powerpc64/bpf: Add powerpc64 JIT support for timed may_goto

When verifier sees a timed may_goto instruction, it emits a call to
arch_bpf_timed_may_goto() with a stack offset in BPF_REG_AX
(powerpc64 R12) and expects the refreshed count value to be returned
in the same register. The verifier doesn't save or restore any registers
before emitting this call.

arch_bpf_timed_may_goto() should act as a trampoline to call
bpf_check_timed_may_goto() with powerpc64 ELF ABI calling convention.

To support this custom calling convention, implement
arch_bpf_timed_may_goto() in assembly and make sure BPF caller saved
registers are preserved, then call bpf_check_timed_may_goto with
the powerpc64 ABI calling convention where first argument and return
value both are in R3. Finally, move the result back into BPF_REG_AX(R12)
before returning.

Also, introduce bpf_jit_emit_func_call() that computes the offset from
kernel_toc_addr(), validates that the target and emits the ADDIS/ADDI
sequence to load the function address before performing the indirect
branch via MTCTR/BCTRL. The existing code in bpf_jit_emit_func_call_rel()
is refactored to use this function.

Acked-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260403105016.74775-1-skb99@linux.ibm.com

Author: Saket Kumar Bhaskar

arch/powerpc/net/Makefile

@@ -3,3 +3,7 @@
 # Arch-specific network modules
 #
 obj-$(CONFIG_BPF_JIT) += bpf_jit_comp.o bpf_jit_comp$(BITS).o
+
+ifdef CONFIG_PPC64
+obj-$(CONFIG_BPF_JIT) += bpf_timed_may_goto.o
+endif

arch/powerpc/net/bpf_jit_comp.c

@@ -517,6 +517,11 @@ bool bpf_jit_supports_subprog_tailcalls(void)
 	return IS_ENABLED(CONFIG_PPC64);
 }
 
+bool bpf_jit_supports_timed_may_goto(void)
+{
+	return IS_ENABLED(CONFIG_PPC64);
+}
+
 bool bpf_jit_supports_kfunc_call(void)
 {
 	return IS_ENABLED(CONFIG_PPC64);

arch/powerpc/net/bpf_jit_comp64.c

@@ -451,10 +451,28 @@ void arch_bpf_stack_walk(bool (*consume_fn)(void *, u64, u64, u64), void *cookie
 	}
 }
 
+static int bpf_jit_emit_func_call(u32 *image, struct codegen_context *ctx, u64 func_addr, int reg)
+{
+	long reladdr = func_addr - kernel_toc_addr();
+
+	if (reladdr > 0x7FFFFFFF || reladdr < -(0x80000000L)) {
+		pr_err("eBPF: address of %ps out of range of kernel_toc.\n", (void *)func_addr);
+		return -ERANGE;
+	}
+
+	EMIT(PPC_RAW_ADDIS(reg, _R2, PPC_HA(reladdr)));
+	EMIT(PPC_RAW_ADDI(reg, reg, PPC_LO(reladdr)));
+	EMIT(PPC_RAW_MTCTR(reg));
+	EMIT(PPC_RAW_BCTRL());
+
+	return 0;
+}
+
 int bpf_jit_emit_func_call_rel(u32 *image, u32 *fimage, struct codegen_context *ctx, u64 func)
 {
 	unsigned long func_addr = func ? ppc_function_entry((void *)func) : 0;
-	long reladdr;
+	long __maybe_unused reladdr;
+	int ret;
 
 	/* bpf to bpf call, func is not known in the initial pass. Emit 5 nops as a placeholder */
 	if (!func) {
@@ -507,16 +525,9 @@ int bpf_jit_emit_func_call_rel(u32 *image, u32 *fimage, struct codegen_context *
 	EMIT(PPC_RAW_BCTRL());
 #else
 	if (core_kernel_text(func_addr)) {
-		reladdr = func_addr - kernel_toc_addr();
-		if (reladdr > 0x7FFFFFFF || reladdr < -(0x80000000L)) {
-			pr_err("eBPF: address of %ps out of range of kernel_toc.\n", (void *)func);
-			return -ERANGE;
-		}
-
-		EMIT(PPC_RAW_ADDIS(_R12, _R2, PPC_HA(reladdr)));
-		EMIT(PPC_RAW_ADDI(_R12, _R12, PPC_LO(reladdr)));
-		EMIT(PPC_RAW_MTCTR(_R12));
-		EMIT(PPC_RAW_BCTRL());
+		ret = bpf_jit_emit_func_call(image, ctx, func_addr, _R12);
+		if (ret)
+			return ret;
 	} else {
 		if (IS_ENABLED(CONFIG_PPC64_ELF_ABI_V1)) {
 			/* func points to the function descriptor */
@@ -1755,6 +1766,35 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, u32 *fimage, struct code
 			if (ret < 0)
 				return ret;
 
