linux-kernel - Re: [PATCH] objtool: Detect non-relocated text references

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Message-ID: <CAMj1kXHQAQwcRFnhiFPySmMEruJPc1gSvWr-1ui-qaiNBV4ZNw@mail.gmail.com>
Date: Fri, 4 Oct 2024 09:59:46 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] objtool: Detect non-relocated text references

On Fri, 4 Oct 2024 at 09:28, Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> On Fri, Oct 04, 2024 at 08:54:16AM +0200, Ard Biesheuvel wrote:
> > On Fri, 4 Oct 2024 at 02:31, Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> > > However there are some "lea xxx(%rip)" cases which don't use relocations
> > > because they're referencing code in the same translation unit.
> >
> > input section
>
> "in the same translation unit and section" ?
>

Yeah, something like that. The point is really that the only way we
might end up in this case is when the LEA offset is known at assembly
time.

> > > However if we enable -fpie, the compiler would start generating them and
> > > there would definitely be bugs in the IBT sealing.
> > >
> >
> > -fpie is guaranteed to break things, but even without it, Clang may
> > issue RIP-relative LEA instructions (or LLD when it performs
> > relaxations), so this is definitely worth addressing even if we don't
> > enable -fpie.
>
> I haven't seen this with Clang either.  Also, objtool runs before the
> linker so LLD relaxations shouldn't matter.
>

Fair enough.