linux-kernel - Re: [PATCH 2/3] x86/callthunks: Handle %rip-relative relocations in call thunk template

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Date:   Thu, 2 Nov 2023 12:44:22 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 2/3] x86/callthunks: Handle %rip-relative relocations in
 call thunk template

On Thu, Nov 02, 2023 at 12:25:47PM +0100, Uros Bizjak wrote:

> @@ -166,13 +168,51 @@ static const u8 nops[] = {
>  	0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90,
>  };
>  
> +#define apply_reloc_n(n_, p_, d_)				\
> +	do {							\
> +		s32 v = *(s##n_ *)(p_);				\
> +		v += (d_);					\
> +		BUG_ON((v >> 31) != (v >> (n_-1)));		\
> +		*(s##n_ *)(p_) = (s##n_)v;			\
> +	} while (0)
> +
> +static __always_inline
> +void apply_reloc(int n, void *ptr, uintptr_t diff)
> +{
> +	switch (n) {
> +	case 4: apply_reloc_n(32, ptr, diff); break;
> +	default: BUG();
> +	}
> +}
> +
> +static void apply_relocation(u8 *buf, size_t len, u8 *dest, u8 *src)
> +{
> +	for (int next, i = 0; i < len; i = next) {
> +		struct insn insn;
> +
> +		if (WARN_ON_ONCE(insn_decode_kernel(&insn, &buf[i])))
> +			return;
> +
> +		next = i + insn.length;
> +
> +		if (insn_rip_relative(&insn))
> +			apply_reloc(insn.displacement.nbytes,
> +				    buf + i + insn_offset_displacement(&insn),
> +				    src - dest);
> +	}
> +}

Isn't it simpler to use apply_relocation() from alternative.c?

Remove static, add decl, stuff like that?