Start of relro segment adjustment

Adjusting the start of the relro segment in order to make it end
exactly on a page boundary runs into difficulties when sections in the
relro segment are aligned;  Adjusting the start by (next_page - end)
sometimes results in more than that adjustment occurring at the end,
overrunning the page boundary.  So when that occurs we try a new lower
start position by masking the adjusted start with the maximum section
alignment.  However, we didn't consider that this masked start address
may in fact be before the initial relro base, which is silly since
that can only increase padding at the relro end.

I've also moved some calculations closer to where they are used, and
comments closer to the relevant statements.

Cherry-pick from master: 6c1aca3e2d408ef4874bd882a7f0e2cd944bbf09
	PR ld/18176
	* ldlang.c (lang_size_sections): When alignment of sections
	results in relro base adjustment being too large, don't go lower
	than the initial value.
	* ldexp.c (fold_binary <DATA_SEGMENT_RELRO_END>): Comment.
	* scripttempl/elf.sc (DATA_SEGMENT_ALIGN): Omit SEGMENT_SIZE
	alignment when SEGMENT_SIZE is the same as MAXPAGESIZE.

3 files changed, 24 insertions, 17 deletions

diff --git a/ld/ldexp.c b/ld/ldexp.c
index b4af893..73283ab 100644
--- a/ld/ldexp.c
+++ b/ld/ldexp.c
@@ -578,6 +578,8 @@ fold_binary (etree_type *tree)
 	  break;
 
 	case DATA_SEGMENT_RELRO_END:
+	  /* Operands swapped!  DATA_SEGMENT_RELRO_END(offset,exp)
+	     has offset in expld.result and exp in lhs.  */
 	  expld.dataseg.relro = exp_dataseg_relro_end;
 	  if (expld.phase == lang_first_phase_enum
 	      || expld.section != bfd_abs_section_ptr)
diff --git a/ld/ldlang.c b/ld/ldlang.c
index 2433acf..92ac9d2 100644
--- a/ld/ldlang.c
+++ b/ld/ldlang.c
@@ -5379,20 +5379,20 @@ 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)
     {
-      /* If DATA_SEGMENT_ALIGN DATA_SEGMENT_RELRO_END pair was seen, try
-	 to put expld.dataseg.relro_end on a (common) page boundary.  */
-      bfd_vma min_base, relro_end, maxpage;
+      bfd_vma initial_base, min_base, relro_end, maxpage;
 
       expld.dataseg.phase = exp_dataseg_relro_adjust;
       maxpage = expld.dataseg.maxpagesize;
-      /* 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);
+      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;
@@ -5417,16 +5417,17 @@ lang_size_sections (bfd_boolean *relax, bfd_boolean check_regions)
 		&& sec->alignment_power > max_alignment_power)
 	      max_alignment_power = sec->alignment_power;
 
-	  if (((bfd_vma) 1 << max_alignment_power) < expld.dataseg.pagesize)
-	    {
-	      /* 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 &= ~((1 << max_alignment_power) - 1);
-	      lang_reset_memory_regions ();
-	      one_lang_size_sections_pass (relax, check_regions);
-	    }
+	  /* 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 &= ~((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;
+	  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;
diff --git a/ld/scripttempl/elf.sc b/ld/scripttempl/elf.sc
index 4368fd9..ffaae4a 100644
--- a/ld/scripttempl/elf.sc
+++ b/ld/scripttempl/elf.sc
@@ -127,7 +127,11 @@ DATA_SEGMENT_ALIGN="ALIGN(${SEGMENT_SIZE}) + (. & (${MAXPAGESIZE} - 1))"
 DATA_SEGMENT_RELRO_END=""
 DATA_SEGMENT_END=""
 if test -n "${COMMONPAGESIZE}"; then
-  DATA_SEGMENT_ALIGN="ALIGN (${SEGMENT_SIZE}) - ((${MAXPAGESIZE} - .) & (${MAXPAGESIZE} - 1)); . = DATA_SEGMENT_ALIGN (${MAXPAGESIZE}, ${COMMONPAGESIZE})"
+  if test "${SEGMENT_SIZE}" != "${MAXPAGESIZE}"; then
+    DATA_SEGMENT_ALIGN="ALIGN (${SEGMENT_SIZE}) - ((${MAXPAGESIZE} - .) & (${MAXPAGESIZE} - 1)); . = DATA_SEGMENT_ALIGN (${MAXPAGESIZE}, ${COMMONPAGESIZE})"
+  else
+    DATA_SEGMENT_ALIGN="DATA_SEGMENT_ALIGN (${MAXPAGESIZE}, ${COMMONPAGESIZE})"
+  fi
   DATA_SEGMENT_END=". = DATA_SEGMENT_END (.);"
   DATA_SEGMENT_RELRO_END=". = DATA_SEGMENT_RELRO_END (${SEPARATE_GOTPLT-0}, .);"
 fi