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

From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: x86@kernel.org, joao@overdrivepizza.com, hjl.tools@gmail.com,
	andrew.cooper3@citrix.com, linux-kernel@vger.kernel.org,
	ndesaulniers@google.com, keescook@chromium.org,
	samitolvanen@google.com
Subject: Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups
Date: Wed, 24 Nov 2021 11:30:37 -0800	[thread overview]
Message-ID: <20211124193037.nu7q4pa3sianzqtc@treble> (raw)
In-Reply-To: <20211122170805.338489412@infradead.org>

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