powerpc64/bpf: Implement JIT support for private stack

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.

Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Acked-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260401103215.104438-1-adubey@linux.ibm.com

Author: Abhishek Dubey

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

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 IS_ENABLED(CONFIG_PPC64);
 }
 
+bool bpf_jit_supports_private_stack(void)
+{
+	return IS_ENABLED(CONFIG_PPC64);
+}
+
 bool bpf_jit_supports_arena(void)
 {
 	return IS_ENABLED(CONFIG_PPC64);

arch/powerpc/net/bpf_jit_comp64.c

@@ -183,6 +183,24 @@ void bpf_jit_realloc_regs(struct codegen_context *ctx)
 {
 }
 
+static void emit_fp_priv_stack(u32 *image, struct codegen_context *ctx)
+{
+	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)
+	 */
+#ifdef CONFIG_SMP
+	/* Load percpu data offset */
+	EMIT(PPC_RAW_LD(bpf_to_ppc(TMP_REG_1), _R13,
+		offsetof(struct paca_struct, data_offset)));
+	EMIT(PPC_RAW_ADD(bpf_to_ppc(BPF_REG_FP),
+		bpf_to_ppc(TMP_REG_1), bpf_to_ppc(BPF_REG_FP)));
+#endif
+	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 +325,16 @@ 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);