elf: Set p_align to the minimum page size if possible

Currently, on 32-bit and 64-bit ARM, it seems that ld generates p_align
values of 0x10000 even if no section alignment is greater than 0x1000.
The issue is more general and probably affects other targets with multiple
page sizes.

While file layout absolutely must take 64K page size into account, that
does not have to be reflected in the p_align value.  If running on a 64K
kernel, the file will be loaded at a 64K page boundary by necessity. On
a 4K kernel, 64K alignment is not needed.

The glibc loader has been fixed to honor p_align:

https://sourceware.org/bugzilla/show_bug.cgi?id=28676

similar to kernel:

commit ce81bb256a224259ab686742a6284930cbe4f1fa
Author: Chris Kennelly <ckennelly@google.com>
Date:   Thu Oct 15 20:12:32 2020 -0700

    fs/binfmt_elf: use PT_LOAD p_align values for suitable start address

This means that on 4K kernels, we will start to do extra work for 64K
p_align, but this pointless for pretty much all binaries (whose section
alignment rarely exceeds 16).

The minimum page size is used, instead of the maximum section alignment
due to this glibc bug:

https://sourceware.org/bugzilla/show_bug.cgi?id=28688

It has been fixed in glibc 2.35.  But linker output must work on existing
glibc binaries.

1. Set p_align to the minimum page size while laying out segments aligning
to the maximum page size or section alignment.  The run-time loader can
align segments to the minimum page size or above, depending on system page
size.
2. If -z max-page-size=NNN is used, p_align will be set to the maximum
page size or the largest section alignment.
3. If a section requires alignment higher than the minimum page size,
don't set p_align to the minimum page size.
4. If a section requires alignment higher than the maximum page size,
set p_align to the section alignment.
5. For objcopy, when the minimum page size != the maximum page size,
p_align may be set to the minimum page size while segments are aligned
to the maximum page size.  In this case, the input p_align will be
ignored and the maximum page size will be used to align the ouput
segments.
6. Update linker to disallow the common page size > the maximum page size.
7. Update linker to avoid the common page size > the maximum page size.
8. Adjust pru_irq_map-1.d to expect p_align == sh_addralign:

Section Headers:
  [Nr] Name   Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]        NULL            00000000 000000 000000 00      0   0  0
  [ 1] .text  PROGBITS        20000000 00007c 000004 00  AX  0   0  4
...
Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  LOAD           0x000074 0x00000000 0x00000000 0x00008 0x00008 RW  0x1
  LOAD           0x00007c 0x20000000 0x20000000 0x00004 0x00004 R E 0x4

vs.

Section Headers:
  [Nr] Name   Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]        NULL            00000000 000000 000000 00      0   0  0
  [ 1] .text  PROGBITS        20000000 00007c 000004 00  AX  0   0  4
...
Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  LOAD           0x000074 0x00000000 0x00000000 0x00008 0x00008 RW  0x1
  LOAD           0x00007c 0x20000000 0x20000000 0x00004 0x00004 R E 0x1

To enable this linker optimization, the backend should define ELF_P_ALIGN
to ELF_MINPAGESIZE.

bfd/

	PR ld/28689
	PR ld/28695
	* elf-bfd.h (elf_backend_data): Add p_align.
	* elf.c (assign_file_positions_for_load_sections): Set p_align
	to the default p_align value while laying out segments aligning
	to maximum page size or section alignment.
	(elf_is_p_align_valid): New function.
	(copy_elf_program_header): Call elf_is_p_align_valid to determine
	if p_align is valid.
	* elfxx-target.h (ELF_P_ALIGN): New.  Default to 0.
	(elfNN_bed): Add ELF_P_ALIGN.
	* elfxx-x86.h (ELF_P_ALIGN): New.  Set to ELF_MINPAGESIZE.

include/

	PR ld/28689
	PR ld/28695
	* bfdlink.h (bfd_link_info): Add maxpagesize_is_set.

ld/

	PR ld/28689
	PR ld/28695
	* emultempl/elf.em (gld${EMULATION_NAME}_handle_option): Set
	link_info.maxpagesize_is_set for -z max-page-size=NNN.
	* ldelf.c (ldelf_after_parse): Disallow link_info.commonpagesize
	> link_info.maxpagesize.
	* testsuite/ld-elf/elf.exp: Pass -z max-page-size=0x4000 to
	linker to build mbind2a and mbind2b.
	* testsuite/ld-elf/header.d: Add -z common-page-size=0x100.
	* testsuite/ld-elf/linux-x86.exp: Add PR ld/28689 tests.
	* testsuite/ld-elf/p_align-1.c: New file.
	* testsuite/ld-elf/page-size-1.d: New test.
	* testsuite/ld-elf/pr26936.d: Add -z common-page-size=0x1000.
	* testsuite/ld-elf/seg.d: Likewise.
	* testsuite/ld-scripts/rgn-at5.d: Likewise.
	* testsuite/ld-pru/pru_irq_map-1.d: Append 1 to name.  Adjust
	expected PT_LOAD segment alignment.
	* testsuite/ld-pru/pru_irq_map-2.d: Append 2 to name.
	* testsuite/ld-scripts/pr23571.d: Add -z max-page-size=0x1000.

