[PATCH] elf: Set p_align to the common page size if possible
H.J. Lu
hjl.tools@gmail.com
Thu Dec 16 14:33:57 GMT 2021
On Thu, Dec 16, 2021 at 12:52 AM Fangrui Song <i@maskray.me> wrote:
>
> On 2021-12-15, H.J. Lu via Binutils wrote:
> >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 common 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:
>
> Maybe it's just me who is very careful on the words: aligning to p_align
> is a new feature, not a bug, as no ABI requires it. No ld.so I know
> (FreeBSD, musl, bionic) does this.
>
> >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 kernel patch has no cost. It just picks a load bias, while the
> glibc's .so loading patch has some costs because there is no alignment
> parameter to mmap... So now, every
>
> * (Linux x86-64) -z noseparate-code (default max-page-size=2MiB) .so incurs some munmap overhead
> * arm/aarch64/powerpc (default max-page-size=65536) .so incurs some munmap overhead...
>
> If I were to do this, I would fix objcopy first, then adjust ld's
> p_align, finally tune glibc's .so loading.
>
> >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).
> >
> >1. Set p_align to common page size while laying out segments aligning to
> >maximum page size or section alignment. The run-time loader can align
> >segments to common page size or maximum page size, depending on system
> >page size.
> >2. If -z max-page-size=NNN is used, p_align will be set to maximum page
> >size or the largest section alignment.
> >3. If a section requires alignment higher than the common page size,
> >don't set p_align to the common 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 commone page size != the maximum page size,
> >p_align may be set to the commone 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.
>
> Does this patch do anything with selecting max-page-size for objcopy?
> https://sourceware.org/bugzilla/show_bug.cgi?id=28689#c4
> Something like gcd({p_vaddr - p_offset}) can be used, but I don't see
> anything related to gcd.
For objcopy, my patch does
static bool
elf_is_p_align_valid (bfd *abfd)
{
...
if (commonpagesize == maxpagesize)
return true;
/* When the common page size != the maximum page size, p_align may
be set to the common 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 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 != commonpagesize
|| vma_page_aligned_bias (segment->p_vaddr,
segment->p_offset,
maxpagesize) != 0))
return true;
return false;
}
1. If p_align isn't set to the common page size, use it as the maximum
page size.
2. Otherwise, use the default maximum page size as the maximum
page size if the segment layout supports it.
objcopy shouldn't change p_align. But it must keep the same segment
layout.
> >6. Update linker to disallow the commone page size > the maximum page
> >size.
> >
> >bfd/
> >
> > PR ld/28689
> > PR ld/28695
> > * elf.c (assign_file_positions_for_load_sections): Set p_align
> > to common page size 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.
> >
> >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.c | 78 +++++++++++++++++++++++++++++--
> > 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 ++
> > 9 files changed, 149 insertions(+), 7 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.c b/bfd/elf.c
> >index e6c6a8a6c05..431084760ab 100644
> >--- a/bfd/elf.c
> >+++ b/bfd/elf.c
> >@@ -5406,6 +5406,8 @@ assign_file_positions_for_load_sections (bfd *abfd,
> > Elf_Internal_Phdr *p;
> > file_ptr off; /* Octets. */
> > bfd_size_type maxpagesize;
> >+ bfd_size_type commonpagesize;
> >+ bool p_align_commonpagesize_p = false;
> > unsigned int alloc, actual;
> > unsigned int i, j;
> > struct elf_segment_map **sorted_seg_map;
> >@@ -5491,12 +5493,19 @@ assign_file_positions_for_load_sections (bfd *abfd,
> > elf_sort_segments);
> >
> > maxpagesize = 1;
> >+ commonpagesize = 1;
> > if ((abfd->flags & D_PAGED) != 0)
> > {
> > if (link_info != NULL)
> >- maxpagesize = link_info->maxpagesize;
> >+ {
> >+ maxpagesize = link_info->maxpagesize;
> >+ commonpagesize = link_info->commonpagesize;
> >+ }
> > else
> >- maxpagesize = bed->maxpagesize;
> >+ {
> >+ maxpagesize = bed->maxpagesize;
> >+ commonpagesize = bed->commonpagesize;
> >+ }
> > }
> >
> > /* Sections must map to file offsets past the ELF file header. */
> >@@ -5560,6 +5569,13 @@ assign_file_positions_for_load_sections (bfd *abfd,
> > segment. */
> > if (m->p_align_valid)
> > maxpagesize = m->p_align;
> >+ else if (link_info == NULL || !link_info->maxpagesize_is_set)
> >+ /* Set p_align to common page size while laying out segments
> >+ aligning to the maximum page size or the largest section
> >+ alignment. The run-time loader can align segments to the
> >+ common page size or the maximum page size, depending on
> >+ system page size. */
> >+ p_align_commonpagesize_p = true;
> >
> > p->p_align = maxpagesize;
> > }
> >@@ -5597,7 +5613,22 @@ 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
> >+ common page size, don't set p_align to the common
> >+ page size. */
> >+ if (align > commonpagesize)
> >+ p_align_commonpagesize_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_commonpagesize_p = true;
> >+ commonpagesize = align;
> >+ }
> > }
> >
> > for (i = 0; i < m->count; i++)
> >@@ -5976,6 +6007,9 @@ assign_file_positions_for_load_sections (bfd *abfd,
> > print_segment_map (m);
> > }
> > }
> >+
> >+ if (p_align_commonpagesize_p)
> >+ p->p_align = commonpagesize;
> > }
> > }
> >
> >@@ -7485,6 +7519,39 @@ 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 commonpagesize = bed->commonpagesize;
> >+
> >+ if (commonpagesize == maxpagesize)
> >+ return true;
> >+
> >+ /* When the commone page size != the maximum page size, p_align may
> >+ be set to the commone 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. */
> >+ 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 != commonpagesize
> >+ || 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 +7566,7 @@ 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 +7587,8 @@ 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 +7631,7 @@ 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/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
>
> There is no test about max-page-size > common-page-size.
--
H.J.
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