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

Hari Bathini
2026-04-02T13:16:47Z

On 02/04/26 11:12 am, 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.
>
> Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
> ---
> arch/powerpc/net/Makefile | 2 +-
> arch/powerpc/net/bpf_jit_comp.c | 5 +++
> arch/powerpc/net/bpf_jit_comp64.c | 59 ++++++++++++++++++++++-----
> arch/powerpc/net/bpf_timed_may_goto.S | 57 ++++++++++++++++++++++++++
> 4 files changed, 111 insertions(+), 12 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..204fc57ac56e 100644
> --- a/arch/powerpc/net/Makefile
> +++ b/arch/powerpc/net/Makefile
> @@ -2,4 +2,4 @@
> #
> # Arch-specific network modules
> #
> -obj-$(CONFIG_BPF_JIT) += bpf_jit_comp.o bpf_jit_comp$(BITS).o
> +obj-$(CONFIG_BPF_JIT) += bpf_jit_comp.o bpf_jit_comp$(BITS).o bpf_timed_may_goto.o

As this is currently only enabled for CONFIG_PPC64, bpf_timed_may_goto.o
should be compiled conditionally for PPC64:

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..d39241444cd9 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;
> +
> + reladdr = func_addr - kernel_toc_addr();

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;
> + int ret;
>
> /* bpf to bpf call, func is not known in the initial pass. Emit 5 nops as a placeholder */
> if (!func) {
> @@ -469,6 +487,7 @@ int bpf_jit_emit_func_call_rel(u32 *image, u32 *fimage, struct codegen_context *
> }
>
> #ifdef CONFIG_PPC_KERNEL_PCREL

> + long reladdr;

While this works with modern C standard, I would be conservative and let
this be at the start of the function itself..

> reladdr = func_addr - local_paca->kernelbase;
>
> /*
> @@ -507,16 +526,9 @@ 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 +1767,31 @@ 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) {

> +#ifdef CONFIG_PPC_KERNEL_PCREL
> + PPC_LI_ADDR(tmp1_reg, func_addr);
> + EMIT(PPC_RAW_MTCTR(tmp1_reg));
> + EMIT(PPC_RAW_BCTRL());
> +#else
> + bpf_jit_emit_func_call(image, ctx, func_addr, tmp1_reg);

The error check is missing for the above function..
Actually, how about:

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());
}

> +#endif
> + 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..0b9afe3cfa1f
> --- /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, 24(r1)

May not matter much for this handwritten function but can we use
32 and on instead, as 32 is the MIN_FRAME_SIZE on ABIv2?

> + std r4, 32(r1)
> + std r5, 40(r1)
> + std r6, 48(r1)
> + std r7, 56(r1)
> + std r8, 64(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, 24(r1)
> + ld r4, 32(r1)
> + ld r5, 40(r1)
> + ld r6, 48(r1)
> + ld r7, 56(r1)
> + ld r8, 64(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)

- Hari