[PATCH v3] elf: Set p_align to the minimum page size if possible

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

Changes in v3:

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

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.

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.
---
 bfd/elf-bfd.h                     |  4 ++
 bfd/elf.c                         | 71 ++++++++++++++++++++++++++++++-
 bfd/elfxx-target.h                |  5 +++
 bfd/elfxx-x86.h                   |  2 +
 include/bfdlink.h                 |  3 ++
 ld/emultempl/elf.em               |  1 +
 ld/ldelf.c                        |  3 ++
 ld/testsuite/ld-elf/elf.exp       |  4 +-
 ld/testsuite/ld-elf/header.d      |  2 +-
 ld/testsuite/ld-elf/linux-x86.exp | 36 ++++++++++++++++
 ld/testsuite/ld-elf/p_align-1.c   | 25 +++++++++++
 ld/testsuite/ld-elf/page-size-1.d |  4 ++
 12 files changed, 155 insertions(+), 5 deletions(-)
 create mode 100644 ld/testsuite/ld-elf/p_align-1.c
 create mode 100644 ld/testsuite/ld-elf/page-size-1.d

diff --git a/bfd/elf-bfd.h b/bfd/elf-bfd.h
index 68e830c6f05..70331408a0b 100644
--- a/bfd/elf-bfd.h
+++ b/bfd/elf-bfd.h
@@ -945,6 +945,10 @@ struct elf_backend_data
   /* The value of commonpagesize to use when -z relro for this backend.  */
   bfd_vma relropagesize;
 
+  /* The p_align value for this backend.  If it is set, p_align of
+      PT_LOAD alignment will be to p_align by default.  */
+  bfd_vma p_align;
+
   /* The BFD flags applied to sections created for dynamic linking.  */
   flagword dynamic_sec_flags;
 
diff --git a/bfd/elf.c b/bfd/elf.c
index 92c06f2e44f..6c440a6dafc 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;
 	}
     }
 
@@ -7485,6 +7515,40 @@ 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
@@ -7499,6 +7563,7 @@ 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);
@@ -7519,6 +7584,8 @@ 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++)
@@ -7561,7 +7628,7 @@ 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
diff --git a/bfd/elfxx-target.h b/bfd/elfxx-target.h
index 4c6b1f20340..d0e2779a061 100644
--- a/bfd/elfxx-target.h
+++ b/bfd/elfxx-target.h
@@ -400,6 +400,10 @@
 # error ELF_MINPAGESIZE > ELF_RELROPAGESIZE
 #endif
 
+#ifndef ELF_P_ALIGN
+#define ELF_P_ALIGN 0
+#endif
+
 #ifndef ELF_DYNAMIC_SEC_FLAGS
 /* Note that we set the SEC_IN_MEMORY flag for these sections.  */
 #define ELF_DYNAMIC_SEC_FLAGS			\
@@ -813,6 +817,7 @@ static const struct elf_backend_data elfNN_bed =
   ELF_MINPAGESIZE,		/* minpagesize */
   ELF_COMMONPAGESIZE,		/* commonpagesize */
   ELF_RELROPAGESIZE,		/* commonpagesize to use with -z relro */
+  ELF_P_ALIGN,			/* p_align */
   ELF_DYNAMIC_SEC_FLAGS,	/* dynamic_sec_flags */
   elf_backend_arch_data,
   elf_info_to_howto,
diff --git a/bfd/elfxx-x86.h b/bfd/elfxx-x86.h
index aab641ab751..0c95f3533af 100644
--- a/bfd/elfxx-x86.h
+++ b/bfd/elfxx-x86.h
@@ -732,6 +732,8 @@ extern void _bfd_x86_elf_link_report_relative_reloc
 #define elf_backend_fixup_gnu_properties \
   _bfd_x86_elf_link_fixup_gnu_properties
 
+#define ELF_P_ALIGN ELF_MINPAGESIZE
+
 /* Return true if H is a __start_SECNAME/__stop_SECNAME symbol for the
    SECNAME section which has been garbage collected by --gc-sections
    -z start-stop-gc.  */
diff --git a/include/bfdlink.h b/include/bfdlink.h
index 566529ee644..8cf34b05c1a 100644
--- a/include/bfdlink.h
+++ b/include/bfdlink.h
@@ -525,6 +525,9 @@ struct bfd_link_info
   /* TRUE if all symbol names should be unique.  */
   unsigned int unique_symbol : 1;
 
+  /* TRUE if maxpagesize is set on command-line.  */
+  unsigned int maxpagesize_is_set : 1;
+
   /* Char that may appear as the first char of a symbol, but should be
      skipped (like symbol_leading_char) when looking up symbols in
      wrap_hash.  Used by PowerPC Linux for 'dot' symbols.  */
diff --git a/ld/emultempl/elf.em b/ld/emultempl/elf.em
index bfaf8130a3e..5eadab9f4b9 100644
--- a/ld/emultempl/elf.em
+++ b/ld/emultempl/elf.em
@@ -721,6 +721,7 @@ fragment <<EOF
 	      || (link_info.maxpagesize & (link_info.maxpagesize - 1)) != 0)
 	    einfo (_("%F%P: invalid maximum page size \`%s'\n"),
 		   optarg + 14);
+	  link_info.maxpagesize_is_set = true;
 	}
       else if (startswith (optarg, "common-page-size="))
 	{
diff --git a/ld/ldelf.c b/ld/ldelf.c
index 529992b02ae..ee09df5f35c 100644
--- a/ld/ldelf.c
+++ b/ld/ldelf.c
@@ -72,6 +72,9 @@ ldelf_after_parse (void)
       link_info.dynamic_undefined_weak = 0;
     }
   after_parse_default ();
+  if (link_info.commonpagesize > link_info.maxpagesize)
+    einfo (_("%F%P: common page size (0x%v) > maximum page size (0x%v)\n"),
+	   link_info.commonpagesize, link_info.maxpagesize);
 }
 