1 files changed, 69 insertions, 2 deletions

diff --git a/bfd/elf.c b/bfd/elf.c
index 7e8a2ec..1003bd2 100644
--- a/bfd/elf.c
+++ b/bfd/elf.c
@@ -5407,6 +5407,8 @@ assign_file_positions_for_load_sections (bfd *abfd,
   Elf_Internal_Phdr *p;
   file_ptr off;  /* Octets.  */
   bfd_size_type maxpagesize;
+  bfd_size_type p_align;
+  bool p_align_p = false;
   unsigned int alloc, actual;
   unsigned int i, j;
   struct elf_segment_map **sorted_seg_map;
@@ -5491,6 +5493,7 @@ assign_file_positions_for_load_sections (bfd *abfd,
     qsort (sorted_seg_map, alloc, sizeof (*sorted_seg_map),
 	   elf_sort_segments);
 
+  p_align = bed->p_align;
   maxpagesize = 1;
   if ((abfd->flags & D_PAGED) != 0)
     {
@@ -5561,6 +5564,15 @@ assign_file_positions_for_load_sections (bfd *abfd,
 	     segment.  */
 	  if (m->p_align_valid)
 	    maxpagesize = m->p_align;
+	  else if (p_align != 0
+		   && (link_info == NULL
+		       || !link_info->maxpagesize_is_set))
+	    /* Set p_align to the default p_align value while laying
+	       out segments aligning to the maximum page size or the
+	       largest section alignment.  The run-time loader can
+	       align segments to the default p_align value or the
+	       maximum page size, depending on system page size.  */
+	    p_align_p = true;
 
 	  p->p_align = maxpagesize;
 	}
@@ -5598,7 +5610,22 @@ assign_file_positions_for_load_sections (bfd *abfd,
 		}
 	      align = (bfd_size_type) 1 << align_power;
 	      if (align < maxpagesize)
-		align = maxpagesize;
+		{
+		  /* If a section requires alignment higher than the
+		     default p_align value, don't set p_align to the
+		     default p_align value.  */
+		  if (align > p_align)
+		    p_align_p = false;
+		  align = maxpagesize;
+		}
+	      else
+		{
+		  /* If a section requires alignment higher than the
+		     maximum page size, set p_align to the section
+		     alignment.  */
+		  p_align_p = true;
+		  p_align = align;
+		}
 	    }
 
 	  for (i = 0; i < m->count; i++)
@@ -5977,6 +6004,9 @@ assign_file_positions_for_load_sections (bfd *abfd,
 		  print_segment_map (m);
 		}
 	    }
+
+	  if (p_align_p)
+	    p->p_align = p_align;
 	}
     }
 
@@ -7484,6 +7514,40 @@ rewrite_elf_program_header (bfd *ibfd, bfd *obfd, bfd_vma maxpagesize)
   return true;
 }
 
+/* Return true if p_align in the ELF program header in ABFD is valid.  */
+
+static bool
+elf_is_p_align_valid (bfd *abfd)
+{
+  unsigned int i;
+  Elf_Internal_Phdr *segment;
+  unsigned int num_segments;
+  const struct elf_backend_data *bed = get_elf_backend_data (abfd);
+  bfd_size_type maxpagesize = bed->maxpagesize;
+  bfd_size_type p_align = bed->p_align;
+
+  /* Return true if the default p_align value isn't set or the maximum
+     page size is the same as the minimum page size.  */
+  if (p_align == 0 || maxpagesize == bed->minpagesize)
+    return true;
+
+  /* When the default p_align value is set, p_align may be set to the
+     default p_align value while segments are aligned to the maximum
+     page size.  In this case, the input p_align will be ignored and
+     the maximum page size will be used to align the output segments.  */
+  segment = elf_tdata (abfd)->phdr;
+  num_segments = elf_elfheader (abfd)->e_phnum;
+  for (i = 0; i < num_segments; i++, segment++)
+    if (segment->p_type == PT_LOAD
+	&& (segment->p_align != p_align
+	    || vma_page_aligned_bias (segment->p_vaddr,
+				      segment->p_offset,
+				      maxpagesize) != 0))
+      return true;
+
+  return false;
+}
+
 /* Copy ELF program header information.  */
 
 static bool
@@ -7498,6 +7562,7 @@ copy_elf_program_header (bfd *ibfd, bfd *obfd)
   unsigned int num_segments;
   bool phdr_included = false;
   bool p_paddr_valid;
+  bool p_palign_valid;
   unsigned int opb = bfd_octets_per_byte (ibfd, NULL);
 
   iehdr = elf_elfheader (ibfd);
@@ -7518,6 +7583,8 @@ copy_elf_program_header (bfd *ibfd, bfd *obfd)
 	break;
       }
 
+  p_palign_valid = elf_is_p_align_valid (ibfd);
+
   for (i = 0, segment = elf_tdata (ibfd)->phdr;
        i < num_segments;
        i++, segment++)
@@ -7560,7 +7627,7 @@ copy_elf_program_header (bfd *ibfd, bfd *obfd)
       map->p_paddr = segment->p_paddr;
       map->p_paddr_valid = p_paddr_valid;
       map->p_align = segment->p_align;
-      map->p_align_valid = 1;
+      map->p_align_valid = p_palign_valid;
       map->p_vaddr_offset = 0;
 
       if (map->p_type == PT_GNU_RELRO