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Re: Gold linker: specify alignment of sections


Ian Lance Taylor <iant@google.com> wrote:
On Wed, Jul 31, 2013 at 10:30 AM, Martin Richtarsky <ml@martinien.de> wrote:

E.g. include the script in rpath-link and also pass it as one of
the object files. This is an implicit linker script as described here:


http://sourceware.org/binutils/docs-2.23.1/ld/Implicit-Linker-Scripts.html#Implicit-Linker-Scripts


Now, when linking in the same way using gold 2.23.1, the
script is rejected with this error:

" LinkerScript.ld: SECTIONS seen after other input files;
  try -T/--script"

So gold wants me to specify a complete linker script, however I
only want to change a couple attributes. Is this possible?

You won't get that error if you put your implicit linker script first.

However, I agree that its complex to specify a SECTIONS clause that
does not describe what to do with all sections. Does that really work with GNU ld? As the docs say, an implicit linker script normally only
includes symbol assignments and INPUT commands and the like.  I
suspect that your implicit linker script is overriding the default
SECTIONS clause, with results that may be hard to understand.

- Passing the script with the -T option. This produces a broken
  executable that fails to start. As I understand this is because
  -T only accepts complete linker scripts, while the above is
  just an implicit linker script.

You didn't really say that the target is.  If this is a GNU/Linux
system, then you probably don't need much to get it working.

My target is GNU/Linux x86_64. When I am using the complete linker script,
I get a crash here:

#0  0x0000000000403770 in cxa_throw_notify_local ()
#1  0x0000000000400f3e in call_gmon_start ()
#2  0x00007fffffff86f8 in ?? ()
#3  0x0000000000400df1 in _init ()
#4  0x00002aaaabec7aa0 in ?? () from /lib64/libc.so.6
#5 0x00000000004017d5 in __libc_csu_init (argc=1, argv=0x7fffffff86f8, envp=0x7fffffff8708)
    at elf-init.c:120
#6  0x00002aaaabed6b52 in __libc_start_main () from /lib64/libc.so.6
#7 0x0000000000400f29 in _start () at ../sysdeps/x86_64/elf/start.S:113



cxa_throw_notify_local is a globally visible function pointer that is initialized to point to another function at startup. Any section I am missing here in my
linker script? (attached)

.gnu.hash
init_array
.ldata

Comparing readelf --sections, the gold executable has the same sections as the ld
except for ".gnu.hash" and two additional: .init_array and .ldata

- So I have tried obtaining a complete linker script. "ld --verbose"
  will print the default linker script. I had to remove a couple
of symbols that were not understood by gold, afterwards the executable produced with the "-T/path/to/LinkerScript.ld" option didn't work anymore.

That may be a bug in gold.

Since all you want to do is set the alignment of the output section,
and the alignment of an output section is determined by the maximum
alignment of all input sections, I suspect you can get the results you want by including a little assembler file in your link, something like

    .text
    .balign 0x40
    .data
    .balign 0x40

Although I admit I'm not sure offhand whether you can set the
alignment of .plt this way.  But then I'm pretty sure you can't set
the alignment of .plt with a linker script anyhow.

I tried it and it worked as far as alignment goes, but it leads to
crashes when changing the alignment for .got, .plt or .got.plt:

(gdb) bt
#0 0x00002aaaaaab66f8 in _dl_relocate_object () from /lib64/ld-linux-x86-64.so.2
#1  0x00002aaaaaaae95e in dl_main () from /lib64/ld-linux-x86-64.so.2
#2 0x00002aaaaaabf97a in _dl_sysdep_start () from /lib64/ld-linux-x86-64.so.2
#3  0x00002aaaaaaac3d1 in _dl_start () from /lib64/ld-linux-x86-64.so.2
#4  0x00002aaaaaaabb08 in _start () from /lib64/ld-linux-x86-64.so.2
#5  0x0000000000000002 in ?? ()
#6  0x00007fffffff9377 in ?? ()
#7  0x00007fffffff939f in ?? ()
#8  0x0000000000000000 in ?? ()


Changing the alignment using the original SECTIONS script together with ld works,
the alignment is correct and the executables are running.


Best regards,
Martin
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    *(.text.unlikely .text.*_unlikely)
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