[PATCH v3] ld: Rewrite lang_size_relro_segment_1

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Jan 24, 2022 at 04:24:33PM +0000, Nick Clifton wrote:
> Hi H.J.
> 
> > Here is the v2 patch to track the maximum section alignment from
> > the RELRO segment.  If the maximum page size >= the maximum
> > section alignment, we can align the RELRO segment to the maximum
> > page size.
> 
> I am having some trouble getting my head around this patch, so please
> bare with me if I ask some silly questions:
> 
>    /* Find the first section in the relro segment. */
>    for (sec = link_info.output_bfd->section_last; sec; sec = sec->prev)
>      if ((sec->flags & SEC_ALLOC) != 0)
>        {
>  	if (last_sec_status != relro_sec_before
>  	    && sec->alignment_power > max_alignment_power)
>  	  max_alignment_power = sec->alignment_power;
> 
>  	if (sec->vma >= seg->base
>  	    && sec->vma < seg->relro_end - seg->relro_offset)
>  	  {
>  	    relro_sec = sec;
>  	    prev_sec = sec->prev;
>  	    last_sec_status = relro_sec_in;
>  	  }
>  	else
>  	  last_sec_status = relro_sec_before;
>        }
> 
> Questions:
>   Should a zero-sized section (with SHF_ALLOC) be treated as a
>   viable candidate ?

Fixed.

> 
>   Once last_sec_status has been set to relro_sec_before, is there
>   any point in continuing the scan ?

Fixed.

> 
>   If the section list contains a section which starts beyond the
>   end of the segment (ie sec->vma >= seg->relro_end - seg->relro_offset)
>   then a) is this an error ?  and b) should its alignment power be
>   considered against max_alignment_power ?

Since sections are scanned backwards from the highest address and
sections whose vma >= seg->relro_end - seg->relro_offset are placed
before the PT_GNU_RELRO segment, these sections are scanned for
max_alignment_power.

> 
> 
>    if (relro_sec)
>      {
>        /* Where do we want to put this section so that it ends as
>  	 desired?  */
> 
>   By "desired" I assume you mean "with the desired alignment, but
>   without any unnecessary padding".  Is that correct ?

I changed it to

            /* Where do we want to put the relro section so that the
               relro segment ends on the page bounary?  */
            bfd_vma start, end, bump;

> 
> 
>       /* Find the first non-empty preceding section.  */
>       for (; prev_sec; prev_sec = prev_sec->prev)
> 	if (prev_sec->size != 0 && (prev_sec->flags & SEC_ALLOC) != 0)
> 	  break;
> 
>   Given that prev_sec is only needed later on, inside an if-statement,
>   you could move this loop - potentially saving a bit of time in some
>   cases.
> 

Fixed.

> 
>   Overall I think that the code could use some more comments, explaining
>   what is going on with some examples of how each part of the algorithm
>   operates.
> 

I added more comments.  Here is the v3 patch.

Thanks.


H.J.
 --
1. Compute the desired PT_GNU_RELRO segment base and find the maximum
section alignment of sections starting from the PT_GNU_RELRO segment.
2. Find the first preceding load section.
3. Don't add the 1-page gap between the first preceding load section and
the relro segment if the maximum page size >= the maximum section
alignment.  Subtract the maximum page size from the desired PT_GNU_RELRO
segment base only if the preceding load section size is > the maximum
page size to avoid moving the PT_GNU_RELRO segment base before the
preceding load section.  Otherwise, simply align the desired PT_GNU_RELRO
segment base.