+			/*
+			 * Call to arch_bpf_timed_may_goto() is emitted by the
+			 * verifier and called with custom calling convention with
+			 * first argument and return value in BPF_REG_AX (_R12).
+			 *
+			 * The generic helper or bpf function call emission path
+			 * may use the same scratch register as BPF_REG_AX to
+			 * materialize the target address. This would clobber AX
+			 * and break timed may_goto semantics.
+			 *
+			 * Emit a minimal indirect call sequence here using a temp
+			 * register and skip the normal post-call return-value move.
+			 */
+
+			if (func_addr == (u64)arch_bpf_timed_may_goto) {
+				ret = 0;
+				if (!IS_ENABLED(CONFIG_PPC_KERNEL_PCREL))
+					ret = bpf_jit_emit_func_call(image, ctx, func_addr,
+								     tmp1_reg);
+
+				if (ret || IS_ENABLED(CONFIG_PPC_KERNEL_PCREL)) {
+					PPC_LI_ADDR(tmp1_reg, func_addr);
+					EMIT(PPC_RAW_MTCTR(tmp1_reg));
+					EMIT(PPC_RAW_BCTRL());
+				}
+
+				break;
+			}
+
 			/* Take care of powerpc ABI requirements before kfunc call */
 			if (insn[i].src_reg == BPF_PSEUDO_KFUNC_CALL) {
 				if (prepare_for_kfunc_call(fp, image, ctx, &insn[i]))

arch/powerpc/net/bpf_timed_may_goto.S

@@ -0,0 +1,57 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (c) 2025 IBM Corporation, Saket Kumar Bhaskar <skb99@linux.ibm.com> */
+
+#include <linux/linkage.h>
+#include <asm/ppc_asm.h>
+
+/*
+ * arch_bpf_timed_may_goto() trampoline for powerpc64
+ *
+ * Custom BPF convention (verifier/JIT):
+ *	- input:  stack offset in BPF_REG_AX (r12)
+ *	- output: updated count in BPF_REG_AX (r12)
+ *
+ * Call bpf_check_timed_may_goto(ptr) with normal powerpc64 ABI:
+ * 	- r3 = ptr, return in r3
+ *
+ * Preserve BPF regs R0-R5 (mapping: r8, r3-r7).
+ */
+
+SYM_FUNC_START(arch_bpf_timed_may_goto)
+	/* Prologue: save LR, allocate frame */
+	mflr    r0
+	std     r0, 16(r1)
+	stdu    r1, -112(r1)
+
+	/* Save BPF registers R0 - R5 (r8, r3-r7) */
+	std     r3, 32(r1)
+	std     r4, 40(r1)
+	std     r5, 48(r1)
+	std     r6, 56(r1)
+	std     r7, 64(r1)
+	std     r8, 72(r1)
+
+	/*
+	 * r3 = BPF_REG_FP + BPF_REG_AX
+	 * BPF_REG_FP is r31; BPF_REG_AX is r12 (stack offset in bytes).
+	 */
+	add     r3, r31, r12
+	bl      bpf_check_timed_may_goto
+
+	/* Put return value back into AX */
+	mr      r12, r3
+
+	/* Restore BPF registers R0 - R5 (r8, r3-r7) */
+	ld      r3, 32(r1)
+	ld      r4, 40(r1)
+	ld      r5, 48(r1)
+	ld      r6, 56(r1)
+	ld      r7, 64(r1)
+	ld      r8, 72(r1)
+
+	/* Epilogue: pop frame, restore LR, return */
+	addi    r1, r1, 112
+	ld      r0, 16(r1)
+	mtlr    r0
+	blr
+SYM_FUNC_END(arch_bpf_timed_may_goto)