[PATCH bpf-next v12 3/3] arm64/cfi,bpf: Support kCFI + BPF on arm64

[Xu Kuohai]

On 7/22/2025 4:20 AM, Sami Tolvanen wrote:
> From: Puranjay Mohan <puranjay12 at gmail.com>
> 
> Currently, bpf_dispatcher_*_func() is marked with `__nocfi` therefore
> calling BPF programs from this interface doesn't cause CFI warnings.
> 
> When BPF programs are called directly from C: from BPF helpers or
> struct_ops, CFI warnings are generated.
> 
> Implement proper CFI prologues for the BPF programs and callbacks and
> drop __nocfi for arm64. Fix the trampoline generation code to emit kCFI
> prologue when a struct_ops trampoline is being prepared.
> 
> Signed-off-by: Puranjay Mohan <puranjay12 at gmail.com>
> Co-developed-by: Maxwell Bland <mbland at motorola.com>
> Signed-off-by: Maxwell Bland <mbland at motorola.com>
> Co-developed-by: Sami Tolvanen <samitolvanen at google.com>
> Signed-off-by: Sami Tolvanen <samitolvanen at google.com>
> Tested-by: Dao Huang <huangdao1 at oppo.com>
> Acked-by: Will Deacon <will at kernel.org>
> ---
>   arch/arm64/include/asm/cfi.h  |  7 +++++++
>   arch/arm64/net/bpf_jit_comp.c | 22 +++++++++++++++++++---
>   2 files changed, 26 insertions(+), 3 deletions(-)
>   create mode 100644 arch/arm64/include/asm/cfi.h
> 
> diff --git a/arch/arm64/include/asm/cfi.h b/arch/arm64/include/asm/cfi.h
> new file mode 100644
> index 000000000000..ab90f0351b7a
> --- /dev/null
> +++ b/arch/arm64/include/asm/cfi.h
> @@ -0,0 +1,7 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_ARM64_CFI_H
> +#define _ASM_ARM64_CFI_H
> +
> +#define __bpfcall
> +
> +#endif /* _ASM_ARM64_CFI_H */
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index 89b1b8c248c6..f4a98c1a1583 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -10,6 +10,7 @@
>   #include <linux/arm-smccc.h>
>   #include <linux/bitfield.h>
>   #include <linux/bpf.h>
> +#include <linux/cfi.h>
>   #include <linux/filter.h>
>   #include <linux/memory.h>
>   #include <linux/printk.h>
> @@ -166,6 +167,12 @@ static inline void emit_bti(u32 insn, struct jit_ctx *ctx)
>   		emit(insn, ctx);
>   }
>   
> +static inline void emit_kcfi(u32 hash, struct jit_ctx *ctx)
> +{
> +	if (IS_ENABLED(CONFIG_CFI_CLANG))
> +		emit(hash, ctx);

I guess this won't work on big-endian cpus, since arm64 instructions
are always stored in little-endian, but data not.

> +}
> +
>   /*
>    * Kernel addresses in the vmalloc space use at most 48 bits, and the
>    * remaining bits are guaranteed to be 0x1. So we can compose the address
> @@ -476,7 +483,6 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
>   	const bool is_main_prog = !bpf_is_subprog(prog);
>   	const u8 fp = bpf2a64[BPF_REG_FP];
>   	const u8 arena_vm_base = bpf2a64[ARENA_VM_START];
> -	const int idx0 = ctx->idx;
>   	int cur_offset;
>   
>   	/*
> @@ -502,6 +508,9 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
>   	 *
>   	 */
>   
> +	emit_kcfi(is_main_prog ? cfi_bpf_hash : cfi_bpf_subprog_hash, ctx);
> +	const int idx0 = ctx->idx;

move the idx0 definition back to its original position to match the
coding style of the rest of the file?

> +
>   	/* bpf function may be invoked by 3 instruction types:
>   	 * 1. bl, attached via freplace to bpf prog via short jump
>   	 * 2. br, attached via freplace to bpf prog via long jump
> @@ -2055,9 +2064,9 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>   		jit_data->ro_header = ro_header;
>   	}
>   
> -	prog->bpf_func = (void *)ctx.ro_image;
> +	prog->bpf_func = (void *)ctx.ro_image + cfi_get_offset();
>   	prog->jited = 1;
> -	prog->jited_len = prog_size;
> +	prog->jited_len = prog_size - cfi_get_offset();
>   
>   	if (!prog->is_func || extra_pass) {
>   		int i;
> @@ -2426,6 +2435,12 @@ static int prepare_trampoline(struct jit_ctx *ctx, struct bpf_tramp_image *im,
>   	/* return address locates above FP */
>   	retaddr_off = stack_size + 8;
>   
> +	if (flags & BPF_TRAMP_F_INDIRECT) {
> +		/*
> +		 * Indirect call for bpf_struct_ops
> +		 */
> +		emit_kcfi(cfi_get_func_hash(func_addr), ctx);
> +	}
>   	/* bpf trampoline may be invoked by 3 instruction types:
>   	 * 1. bl, attached to bpf prog or kernel function via short jump
>   	 * 2. br, attached to bpf prog or kernel function via long jump
> @@ -2942,6 +2957,7 @@ void bpf_jit_free(struct bpf_prog *prog)
>   					   sizeof(jit_data->header->size));
>   			kfree(jit_data);
>   		}
> +		prog->bpf_func -= cfi_get_offset();
>   		hdr = bpf_jit_binary_pack_hdr(prog);
>   		bpf_jit_binary_pack_free(hdr, NULL);
>   		WARN_ON_ONCE(!bpf_prog_kallsyms_verify_off(prog));