	PR ld/28743
	* ldlang.c (lang_size_relro_segment_1): Rewrite.
	* testsuite/ld-x86-64/pr28743-1.d: New file.
	* testsuite/ld-x86-64/pr28743-1.s: Likewise.
	* testsuite/ld-x86-64/x86-64.exp: Run pr28743-1.
---
 ld/ldlang.c                        | 90 ++++++++++++++++--------------
 ld/testsuite/ld-x86-64/pr28743-1.d | 50 +++++++++++++++++
 ld/testsuite/ld-x86-64/pr28743-1.s | 16 ++++++
 ld/testsuite/ld-x86-64/x86-64.exp  |  1 +
 4 files changed, 114 insertions(+), 43 deletions(-)
 create mode 100644 ld/testsuite/ld-x86-64/pr28743-1.d
 create mode 100644 ld/testsuite/ld-x86-64/pr28743-1.s

diff --git a/ld/ldlang.c b/ld/ldlang.c
index 93fcfc4cbc7..25eac62a63a 100644
--- a/ld/ldlang.c
+++ b/ld/ldlang.c
@@ -6370,94 +6370,98 @@ lang_size_segment (seg_align_type *seg)
 static bfd_vma
 lang_size_relro_segment_1 (seg_align_type *seg)
 {
-  bfd_vma relro_end, desired_end;
-  asection *sec, *prev_sec = NULL;
-  bool remove_page_gap = false;
+  bfd_vma relro_end, desired_relro_base;
+  asection *sec, *relro_sec = NULL;
   unsigned int max_alignment_power = 0;
+  bool seen_reloc_section = false;
+  bool desired_relro_base_reduced = false;
 
   /* Compute the expected PT_GNU_RELRO/PT_LOAD segment end.  */
   relro_end = ((seg->relro_end + seg->pagesize - 1)
 	       & ~(seg->pagesize - 1));
 
   /* Adjust by the offset arg of XXX_SEGMENT_RELRO_END.  */
-  desired_end = relro_end - seg->relro_offset;
+  desired_relro_base = relro_end - seg->relro_offset;
 
-  /* For sections in the relro segment..  */
+  /* For sections in the relro segment.  */
   for (sec = link_info.output_bfd->section_last; sec; sec = sec->prev)
     if ((sec->flags & SEC_ALLOC) != 0)
       {
+	/* Record the maximum alignment for all sections starting from
+	   the relro segment.  */
 	if (sec->alignment_power > max_alignment_power)
 	  max_alignment_power = sec->alignment_power;
 
 	if (sec->vma >= seg->base
 	    && sec->vma < seg->relro_end - seg->relro_offset)
 	  {
-	    /* Where do we want to put this section so that it ends as
-	       desired?  */
+	    /* Where do we want to put the relro section so that the
+	       relro segment ends on the page bounary?  */
 	    bfd_vma start, end, bump;
 
 	    end = start = sec->vma;
 	    if (!IS_TBSS (sec))
 	      end += TO_ADDR (sec->size);
-	    bump = desired_end - end;
+	    bump = desired_relro_base - end;
 	    /* We'd like to increase START by BUMP, but we must heed
 	       alignment so the increase might be less than optimum.  */
 	    start += bump;
 	    start &= ~(((bfd_vma) 1 << sec->alignment_power) - 1);
 	    /* This is now the desired end for the previous section.  */
-	    desired_end = start;
-	    prev_sec = sec->prev;
+	    desired_relro_base = start;
+	    relro_sec = sec;
+	    seen_reloc_section = true;
 	  }
-      }
-
-  seg->phase = exp_seg_relro_adjust;
-  ASSERT (desired_end >= seg->base);
-
-  for (; prev_sec; prev_sec = prev_sec->prev)
-    if ((prev_sec->flags & SEC_ALLOC) != 0)
-      {
-	if (prev_sec->alignment_power > max_alignment_power)
-	  max_alignment_power = prev_sec->alignment_power;
-
-	if (prev_sec->size != 0)
+	else if (seen_reloc_section)
 	  {
-	    /* The 1-page gap before the RELRO segment may be removed.  */
-	    remove_page_gap = ((prev_sec->vma + prev_sec->size
-				+ seg->maxpagesize) < desired_end);
-
+	    /* Stop searching if we see a non-relro section after seeing
+	       relro sections.  */
 	    break;
 	  }
       }
 
