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From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Tue, 25 Feb 2020 13:29:51 -0500
To:     Kees Cook <keescook@chromium.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Fangrui Song <maskray@google.com>,
        Borislav Petkov <bp@alien8.de>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Michael Matz <matz@suse.de>
Subject: Re: --orphan-handling=warn
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On Mon, Feb 24, 2020 at 09:35:04PM -0800, Kees Cook wrote:
> 
> Actually, it's rather opposed to the FGKASLR series, as for that, I need
> some kind of linker script directive like this:
> 
> 	/PASSTHRU/ : {
> 		*(.text.*)
> 	}
> 
> Where "PASSTHRU" would create a 1-to-1 input-section to output-section
> with the same name, flags, etc.
> 
> ld.bfd's handling of orphan sections named .text.* is to put them each
> as a separate output section, after the existing .text output section.
> 
> ld.lld's handling of orphan sections named .text.* is to put them into
> the .text output section.

This doesn't match ld's documentation [1] of how orphan sections are to
be handled, it's supposed to append it into an existing output section
only if the names match exactly, creating a new one if that isn't so. If
ld.lld is to be a drop-in replacement for ld.bfd, this probably needs to
change?

Also ld.lld doesn't seem to support the --unique option, I think you'll
also want that for FGKASLR to avoid merging static functions with the
same name from unrelated source files.

[1] https://sourceware.org/binutils/docs/ld/Orphan-Sections.html

> 
> For FGKASLR (as it is currently implemented[2]), the sections need to be
> individually named output sections (as bfd does it). *However*, with the
> "warn on orphans" patch, FGKASLR's intentional orphaning will backfire
> (I guess the warning could be turned off, but I'd like lld to handle
> FGKASLR at some point.)
> 
> Note that cheating and doing the 1-to-1 mapping by handy with a 40,000
> entry linker script ... made ld.lld take about 15 minutes to do the
> final link. :(

Out of curiosity, how long does ld.bfd take on that linker script :)