linux-kernel - Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups

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Message-ID: <20211124193037.nu7q4pa3sianzqtc@treble>
Date:   Wed, 24 Nov 2021 11:30:37 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com
Subject: Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups

On Mon, Nov 22, 2021 at 06:03:07PM +0100, Peter Zijlstra wrote:
> +static int validate_ibt_reloc(struct objtool_file *file, struct reloc *reloc, char **name)
> +{
> +	struct instruction *dest;
> +	struct section *sec;
> +	unsigned long off;
> +
> +	sec = reloc->sym->sec;
> +	off = reloc->sym->offset + reloc->addend;
> +
> +	dest = find_insn(file, sec, off);
> +	if (!dest)
> +		return 0;
> +
> +	if (name && dest->func)
> +		*name = dest->func->name;

I think these checks can be further narrowed down by only looking for
X86_64_64 relocs.

> +	list_for_each_entry(insn, &file->endbr_list, call_node) {
> +		if (ibt_seal) {
> +			elf_write_insn(file->elf, insn->sec,
> +				       insn->offset, insn->len,
> +				       arch_nop_insn(insn->len));
> +		}

Like the retpoline rewriting, I'd much rather have objtool create
annotations which the kernel can then read and patch itself.

e.g. have '.ibt.direct_call_sites' and '.ibt.unused_endbr_insns'
sections.

-- 
Josh