-  if (remove_page_gap)
+  if (relro_sec != NULL
+      && seg->maxpagesize >= (1U << max_alignment_power))
     {
-      /* Find the maximum section alignment.  */
-      for (sec = prev_sec; sec; sec = sec->prev)
-	if ((sec->flags & SEC_ALLOC) != 0
-	    && sec->alignment_power > max_alignment_power)
-	  max_alignment_power = sec->alignment_power;
+      asection *prev_sec;
 
-      /* Remove the 1-page gap before the RELRO segment only if the
-	 maximum page size >= the maximum section alignment.  */
-      if (seg->maxpagesize >= (1U << max_alignment_power))
+       /* Find the first preceding load section.  */
+      for (prev_sec = relro_sec->prev;
+	   prev_sec != NULL;
+	   prev_sec = prev_sec->prev)
+	if ((prev_sec->flags & SEC_ALLOC) != 0)
+	  break;
+
+      if ((prev_sec->vma + prev_sec->size + seg->maxpagesize)
+	  < desired_relro_base)
 	{
-	  /* If the preceding section size is greater than the maximum
-	     page size, subtract the maximum page size.  Otherwise,
-	     align the RELRO segment to the maximum page size.  */
+	  desired_relro_base_reduced = true;
+
+	  /* Don't add the 1-page gap before the relro segment if the
+	     maximum page size >= the maximum section alignment.  */
 	  if (prev_sec->size > seg->maxpagesize)
 	    {
-	      desired_end -= seg->maxpagesize;
+	      /* Subtract the maximum page size only if the preceding
+	         load section size is > the maximum page size to avoid
+		 moving the relro segment base before the preceding
+		 load section.  */
+	      desired_relro_base -= seg->maxpagesize;
 	      relro_end -= seg->maxpagesize;
 	    }
 	  else
 	    {
-	      desired_end &= ~(seg->maxpagesize - 1);
+	      /* Align the relro segment if there is a 1-page gap
+		 before the relro segment.  */
+	      desired_relro_base &= ~(seg->maxpagesize - 1);
 	      relro_end &= ~(seg->maxpagesize - 1);
 	    }
-	  }
-      }
+	}
+    }
 
-  seg->base = desired_end;
+  seg->phase = exp_seg_relro_adjust;
+  ASSERT (desired_relro_base_reduced
+	  || desired_relro_base >= seg->base);
+  seg->base = desired_relro_base;
   return relro_end;
 }
 
