Rewrite relro adjusting code

The linker tries to put the end of the last section in the relro
segment exactly on a page boundary, because the relro segment itself
must end on a page boundary.  If for any reason this can't be done,
padding is inserted.  Since the end of the relro segment is typically
between .got and .got.plt, padding effectively increases the size of
the GOT.  This isn't nice for targets and code models with limited GOT
addressing.

The problem with the current code is that it doesn't cope very well
with aligned sections in the relro segment.  When making .got aligned
to a 256 byte boundary for PowerPC64, I found that often the initial
alignment attempt failed and the fallback attempt to be less than
adequate.  This is a particular problem for PowerPC64 since the
distance between .got and .plt affects the size of plt call stubs,
leading to "stubs don't match calculated size" errors.

So this rewrite takes a direct approach to calculating a new relro
base.  Starting from the last section in the segment, we calculate
where it must start to position its end on the boundary, or as near as
possible considering alignment requirements.  The new start then
becomes the goal for the previous section to end, and so on for all
sections.  This of course ignores the possibility that user scripts
will place . = ALIGN(xxx); in the relro segment, or provide section
address expressions.  In those cases we might fail, but the old code
probably did too, and a fallback is provided.

ld/
	* ldexp.h (struct ldexp_control): Delete dataseg.min_base.  Add
	data_seg.relro_offset.
	* ldexp.c (fold_binary <DATA_SEGMENT_ALIGN>): Don't set min_base.
	(fold_binary <DATA_SEGMENT_RELRO_END>): Do set relro_offset.
	* ldlang.c (lang_size_sections): Rewrite code adjusting relro
	segment base to line up last section on page boundary.
ld/testsuite/
	* ld-x86-64/pr18176.d: Update.

1 files changed, 34 insertions, 39 deletions

diff --git a/ld/ldlang.c b/ld/ldlang.c
index b074169..c96c21f 100644
--- a/ld/ldlang.c
+++ b/ld/ldlang.c
@@ -5382,56 +5382,51 @@ lang_size_sections (bfd_boolean *relax, bfd_boolean check_regions)
   if (expld.dataseg.phase == exp_dataseg_end_seen
       && link_info.relro && expld.dataseg.relro_end)
     {
-      bfd_vma initial_base, min_base, relro_end, maxpage;
+      bfd_vma initial_base, relro_end, desired_end;
+      asection *sec;
 
-      expld.dataseg.phase = exp_dataseg_relro_adjust;
-      maxpage = expld.dataseg.maxpagesize;
-      initial_base = expld.dataseg.base;
-      /* Try to put expld.dataseg.relro_end on a (common) page boundary.  */
-      expld.dataseg.base += (-expld.dataseg.relro_end
-			     & (expld.dataseg.pagesize - 1));
       /* Compute the expected PT_GNU_RELRO segment end.  */
       relro_end = ((expld.dataseg.relro_end + expld.dataseg.pagesize - 1)
 		   & ~(expld.dataseg.pagesize - 1));
-      /* MIN_BASE is the absolute minimum address we are allowed to start the
-	 read-write segment (byte before will be mapped read-only).  */
-      min_base = (expld.dataseg.min_base + maxpage - 1) & ~(maxpage - 1);
-      if (min_base + maxpage < expld.dataseg.base)
-	{
-	  expld.dataseg.base -= maxpage;
-	  relro_end -= maxpage;
-	}
+
+      /* Adjust by the offset arg of DATA_SEGMENT_RELRO_END.  */
+      desired_end = relro_end - expld.dataseg.relro_offset;
+
+      /* For sections in the relro segment..  */
+      for (sec = link_info.output_bfd->section_last; sec; sec = sec->prev)
+	if (!IGNORE_SECTION (sec)
+	    && sec->vma >= expld.dataseg.base
+	    && sec->vma < expld.dataseg.relro_end - expld.dataseg.relro_offset)
+	  {
+	    /* Where do we want to put this section so that it ends as
+	       desired?  */
+	    bfd_vma start = sec->vma;
+	    bfd_vma end = start + sec->size;
+	    bfd_vma bump = desired_end - end;
+	    /* We'd like to increase START by BUMP, but we must heed
+	       alignment so the increase might be less than optimum.  */
+	    start += bump & ~(((bfd_vma) 1 << sec->alignment_power) - 1);
+	    /* This is now the desired end for the previous section.  */
+	    desired_end = start;
+	  }
+
+      expld.dataseg.phase = exp_dataseg_relro_adjust;
+      ASSERT (desired_end >= expld.dataseg.base);
+      initial_base = expld.dataseg.base;
+      expld.dataseg.base = desired_end;
       lang_reset_memory_regions ();
       one_lang_size_sections_pass (relax, check_regions);
+
       if (expld.dataseg.relro_end > relro_end)
 	{
-	  /* The alignment of sections between DATA_SEGMENT_ALIGN
-	     and DATA_SEGMENT_RELRO_END can cause excessive padding to
-	     be inserted at DATA_SEGMENT_RELRO_END.  Try to start a
-	     bit lower so that the section alignments will fit in.  */
-	  asection *sec;
-	  unsigned int max_alignment_power = 0;
-
-	  /* Find maximum alignment power of sections between
-	     DATA_SEGMENT_ALIGN and DATA_SEGMENT_RELRO_END.  */
-	  for (sec = link_info.output_bfd->sections; sec; sec = sec->next)
-	    if (sec->vma >= expld.dataseg.base
-		&& sec->vma < expld.dataseg.relro_end
-		&& sec->alignment_power > max_alignment_power)
-	      max_alignment_power = sec->alignment_power;
-
-	  /* Aligning the adjusted base guarantees the padding
-	     between sections won't change.  This is better than
-	     simply subtracting 1 << max_alignment_power which is
-	     what we used to do here.  */
-	  expld.dataseg.base &= ~(((bfd_vma) 1 << max_alignment_power) - 1);
-	  /* It doesn't make much sense to go lower than the initial
-	     base.  That can only increase padding.  */
-	  if (expld.dataseg.base < initial_base)
-	    expld.dataseg.base = initial_base;
+	  /* Assignments to dot, or to output section address in a
+	     user script have increased padding over the original.
+	     Revert.  */
+	  expld.dataseg.base = initial_base;
 	  lang_reset_memory_regions ();
 	  one_lang_size_sections_pass (relax, check_regions);
 	}
+
       link_info.relro_start = expld.dataseg.base;
       link_info.relro_end = expld.dataseg.relro_end;
     }