linux-kernel - Re: [PATCH v2 36/39] objtool: Find unused ENDBR instructions

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Date:   Sat, 26 Feb 2022 19:46:13 -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, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 36/39] objtool: Find unused ENDBR instructions

On Thu, Feb 24, 2022 at 03:52:14PM +0100, Peter Zijlstra wrote:
> +#ifdef CONFIG_X86_KERNEL_IBT
> +	. = ALIGN(8);
> +	.ibt_endbr_sites : AT(ADDR(.ibt_endbr_sites) - LOAD_OFFSET) {
> +		__ibt_endbr_sites = .;
> +		*(.ibt_endbr_sites)
> +		__ibt_endbr_sites_end = .;
> +	}
> +#endif

".ibt_endbr_superfluous" maybe?  It's not *all* the endbr sites.

> +
>  	/*
>  	 * struct alt_inst entries. From the header (alternative.h):
>  	 * "Alternative instructions for different CPU types or capabilities"
> --- a/tools/objtool/builtin-check.c
> +++ b/tools/objtool/builtin-check.c
> @@ -21,7 +21,7 @@
>  
>  bool no_fp, no_unreachable, retpoline, module, backtrace, uaccess, stats,
>       lto, vmlinux, mcount, noinstr, backup, sls, dryrun,
> -     ibt, ibt_fix_direct;
> +     ibt, ibt_fix_direct, ibt_seal;
>  
>  static const char * const check_usage[] = {
>  	"objtool check [<options>] file.o",
> @@ -50,6 +50,7 @@ const struct option check_options[] = {
>  	OPT_BOOLEAN(0, "dry-run", &dryrun, "don't write the modifications"),
>  	OPT_BOOLEAN(0, "ibt", &ibt, "validate ENDBR placement"),
>  	OPT_BOOLEAN(0, "ibt-fix-direct", &ibt_fix_direct, "fixup direct jmp/call to ENDBR"),
> +	OPT_BOOLEAN(0, "ibt-seal", &ibt_seal, "list superfluous ENDBR instructions"),

s/list/annotate/ ?

Not sure "ibt-seal" is the appropriate name since the "seal" is done at
boot time.

Do we really need a separate option anyway?  To get the full benefits of
IBT you might as well enable it...  And always enabling it helps flush
out bugs quicker.

-- 
Josh