diff --git a/ld/testsuite/ld-x86-64/pr28743-1.d b/ld/testsuite/ld-x86-64/pr28743-1.d
new file mode 100644
index 00000000000..4a20f8e3900
--- /dev/null
+++ b/ld/testsuite/ld-x86-64/pr28743-1.d
@@ -0,0 +1,50 @@
+#as: --64
+#ld: -melf_x86_64 -shared -z relro -z now --hash-style=sysv -z max-page-size=0x1000 -z noseparate-code $NO_DT_RELR_LDFLAGS
+#readelf: -S -l --wide
+#target: x86_64-*-linux*
+
+There are 17 section headers, starting at offset 0x101228:
+
+Section Headers:
+  \[Nr\] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
+  \[ 0\]                   NULL            0000000000000000 000000 000000 00      0   0  0
+  \[ 1\] .hash             HASH            0000000000000120 000120 000018 04   A  2   0  8
+  \[ 2\] .dynsym           DYNSYM          0000000000000138 000138 000048 18   A  3   1  8
+  \[ 3\] .dynstr           STRTAB          0000000000000180 000180 00000a 00   A  0   0  1
+  \[ 4\] .rela.dyn         RELA            0000000000000190 000190 000018 18   A  2   0  8
+  \[ 5\] .plt              PROGBITS        00000000000001b0 0001b0 000010 10  AX  0   0 16
+  \[ 6\] .plt.got          PROGBITS        00000000000001c0 0001c0 000008 08  AX  0   0  8
+  \[ 7\] .text             PROGBITS        00000000000001c8 0001c8 00000c 00  AX  0   0  1
+  \[ 8\] .eh_frame         PROGBITS        00000000000001d8 0001d8 00006c 00   A  0   0  8
+  \[ 9\] .init_array       INIT_ARRAY      00000000000ffff8 0ffff8 000004 08  WA  0   0  8
+  \[10\] .fini_array       FINI_ARRAY      0000000000100000 100000 000100 08  WA  0   0 1048576
+  \[11\] .dynamic          DYNAMIC         0000000000100100 100100 000150 10  WA  3   0  8
+  \[12\] .got              PROGBITS        0000000000100250 100250 000020 08  WA  0   0  8
+  \[13\] .data             PROGBITS        0000000000101000 101000 000100 00  WA  0   0  1
+  \[14\] .symtab           SYMTAB          0000000000000000 101100 000078 18     15   3  8
+  \[15\] .strtab           STRTAB          0000000000000000 101178 000029 00      0   0  1
+  \[16\] .shstrtab         STRTAB          0000000000000000 1011a1 000080 00      0   0  1
+Key to Flags:
+  W \(write\), A \(alloc\), X \(execute\), M \(merge\), S \(strings\), I \(info\),
+  L \(link order\), O \(extra OS processing required\), G \(group\), T \(TLS\),
+  C \(compressed\), x \(unknown\), o \(OS specific\), E \(exclude\),
+  D \(mbind\), l \(large\), p \(processor specific\)
+
+Elf file type is DYN \(Shared object file\)
+Entry point 0x0
+There are 4 program headers, starting at offset 64
+
+Program Headers:
+  Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
+  LOAD           0x000000 0x0000000000000000 0x0000000000000000 0x000244 0x000244 R E 0x1000
+  LOAD           0x0ffff8 0x00000000000ffff8 0x00000000000ffff8 0x001108 0x001108 RW  0x100000
+  DYNAMIC        0x100100 0x0000000000100100 0x0000000000100100 0x000150 0x000150 RW  0x8
+  GNU_RELRO      0x0ffff8 0x00000000000ffff8 0x00000000000ffff8 0x001008 0x001008 R   0x1
+
+ Section to Segment mapping:
+  Segment Sections...
+   00     .hash .dynsym .dynstr .rela.dyn .plt .plt.got .text .eh_frame 
+   01     .init_array .fini_array .dynamic .got .data 
+   02     .dynamic 
+   03     .init_array .fini_array .dynamic .got 
+#pass
diff --git a/ld/testsuite/ld-x86-64/pr28743-1.s b/ld/testsuite/ld-x86-64/pr28743-1.s
new file mode 100644
index 00000000000..2e9a1503741
--- /dev/null
+++ b/ld/testsuite/ld-x86-64/pr28743-1.s
@@ -0,0 +1,16 @@
+	.text
+	.globl foo
+	.type foo, @function
+foo:
+	.cfi_startproc
+	call	func@plt
+	movq	func@GOTPCREL(%rip), %rax
+	.cfi_endproc
+	.section .init_array,"aw",@init_array
+	.p2align 3
+	.zero 0x4
+	.section .fini_array,"aw",@fini_array
+	.p2align 20
+	.zero 0x100
+	.data
+	.zero 0x100
diff --git a/ld/testsuite/ld-x86-64/x86-64.exp b/ld/testsuite/ld-x86-64/x86-64.exp
index 3bfc5e6e467..5ef66a237ef 100644
--- a/ld/testsuite/ld-x86-64/x86-64.exp
+++ b/ld/testsuite/ld-x86-64/x86-64.exp
@@ -511,6 +511,7 @@ run_dump_test "dt-relr-1a"
 run_dump_test "dt-relr-1a-x32"
 run_dump_test "dt-relr-1b"
 run_dump_test "dt-relr-1b-x32"
+run_dump_test "pr28743-1"
 
 if ![istarget "x86_64-*-linux*"] {
     return
-- 
2.34.1