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Message-ID: <aHEfJZjW9dTXCgw3@willie-the-truck>
Date: Fri, 11 Jul 2025 15:26:45 +0100
From: Will Deacon <will@...nel.org>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: bpf@...r.kernel.org, Puranjay Mohan <puranjay@...nel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Catalin Marinas <catalin.marinas@....com>,
Andrii Nakryiko <andrii@...nel.org>,
Mark Rutland <mark.rutland@....com>,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
Maxwell Bland <mbland@...orola.com>,
Puranjay Mohan <puranjay12@...il.com>,
Dao Huang <huangdao1@...o.com>
Subject: Re: [PATCH bpf-next v9 2/2] arm64/cfi,bpf: Support kCFI + BPF on
arm64
On Mon, May 05, 2025 at 10:34:40PM +0000, Sami Tolvanen wrote:
> From: Puranjay Mohan <puranjay12@...il.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@...il.com>
> Co-developed-by: Maxwell Bland <mbland@...orola.com>
> Signed-off-by: Maxwell Bland <mbland@...orola.com>
> Co-developed-by: Sami Tolvanen <samitolvanen@...gle.com>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> Tested-by: Dao Huang <huangdao1@...o.com>
> ---
> arch/arm64/include/asm/cfi.h | 23 +++++++++++++++++++++++
> arch/arm64/kernel/alternative.c | 25 +++++++++++++++++++++++++
> arch/arm64/net/bpf_jit_comp.c | 22 +++++++++++++++++++---
> 3 files changed, 67 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..670e191f8628
> --- /dev/null
> +++ b/arch/arm64/include/asm/cfi.h
> @@ -0,0 +1,23 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_ARM64_CFI_H
> +#define _ASM_ARM64_CFI_H
> +
> +#ifdef CONFIG_CFI_CLANG
> +#define __bpfcall
> +static inline int cfi_get_offset(void)
> +{
> + return 4;
Needs a comment.
> +}
> +#define cfi_get_offset cfi_get_offset
> +extern u32 cfi_bpf_hash;
> +extern u32 cfi_bpf_subprog_hash;
> +extern u32 cfi_get_func_hash(void *func);
> +#else
> +#define cfi_bpf_hash 0U
> +#define cfi_bpf_subprog_hash 0U
> +static inline u32 cfi_get_func_hash(void *func)
> +{
> + return 0;
> +}
> +#endif /* CONFIG_CFI_CLANG */
> +#endif /* _ASM_ARM64_CFI_H */
This looks like an awful lot of boiler plate to me. The only thing you
seem to need is the CFI offset -- why isn't that just a constant that we
can define (or a Kconfig symbol?).
> diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
> index 8ff6610af496..71c153488dad 100644
> --- a/arch/arm64/kernel/alternative.c
> +++ b/arch/arm64/kernel/alternative.c
> @@ -8,11 +8,13 @@
>
> #define pr_fmt(fmt) "alternatives: " fmt
>
> +#include <linux/cfi_types.h>
> #include <linux/init.h>
> #include <linux/cpu.h>
> #include <linux/elf.h>
> #include <asm/cacheflush.h>
> #include <asm/alternative.h>
> +#include <asm/cfi.h>
> #include <asm/cpufeature.h>
> #include <asm/insn.h>
> #include <asm/module.h>
> @@ -298,3 +300,26 @@ noinstr void alt_cb_patch_nops(struct alt_instr *alt, __le32 *origptr,
> updptr[i] = cpu_to_le32(aarch64_insn_gen_nop());
> }
> EXPORT_SYMBOL(alt_cb_patch_nops);
> +
> +#ifdef CONFIG_CFI_CLANG
> +struct bpf_insn;
> +
> +/* Must match bpf_func_t / DEFINE_BPF_PROG_RUN() */
> +extern unsigned int __bpf_prog_runX(const void *ctx,
> + const struct bpf_insn *insn);
> +DEFINE_CFI_TYPE(cfi_bpf_hash, __bpf_prog_runX);
> +
> +/* Must match bpf_callback_t */
> +extern u64 __bpf_callback_fn(u64, u64, u64, u64, u64);
> +DEFINE_CFI_TYPE(cfi_bpf_subprog_hash, __bpf_callback_fn);
> +
> +u32 cfi_get_func_hash(void *func)
> +{
> + u32 hash;
> +
> + if (get_kernel_nofault(hash, func - cfi_get_offset()))
> + return 0;
> +
> + return hash;
> +}
> +#endif /* CONFIG_CFI_CLANG */
I don't think this should be in alternative.c. It's probably better off
either as a 'static inline' in the new cfi.h header.
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index 70d7c89d3ac9..3b3691e88dd5 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -9,6 +9,7 @@
>
> #include <linux/bitfield.h>
> #include <linux/bpf.h>
> +#include <linux/cfi.h>
> #include <linux/filter.h>
> #include <linux/memory.h>
> #include <linux/printk.h>
> @@ -164,6 +165,12 @@ 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);
> +}
> +
> /*
> * 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
> @@ -474,7 +481,6 @@ 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;
>
> /*
> @@ -500,6 +506,9 @@ 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;
> +
> /* 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
> @@ -2009,9 +2018,9 @@ 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();
Why do we add the offset even when CONFIG_CFI_CLANG is not enabled?
Will
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