public inbox for
 help / color / mirror / Atom feed
From: Marcus Shawcroft <>
To: Mark Salter <>
Cc: Richard Henderson <>,
	libc-ports <>
Subject: Re: aarch64 prelink issue
Date: Wed, 26 Jun 2013 15:36:00 -0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 26 June 2013 13:42, Mark Salter <> wrote:
> On Mon, 2013-06-24 at 10:18 -0700, Richard Henderson wrote:
>> Try the x86_64 solution:
>>   /* This produces an IP-relative reloc which is resolved at link time. */
>>   extern const ElfW(Addr) _GLOBAL_OFFSET_TABLE_[] attribute_hidden;
>>   return _GLOBAL_OFFSET_TABLE_[0];
>> with the extra hidden attribute, this should result in an ADR(P)
>> reference to the _G_O_T_.
> This isn't working for aarch64. It doesn't use a PC-relative reference
> and it tries reading from the GOT at the link time absolute address.

It looks to me that Richard's proposal gives the correct sequence in

        adrp    x0, _GLOBAL_OFFSET_TABLE_
        ldr     x0, [x0,#:lo12:_GLOBAL_OFFSET_TABLE_]

... but, this exposes an issue in the static linker:

Currently the static linker produces:

.gotplt[0] &_DYNAMIC                             <--DT_PLTGOT
.gotplt[1] reserved for (&linkmap)
.gotplt[2] reserved fpr (resolver)

This layout is broken wrt the placement of &_DYNAMIC.  The correct
layout should be:

.got[0] &_DYNAMIC                                <-_GLOBAL_OFFSET_TABLE_
.gotplt[0] reserved for (&linkmap)       <--DT_PLTGOT
.gotplt[1] reserved fpr (resolver)

Hence when we use Richard's suggestion we end up picking junk out of .got[0]

I'm still looking at this, but at the moment I think we can fix the
static linker to emit:

.got[0] &_DYNAMIC                                <-_GLOBAL_OFFSET_TABLE_
.gotplt[0] reserved for (&linkmap)       <--DT_PLTGOT
.gotplt[1] reserved for (resolver)
.gotplt[2] reserved

With this change, I believe that we do not introduce an
incompatibility between existing binaries / .so's and the modified because the code within elf_machine_dynamic() is only
instantiated within and is only used to inspect's own

The .gotplt[2] entry remains reserved in the new layout to ensure that
an unmodified can load a binary built using a modified static ld
without clobbering  the first real PLT entry.

I'd really appreciate some feedback on the sanity of this proposal.


  reply	other threads:[~2013-06-26 15:36 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-06-24 16:41 Mark Salter
2013-06-24 17:18 ` Richard Henderson
2013-06-26 12:43   ` Mark Salter
2013-06-26 15:36     ` Marcus Shawcroft [this message]
2013-06-26 16:48       ` Richard Henderson

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).