Re: [PATCH v2] powerpc64/bpf: Add powerpc64 JIT support for timed may_goto

[Hari Bathini <hbathini@linux.ibm.com>]

On 02/04/26 9:40 pm, Saket Kumar Bhaskar wrote:
> 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.
> 

Looks good to me.

Acked-by: Hari Bathini <hbathini@linux.ibm.com>

> Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
> ---
> This patch has been rebased on top of these 3 patches on powerpc-next in
> the following sequence:
> 1. https://lore.kernel.org/bpf/20260401103215.104438-1-adubey@linux.ibm.com/
> 2. https://lore.kernel.org/bpf/20260401141043.41513-1-adubey@linux.ibm.com/
> 3. https://lore.kernel.org/bpf/20260401152133.42544-1-adubey@linux.ibm.com/
> 
> Changes since v1:
> - Compile bpf_timed_may_goto.o conditionally for PPC64. (Hari)
> - Moved reladdr at the beginning of the function. (Hari)
> - Added error check for bpf_jit_emit_func_call() in
>    bpf_jit_build_body(). (Hari)
> - Save registers after 32 bytes as MIN_FRAME_SIZE is 32 bytes for
>    powerpc ABIv2. (Hari)
> ---
>   arch/powerpc/net/Makefile             |  4 ++
>   arch/powerpc/net/bpf_jit_comp.c       |  5 +++
>   arch/powerpc/net/bpf_jit_comp64.c     | 62 ++++++++++++++++++++++-----
>   arch/powerpc/net/bpf_timed_may_goto.S | 57 ++++++++++++++++++++++++
>   4 files changed, 117 insertions(+), 11 deletions(-)
>   create mode 100644 arch/powerpc/net/bpf_timed_may_goto.S
> 
> diff --git a/arch/powerpc/net/Makefile b/arch/powerpc/net/Makefile
> index 8e60af32e51e..87395d7e2672 100644
> --- a/arch/powerpc/net/Makefile
> +++ b/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
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index 50103b3794fb..9b2b456b0765 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -537,6 +537,11 @@ 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);
> diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
> index db364d9083e7..dab106cae22b 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/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]))
> diff --git a/arch/powerpc/net/bpf_timed_may_goto.S b/arch/powerpc/net/bpf_timed_may_goto.S
> new file mode 100644
> index 000000000000..6fd8b1c9f4ac
> --- /dev/null
> +++ b/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)