powerpc64/bpf: Additional NVR handling for bpf_throw

The bpf_throw() function never returns, if it has clobbered
any callee-saved register, those will remain clobbered. The
prologue must take care of saving all callee-saved registers
in the frame of exception boundary program. Later these
additional non volatile registers R14-R25 along with other
NVRs are restored back in the epilogue of exception callback.

To achieve above objective, the frame size is determined
dynamically to accommodate additional non volatile registers
in exception boundary's frame.
For non-exception boundary program, the frame size remains
optimal. The additional instructions to save & restore r14-r25
registers are emitted only during exception boundary and
exception callback program respectively.

Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260124075223.6033-7-adubey@linux.ibm.com

Author: Abhishek Dubey

arch/powerpc/net/bpf_jit_comp64.c

@@ -32,21 +32,37 @@
  *
  *		[	prev sp		] <-------------
  *		[    tail_call_info	] 8		|
- *		[   nv gpr save area	] 6*8		|
+ *		[   nv gpr save area	] 6*8 + (12*8)	|
  *		[    local_tmp_var	] 24		|
  * fp (r31) -->	[   ebpf stack space	] upto 512	|
  *		[     frame header	] 32/112	|
  * sp (r1) --->	[    stack pointer	] --------------
+ *
+ * Additional (12*8) in 'nv gpr save area' only in case of
+ * exception boundary.
  */
 
 /* for bpf JIT code internal usage */
 #define BPF_PPC_STACK_LOCALS	24
+/*
+ * for additional non volatile registers(r14-r25) to be saved
+ * at exception boundary
+ */
+#define BPF_PPC_EXC_STACK_SAVE (12*8)
+
 /* stack frame excluding BPF stack, ensure this is quadword aligned */
 #define BPF_PPC_STACKFRAME	(STACK_FRAME_MIN_SIZE + \
 				 BPF_PPC_STACK_LOCALS + \
 				 BPF_PPC_STACK_SAVE   + \
 				 BPF_PPC_TAILCALL)
 
+/*
+ * same as BPF_PPC_STACKFRAME with save area for additional
+ * non volatile registers saved at exception boundary.
+ * This is quad-word aligned.
+ */
+#define BPF_PPC_EXC_STACKFRAME (BPF_PPC_STACKFRAME + BPF_PPC_EXC_STACK_SAVE)
+
 /* BPF register usage */
 #define TMP_REG_1	(MAX_BPF_JIT_REG + 0)
 #define TMP_REG_2	(MAX_BPF_JIT_REG + 1)
@@ -88,10 +104,16 @@ static inline bool bpf_has_stack_frame(struct codegen_context *ctx)
 	 * - we call other functions (kernel helpers), or
 	 * - the bpf program uses its stack area
 	 * The latter condition is deduced from the usage of BPF_REG_FP
+	 *
+	 * bpf_throw() leads to exception callback from a BPF (sub)program.
+	 * The (sub)program is always marked as SEEN_FUNC, creating a stack
+	 * frame. The exception callback uses the frame of the exception
+	 * boundary, so the exception boundary program must have a frame.
 	 */
 	return ctx->seen & SEEN_FUNC ||
 	       bpf_is_seen_register(ctx, bpf_to_ppc(BPF_REG_FP)) ||
-	       ctx->exception_cb;
+	       ctx->exception_cb ||
+	       ctx->exception_boundary;
 }
 
 /*
@@ -103,9 +125,12 @@ static inline bool bpf_has_stack_frame(struct codegen_context *ctx)
  *		[	  ...       	] 		|
  * sp (r1) --->	[    stack pointer	] --------------
  *		[    tail_call_info	] 8
- *		[   nv gpr save area	] 6*8
+ *		[   nv gpr save area	] 6*8 + (12*8)
  *		[    local_tmp_var	] 24
  *		[   unused red zone	] 224
+ *
+ * Additional (12*8) in 'nv gpr save area' only in case of
+ * exception boundary.
  */
 static int bpf_jit_stack_local(struct codegen_context *ctx)
 {
@@ -114,7 +139,12 @@ static int bpf_jit_stack_local(struct codegen_context *ctx)
 		return STACK_FRAME_MIN_SIZE + ctx->stack_size;
 	} else {
 		/* Stack layout with redzone */
-		return -(BPF_PPC_TAILCALL + BPF_PPC_STACK_SAVE + BPF_PPC_STACK_LOCALS);
+		return -(BPF_PPC_TAILCALL
+			+BPF_PPC_STACK_SAVE
+			+(ctx->exception_boundary || ctx->exception_cb ?
+						BPF_PPC_EXC_STACK_SAVE : 0)
+			+BPF_PPC_STACK_LOCALS
+			);
 	}
 }
 
