | 74 +++++++++++++++++++++++++++++++
>   arch/powerpc/net/bpf_jit_comp64.c | 38 +++++++++++++---
>   3 files changed, 112 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
> index 7354e1d72f79..5a115c54e43a 100644
> --- a/arch/powerpc/net/bpf_jit.h
> +++ b/arch/powerpc/net/bpf_jit.h
> @@ -178,8 +178,13 @@ struct codegen_context {
>   	bool is_subprog;
>   	bool exception_boundary;
>   	bool exception_cb;
> +	u64 priv_sp;

Are you sure you want a u64, not a long ?

>   };
>   
> +/* 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..7a78e03d482f 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -129,6 +129,39 @@ 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 %sx\n",
> +			bpf_jit_get_prog_name(fp));
> +			break;
> +		}
> +	}
> +}
> +
>   struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
>   {
>   	u32 proglen;
> @@ -140,6 +173,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
>   	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;
> @@ -173,6 +208,25 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
>   		fp->aux->jit_data = jit_data;
>   	}
>   
> +	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 +263,7 @@ 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 ? (u64)priv_stack_ptr : 0;

priv_stack_ptr is a pointer, ie a long.
priv_sp is a u64, ie a long long.

This will mismatch on powerpc/32

>   
>   	/* Scouting faux-generate pass 0 */
>   	if (bpf_jit_build_body(fp, NULL, NULL, &cgctx, addrs, 0, false)) {
> @@ -306,7 +361,12 @@ 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) {
> +			free_percpu(priv_stack_ptr);
> +			fp->aux->priv_stack_ptr = NULL;

Shouldn't those two lines be swapped ?

> +		}
>   		kfree(addrs);
> +out_priv_stack:
>   		kfree(jit_data);
>   		fp->aux->jit_data = NULL;
>   	} else {
> @@ -419,6 +479,8 @@ 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 +494,13 @@ 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 +522,11 @@ 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..206ef43b4090 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c
> @@ -183,6 +183,20 @@ void bpf_jit_realloc_regs(struct codegen_context *ctx)
>   {
>   }
>   
> +static void emit_fp_priv_stack(u32 *image, struct codegen_context *ctx, void __percpu *ptr)
> +{
> +	/* 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), (u64)ptr);
> +	/*
> +	 * Set frame pointer with percpu allocated
> +	 * buffer for private stack.
> +	 */
> +	EMIT(PPC_RAW_ADD(bpf_to_ppc(BPF_REG_FP),
> +			bpf_to_ppc(TMP_REG_1), bpf_to_ppc(BPF_REG_FP)));
> +}
> +
>   /*
>    * For exception boundary & exception_cb progs:
>    *     return increased size to accommodate additional NVRs.
> @@ -197,6 +211,12 @@ static int bpf_jit_stack_size(struct codegen_context *ctx)
>   void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
>   {
>   	int i;
> +	void __percpu *priv_frame_ptr = NULL;
> +
> +	if (ctx->priv_sp) {
> +		priv_frame_ptr = (void*) ctx->priv_sp + PRIV_STACK_GUARD_SZ +
> +					round_up(ctx->stack_size, 16);
> +	}
>   
>   	/* Instruction for trampoline attach */
>   	EMIT(PPC_RAW_NOP());
> @@ -251,7 +271,7 @@ void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
>   		}
>   
>   		EMIT(PPC_RAW_STDU(_R1, _R1,
> -				-(bpf_jit_stack_size(ctx) + ctx->stack_size)));
> +			-(bpf_jit_stack_size(ctx) + (ctx->priv_sp ? 0 : ctx->stack_size))));
>   	}
>   
>   	/*
> @@ -307,9 +327,16 @@ 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 && priv_frame_ptr) {
> +			/* Set up private stack pointer */
> +			emit_fp_priv_stack(image, ctx, priv_frame_ptr);
> +		} 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);
> @@ -339,7 +366,8 @@ static void bpf_jit_emit_common_epilogue(u32 *image, struct codegen_context *ctx
>   
>   	/* Tear down our stack frame */
>   	if (bpf_has_stack_frame(ctx)) {
> -		EMIT(PPC_RAW_ADDI(_R1, _R1, bpf_jit_stack_size(ctx) + ctx->stack_size));
> +		EMIT(PPC_RAW_ADDI(_R1, _R1,
> +			bpf_jit_stack_size(ctx) + (ctx->priv_sp ? 0 : ctx->stack_size)));
>   
>   		if (ctx->seen & SEEN_FUNC || ctx->exception_cb) {
>   			EMIT(PPC_RAW_LD(_R0, _R1, PPC_LR_STKOFF));