Re: Invalid executable being created by custom linker scripts

[Alan Modra <amodra at bigpond dot net dot au>]

On Sat, Nov 22, 2008 at 10:12:06AM +1030, Alan Modra wrote:
> Mel Gorman wrote on 22/11/2008 02:47:48 AM:
> >   INTERP         0x0000000000000200 0x0000000001000200 0x0000000001000200
> >                  0x0000000000000040 0x0000000000000040  R      4
> >       [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
> >
> > At first glance, it looks ok, but the length is different. On working
> > binaries, it's 0x1c corresponding to the full length of the path, but
> here
> > it is 0x40. Do you think the different length is the problem and if so,
> > have you any idea why this might occur?
> 
> You are asking a kernel question here, since it is the kernel that looks at
> this header.  On investigating fs/binfmt_elf.c, I find that the kernel
> checks that the string is NULL terminated.  It does so by testing the last
> byte covered by the PT_INTERP header, ie. at a file offset of 0x23f in this
> case.  But this is not even in the .interp section!  It's somewhere in
> .note.gnu.build-id and most likely non-zero.  So this is indeed why your
> executable does not run.
> 
> The solution to your problem is to add .note.gnu.build-id to your linker
> script, and specify the correct phdrs for the section.  When
> .note.gnu.build-id is an orphan, the linker looks through existing sections
> to find one like it, and in this case chooses to place the orphan
> .note.gnu.build-id after .interp.  The orphan section also inherits the
> phdr assignment, if any, of the previous section, which is why PT_INTERP
> wrongly has both .interp and .note.gnu.build-id.
> 
> Custom linker scripts are a pain..

It's probably worth special casing PT_INTERP in the linker to stop
this happening, since orphan loadable note sections tend to be placed
after .interp.

	* ldlang.c (lang_record_phdrs): Don't add orphans to PT_INTERP header.

Index: ld/ldlang.c
===================================================================
RCS file: /cvs/src/src/ld/ldlang.c,v
retrieving revision 1.296
diff -u -p -r1.296 ldlang.c
--- ld/ldlang.c	25 Oct 2008 09:54:01 -0000	1.296
+++ ld/ldlang.c	24 Nov 2008 07:50:10 -0000
@@ -6669,6 +6669,10 @@ lang_record_phdrs (void)
 		  || (os->bfd_section->flags & SEC_ALLOC) == 0)
 		continue;
 
+	      /* Don't add orphans to PT_INTERP header.  */
+	      if (l->type == 3)
+		continue;
+
 	      if (last == NULL)
 		{
 		  lang_output_section_statement_type * tmp_os;

-- 
Alan Modra
Australia Development Lab, IBM