@@ -125,9 +155,19 @@ int bpf_jit_stack_tailcallinfo_offset(struct codegen_context *ctx)
 
 static int bpf_jit_stack_offsetof(struct codegen_context *ctx, int reg)
 {
-	if (reg >= BPF_PPC_NVR_MIN && reg < 32)
+	int min_valid_nvreg = BPF_PPC_NVR_MIN;
+	/* Default frame size for all cases except exception boundary */
+	int frame_nvr_size = BPF_PPC_STACKFRAME;
+
+	/* Consider all nv regs for handling exceptions */
+	if (ctx->exception_boundary || ctx->exception_cb) {
+		min_valid_nvreg = _R14;
+		frame_nvr_size = BPF_PPC_EXC_STACKFRAME;
+	}
+
+	if (reg >= min_valid_nvreg && reg < 32)
 		return (bpf_has_stack_frame(ctx) ?
-			(BPF_PPC_STACKFRAME + ctx->stack_size) : 0)
+			(frame_nvr_size + ctx->stack_size) : 0)
 				- (8 * (32 - reg)) - BPF_PPC_TAILCALL;
 
 	pr_err("BPF JIT is asking about unknown registers");
@@ -138,6 +178,17 @@ void bpf_jit_realloc_regs(struct codegen_context *ctx)
 {
 }
 
+/*
+ * For exception boundary & exception_cb progs:
+ *     return increased size to accommodate additional NVRs.
+ */
+static int bpf_jit_stack_size(struct codegen_context *ctx)
+{
+	return ctx->exception_boundary || ctx->exception_cb ?
+					BPF_PPC_EXC_STACKFRAME :
+					BPF_PPC_STACKFRAME;
+}
+
 void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
 {
 	int i;
@@ -198,7 +249,19 @@ void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
 			EMIT(PPC_RAW_STD(_R0, _R1, PPC_LR_STKOFF));
 		}
 
-		EMIT(PPC_RAW_STDU(_R1, _R1, -(BPF_PPC_STACKFRAME + ctx->stack_size)));
+		EMIT(PPC_RAW_STDU(_R1, _R1,
+				-(bpf_jit_stack_size(ctx) + ctx->stack_size)));
+	}
+
+	/*
+	 * Program acting as exception boundary pushes R14..R25 in addition to
+	 * BPF callee-saved non volatile registers. Exception callback uses
+	 * the boundary program's stack frame, recover additionally saved
+	 * registers in epilogue of exception callback.
+	 */
+	if (ctx->exception_boundary) {
+		for (i = _R14; i <= _R25; i++)
+			EMIT(PPC_RAW_STD(i, _R1, bpf_jit_stack_offsetof(ctx, i)));
 	}
 
 	if (!ctx->exception_cb) {
@@ -248,9 +311,19 @@ static void bpf_jit_emit_common_epilogue(u32 *image, struct codegen_context *ctx
 		EMIT(PPC_RAW_LD(bpf_to_ppc(ARENA_VM_START), _R1,
 				bpf_jit_stack_offsetof(ctx, bpf_to_ppc(ARENA_VM_START))));
 
+	if (ctx->exception_cb) {
+		/*
+		 * Recover additionally saved non volatile registers from stack
+		 * frame of exception boundary program.
+		 */
+		for (i = _R14; i <= _R25; i++)
+			EMIT(PPC_RAW_LD(i, _R1, bpf_jit_stack_offsetof(ctx, i)));
+	}
+
 	/* Tear down our stack frame */
 	if (bpf_has_stack_frame(ctx)) {
-		EMIT(PPC_RAW_ADDI(_R1, _R1, BPF_PPC_STACKFRAME + ctx->stack_size));
+		EMIT(PPC_RAW_ADDI(_R1, _R1, bpf_jit_stack_size(ctx) + ctx->stack_size));
+
 		if (ctx->seen & SEEN_FUNC || ctx->exception_cb) {
 			EMIT(PPC_RAW_LD(_R0, _R1, PPC_LR_STKOFF));
 			EMIT(PPC_RAW_MTLR(_R0));