linux-kernel - Re: [PATCH] lkdtm: support llvm-objcopy

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Date:   Wed, 15 May 2019 09:42:56 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Nathan Chancellor <nathanchance@...il.com>,
        Jordan Rupprect <rupprecht@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lkdtm: support llvm-objcopy

On Tue, May 14, 2019 at 01:24:37PM -0700, Nick Desaulniers wrote:
> On Tue, May 14, 2019 at 11:11 AM Kees Cook <keescook@...omium.org> wrote:
> >
> > On Mon, May 13, 2019 at 04:50:05PM -0700, Nick Desaulniers wrote:
> > > On Mon, May 13, 2019 at 4:29 PM Nathan Chancellor
> > > <natechancellor@...il.com> wrote:
> > > >
> > > > On Mon, May 13, 2019 at 03:21:09PM -0700, 'Nick Desaulniers' via Clang Built Linux wrote:
> > > > > With CONFIG_LKDTM=y and make OBJCOPY=llvm-objcopy, llvm-objcopy errors:
> > > > > llvm-objcopy: error: --set-section-flags=.text conflicts with
> > > > > --rename-section=.text=.rodata
> > > > >
> > > > > Rather than support setting flags then renaming sections vs renaming
> > > > > then setting flags, it's simpler to just change both at the same time
> > > > > via --rename-section.
> 
> I'm not sure I want to call it a bug in the initial implementation.  I've filed:
> https://sourceware.org/bugzilla/show_bug.cgi?id=24554 for
> clarification.  Jordan, I hope you can participate in any discussion
> there?

Based on the hint from Alan Modra, it seems PROGBITS/NOBITS can be
controlled with the presence/absence of the "load" section flag. This
appears to work for both BFD and LLVM:

$ objcopy --rename-section .text=.rodata,alloc,readonly,load rodata.o rodata_objcopy.o
$ readelf -WS rodata_objcopy.o | grep \.rodata
 [ 1] .rodata    PROGBITS     0000000000000000 000040 000002 00   A  0   0 16

$ llvm-objcopy --rename-section .text=.rodata,alloc,readonly,load rodata.o rodata_objcopy.o
$ readelf -WS rodata_objcopy.o | grep \.rodata
 [ 1] .rodata    PROGBITS     0000000000000000 000040 000002 00   A  0   0 16

So, that's an easy change that could be folded into the original version
of this patch (no need for my two-phase work-around).

-- 
Kees Cook