 /* Handle the generation of DT_NEEDED tags.  */
diff --git a/ld/testsuite/ld-elf/elf.exp b/ld/testsuite/ld-elf/elf.exp
index 01d22faad9a..ae8f76db1c7 100644
--- a/ld/testsuite/ld-elf/elf.exp
+++ b/ld/testsuite/ld-elf/elf.exp
@@ -365,7 +365,7 @@ if { [istarget *-*-linux*]
     run_ld_link_exec_tests [list \
 	[list \
 	    "Run mbind2a" \
-	    "$NOPIE_LDFLAGS -Wl,-z,common-page-size=0x4000" \
+	    "$NOPIE_LDFLAGS -Wl,-z,common-page-size=0x4000,-z,max-page-size=0x4000" \
 	    "" \
 	    { mbind2a.s mbind2b.c } \
 	    "mbind2a" \
@@ -374,7 +374,7 @@ if { [istarget *-*-linux*]
 	] \
 	[list \
 	    "Run mbind2b" \
-	    "-static -Wl,-z,common-page-size=0x4000" \
+	    "-static -Wl,-z,common-page-size=0x4000,-z,max-page-size=0x4000" \
 	    "" \
 	    { mbind2a.s mbind2b.c } \
 	    "mbind2b" \
diff --git a/ld/testsuite/ld-elf/header.d b/ld/testsuite/ld-elf/header.d
index c4d174a98da..67f0c981920 100644
--- a/ld/testsuite/ld-elf/header.d
+++ b/ld/testsuite/ld-elf/header.d
@@ -1,5 +1,5 @@
 # target: *-*-linux* *-*-gnu* *-*-vxworks arm*-*-uclinuxfdpiceabi
-# ld: -T header.t -z max-page-size=0x100
+# ld: -T header.t -z max-page-size=0x100 -z common-page-size=0x100
 # objdump: -hpw
 
 #...
diff --git a/ld/testsuite/ld-elf/linux-x86.exp b/ld/testsuite/ld-elf/linux-x86.exp
index ee03b565faf..25a8d0411d9 100644
--- a/ld/testsuite/ld-elf/linux-x86.exp
+++ b/ld/testsuite/ld-elf/linux-x86.exp
@@ -185,6 +185,42 @@ run_ld_link_exec_tests [list \
 	"" \
 	"tmpdir/indirect-extern-access-2.so" \
     ] \
+    [list \
+	"Run p_align-1a without PIE" \
+	"$NOPIE_LDFLAGS" \
+	"" \
+	{ p_align-1.c } \
+	"p_align-1a" \
+	"pass.out" \
+	"$NOPIE_CFLAGS" \
+    ] \
+    [list \
+	"Run p_align-1b with PIE" \
+	"-pie" \
+	"" \
+	{ p_align-1.c } \
+	"p_align-1b" \
+	"pass.out" \
+	"-fpie" \
+    ] \
+    [list \
+	"Run p_align-1c with -Wl,-z,max-page-size=0x1000 without PIE" \
+	"$NOPIE_LDFLAGS -Wl,-z,max-page-size=0x1000" \
+	"" \
+	{ p_align-1.c } \
+	"p_align-1c" \
+	"pass.out" \
+	"$NOPIE_CFLAGS" \
+    ] \
+    [list \
+	"Run p_align-1d with -Wl,-z,max-page-size=0x1000 with PIE" \
+	"-pie -Wl,-z,max-page-size=0x1000" \
+	"" \
+	{ p_align-1.c } \
+	"p_align-1d" \
+	"pass.out" \
+	"-fpie" \
+    ] \
 ]
 
 proc elfedit_test { options test output } {
diff --git a/ld/testsuite/ld-elf/p_align-1.c b/ld/testsuite/ld-elf/p_align-1.c
new file mode 100644
index 00000000000..6579cd74e52
--- /dev/null
+++ b/ld/testsuite/ld-elf/p_align-1.c
@@ -0,0 +1,25 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#ifndef ALIGN
+# define ALIGN 0x800000
+#endif
+
+int
+__attribute__ ((weak))
+is_aligned (void *p, int align)
+{
+  return (((uintptr_t) p) & (align - 1)) == 0;
+}
+
+int foo __attribute__ ((aligned (ALIGN))) = 1;
+
+int
+main (void)
+{
+  if (!is_aligned (&foo, ALIGN))
+    abort ();
+  printf ("PASS\n");
+  return 0;
+}
diff --git a/ld/testsuite/ld-elf/page-size-1.d b/ld/testsuite/ld-elf/page-size-1.d
new file mode 100644
index 00000000000..04d2153b2f9
--- /dev/null
+++ b/ld/testsuite/ld-elf/page-size-1.d
@@ -0,0 +1,4 @@
+#source: dummy.s
+#ld: -z common-page-size=0x4000 -z max-page-size=0x1000
+#error: common page size \(0x4000\) > maximum page size \(0x1000\)
+#target: *-*-linux-gnu *-*-gnu* arm*-*-uclinuxfdpiceabi
-- 
2.33.1