linux-kernel - Re: [PATCH 01/14] x86/cfi: Wreck things...

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Message-ID: <20240927231527.em2a4cjbnpos6c4u@treble>
Date: Fri, 27 Sep 2024 16:15:27 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, alyssa.milburn@...el.com,
	scott.d.constable@...el.com, joao@...rdrivepizza.com,
	andrew.cooper3@...rix.com, jose.marchesi@...cle.com,
	hjl.tools@...il.com, ndesaulniers@...gle.com,
	samitolvanen@...gle.com, nathan@...nel.org, ojeda@...nel.org,
	kees@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH 01/14] x86/cfi: Wreck things...

On Fri, Sep 27, 2024 at 09:48:57PM +0200, Peter Zijlstra wrote:
> vmlinux.o: warning: objtool: .export_symbol+0x3c9f0: data relocation to !ENDBR: entry_untrain_ret+0x0
> 
> Which states that while these functions are exported and (directly)
> callable, they cannot be called indirectly. There are two solutions:

IIRC, exported symbols are by far the most common "need" for ENDBR.  But
presumably the vast majority of them aren't being indirect called.

>  - exclude the .export_symbol section from validation; effectively
>    saying that having linkable but not indirectly callable exports are
>    fine by default, or

This is confusingly inconsistent IMO.

>  - make all of those use SYM_TYPED_FUNC_START to restore the
>    traditional (and expected, but less secure?) behaviour.

Why not just make SYM_FUNC_START imply "typed"?  That's consistent with
what the compiler does anyway right?

Even better, require exported+indirect-called symbols to use
EXPORT_SYMBOL_TYPED, otherwise they get sealed.  I suppose we'd need to
add some module-to-vmlinux ENDBR validation to make sure modules don't
get broken by this.

-- 
Josh