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

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Message-ID: <CABCJKucpfQ2nsmo8npc96SAQeQbTJiMcseWA1L-oNEJzF=Hu0g@mail.gmail.com>
Date: Tue, 22 Jul 2025 07:33:40 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Xu Kuohai <xukuohai@...weicloud.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>, Will Deacon <will@...nel.org>, 
	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 v12 3/3] arm64/cfi,bpf: Support kCFI + BPF on arm64

Hi,

On Mon, Jul 21, 2025 at 8:44 PM Xu Kuohai <xukuohai@...weicloud.com> wrote:
>
> On 7/22/2025 4:20 AM, Sami Tolvanen wrote:
> > +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.

Nice catch! I'll send a new version with your suggested fix.

> > +}
> > +
> >   /*
> >    * 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?

The const definition needs to happen after emit_kcfi, because we
increment ctx->idx when CFI is enabled.

Sami