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.
git commit 994b251328 "Ignore section symbols when matching linkonce
with comdat" cured the reason why this test used to fail on mips.
* testsuite/ld-elf/pr26936.d: No longer xfail mips.
Required for the expected "CU:" to be emitted for long
source-paths. See binutils/dwarf.c:
if (do_wide || strlen (directory) < 76)
printf (_("CU: %s/%s:\n"), directory, file_table[0].name);
else
printf ("%s:\n", file_table[0].name);
Linkonce sections and comdat groups can be mixed only if comdat groups
have only a single member with matching symbol table entries. Xfail
ld/26936 test:
1. If comdat groups always have more than one member.
2. If symbol table entries in linkonce and comdat group don't match.
3. If the assembly source file is renamed.
PR ld/26936
* testsuite/ld-elf/pr26936.d: Xfail targets which don't support
mixing linkonce and comdat sections.
Many targets fail this test due to -z noseparate-code not being
supported, or _start not being the proper entry symbol, or "as -g"
something other than "generate debug".
PR 26936
* testsuite/ld-elf/pr26936.d: Pass --gen-debug to gas rather than -g.
Only run when -shared -z options are supported.
* testsuite/ld-elf/pr26936b.s: Define more entry symbols.
When mixing linkonce and comdat sections, we need to keep searching to
get the real kept section.
bfd/
PR ld/26936
* elflink.c (_bfd_elf_check_kept_section): Get the real kept
section.
ld/
PR ld/26936
* testsuite/ld-elf/pr26936.d: New file.
* testsuite/ld-elf/pr26936a.s: Likewise.
* testsuite/ld-elf/pr26936b.s: Likewise.
* testsuite/ld-elf/pr26936c.s: Likewise.
