From: Florian Weimer <fweimer@redhat.com>
To: Fangrui Song <i@maskray.me>
Cc: Mark Wielaard <mark@klomp.org>, "H.J. Lu" <hjl.tools@gmail.com>,
Cary Coutant <ccoutant@gmail.com>, "Zhang\,
Annita" <annita.zhang@intel.com>, "Liu\,
Hongtao" <hongtao.liu@intel.com>,
gnu-gabi <gnu-gabi@sourceware.org>,
GNU C Library <libc-alpha@sourceware.org>,
Binutils <binutils@sourceware.org>
Subject: Re: binutils ld and new PT_GNU_PROPERTY segment
Date: Wed, 01 Apr 2020 10:46:58 +0200 [thread overview]
Message-ID: <87tv231tkt.fsf@oldenburg2.str.redhat.com> (raw)
In-Reply-To: <20200222051913.meiied65a5daylvk@google.com> (Fangrui Song's message of "Fri, 21 Feb 2020 21:19:13 -0800")
* Fangrui Song:
> Below is my understanding of these matters. Hope they will be useful for
> interested stakeholders (for example, AArch64 devs, though PT_GNU_PROPERTY is
> currently driven by x86) who don't follow the discussions so closely.
>
> 1. We need PT_GNU_PROPERTY.
> Old linkers don't know the special processing on input .note.gnu.property sections.
> The output .note.gnu.property does not take -z ibt/-z shstk/-z force-bti/-z pac-plt into account =>
> invalid.
> The produced PT_NOTE may contain multiple NT_GNU_PROPERTY_TYPE_0 => invalid [3]
In practice, we can recognize binaries produced by old linkers when the
object has been linked on a CET-enabled distribution because the
produced notes are always invalid. The glibc dynamic loader already
checks for this and does not enable CET in this case.
ld -r involving exactly one CET-enabled object and one or more non-CET
objects is still problematic, of course, but it seems an unlikely
outcome. The only way I can see this happening is with a CET-by-default
GCC (such as the one Ubuntu uses), but then you still would have to use
another linker (not /usr/bin/ld). So even that seems like a fringe
issue to me.
In short, we looked at this situation, and still think that it's
supportable.
> Also note that sh_addralign(.note.gnu.property)=8 on a 64-bit
> platform, while
> sh_addralign(.note.gnu.build-id)=sh_addralign(.note.ABI-tag)=...=4
> (ancient mistake made by at least Linux/FreeBSD/NetBSD/...) GNU ld
> before PR ld/23658 may create corrupted PT_NOTE.
Yes, we ran into some of these issues in our distribution and had to
rebuild a few objects.
> For at least the above reasons, loaders are better not interpreting PT_NOTE.
> glibc/sysdeps/x86/dl-prop.h is currently interpreting PT_NOTE => it
> should be fixed.
I'm not sure we can do that for backwards compatibility reasons. The
ABI has been out there for several years now.
> Given point 1 and 3, this comment deserves a reconsideration:
>
> > Binaries with .note.gnu.property section have been put into many
> > OS releases. We must support them.
>
> 2. .note.gnu.property behaves strangely, unlike a regular SHT_NOTE.
> For a .note.gnu.property aware linker (newer GNU ld, newer lld),
> .note.gnu.property input sections are dropped.
>
> (We have .note.GNU-stack and .note.GNU-split-stack which both require special processing, but
> they are SHT_PROGBITS.)
>
> 3. We need SHT_GNU_PROPERTY.
> The output .note.gnu.property being SHT_NOTE causes linkers to place the section in both PT_NOTE
> and PT_GNU_PROPERTY.
> PT_NOTE, as explained by point 1 above, can cause trouble to old loaders.
> Have we proved that "older linker-produced concatenated PT_NOTE cannot cause trouble to loaders interpreting PT_NOTE"?
>
> SHT_GNU_PROPERTY does not contribute to PT_NOTE and will not cause any problem to old loaders
> interpreting PT_NOTE.
Yes, I agree that it's desirable to add SHT_GNU_PROPERTY.
Thanks,
Florian
next prev parent reply other threads:[~2020-04-01 8:47 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-01 0:00 Mark Wielaard
2020-01-01 0:00 ` H.J. Lu
2020-01-01 0:00 ` Fangrui Song via gnu-gabi
2020-01-01 0:00 ` Zhang, Annita
2020-01-01 0:00 ` Mark Wielaard
2020-01-01 0:00 ` H.J. Lu
2020-01-01 0:00 ` Mark Wielaard
2020-01-01 0:00 ` H.J. Lu
2020-01-01 0:00 ` H.J. Lu
2020-01-01 0:00 ` Mark Wielaard
2020-01-01 0:00 ` H.J. Lu
2020-01-01 0:00 ` Fangrui Song
2020-01-01 0:00 ` H.J. Lu
2020-01-01 0:00 ` Mark Wielaard
2020-01-01 0:00 ` H.J. Lu
2020-01-01 0:00 ` Mark Wielaard
2020-01-01 0:00 ` H.J. Lu
2020-01-01 0:00 ` Fangrui Song
2020-01-01 0:00 ` H.J. Lu
2020-01-01 0:00 ` Mark Wielaard
2020-01-01 0:00 ` Fangrui Song
2020-04-01 8:46 ` Florian Weimer [this message]
2020-04-01 9:22 ` Szabolcs Nagy
2020-04-01 9:29 ` Florian Weimer
2020-04-01 10:10 ` Szabolcs Nagy
2020-04-01 10:21 ` Florian Weimer
2020-01-01 0:00 ` Mark Wielaard
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