Currently, on 32-bit and 64-bit Arm, it seems that BFD ld generates p_align values of 65536 even if no section alignment is greater than 4096. 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. We are going to fix the glibc loader to honor p_align (bug 28676). 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). A further complication is glibc bug 28688. So I'm not entirely sure anymore what we should do here.
I think p_align should be set to ELF_MINPAGESIZE by default.
One problem: Elf file type is EXEC (Executable file) Entry point 0x600000 There are 7 program headers, starting at offset 64 Program Headers: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align LOAD 0x000000 0x0000000000400000 0x0000000000400000 0x0001f8 0x0001f8 R 0x1000 LOAD 0x200000 0x0000000000600000 0x0000000000600000 0x000001 0x000001 R E 0x1000 LOAD 0x400000 0x0000000000800000 0x0000000000800000 0x002000 0x002000 RW 0x1000 NOTE 0x0001c8 0x00000000004001c8 0x00000000004001c8 0x000030 0x000030 R 0x8 TLS 0x400000 0x0000000000800000 0x0000000000800000 0x001000 0x001000 R 0x10 GNU_PROPERTY 0x0001c8 0x00000000004001c8 0x00000000004001c8 0x000030 0x000030 R 0x8 GNU_RELRO 0x400000 0x0000000000800000 0x0000000000800000 0x001000 0x001000 R 0x1 But objcopy will take p_align as the maximum page size: Elf file type is EXEC (Executable file) Entry point 0x600000 There are 7 program headers, starting at offset 64 Program Headers: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align LOAD 0x000000 0x0000000000400000 0x0000000000400000 0x0001f8 0x0001f8 R 0x1000 LOAD 0x001000 0x0000000000600000 0x0000000000600000 0x000001 0x000001 R E 0x1000 LOAD 0x002000 0x0000000000800000 0x0000000000800000 0x002000 0x002000 RW 0x1000 NOTE 0x0001c8 0x00000000004001c8 0x00000000004001c8 0x000030 0x000030 R 0x8 TLS 0x002000 0x0000000000800000 0x0000000000800000 0x001000 0x001000 R 0x10 GNU_PROPERTY 0x0001c8 0x00000000004001c8 0x00000000004001c8 0x000030 0x000030 R 0x8 GNU_RELRO 0x002000 0x0000000000800000 0x0000000000800000 0x001000 0x001000 R 0x1 File offsets are changed.
Created attachment 13850 [details] A patch Does this work?
(In reply to H.J. Lu from comment #2) > One problem: > [...] > File offsets are changed. For objcopy, it is important to not transform the file to decrease the intrinsic max-page-size. This would make the output not runnable on the originally supported systems. If ld decreases p_align to reduce wasted memory pages, can you describe how the objcopy problem (inferring max-page-size) will be solved? Note that max-page-size can be trick to infer in the -z noseparate-code case.
(In reply to Fangrui Song from comment #4) > (In reply to H.J. Lu from comment #2) > > One problem: > > [...] > > File offsets are changed. > > For objcopy, it is important to not transform the file to decrease the > intrinsic max-page-size. This would make the output not runnable on the > originally supported systems. > > If ld decreases p_align to reduce wasted memory pages, can you describe how > the objcopy problem (inferring max-page-size) will be solved? > Note that max-page-size can be trick to infer in the -z noseparate-code case. Maybe something like gcd({p_vaddr - p_offset}). If the gcd is 0, pick p_align
(In reply to Fangrui Song from comment #5) > (In reply to Fangrui Song from comment #4) > > (In reply to H.J. Lu from comment #2) > > > One problem: > > > [...] > > > File offsets are changed. > > > > For objcopy, it is important to not transform the file to decrease the > > intrinsic max-page-size. This would make the output not runnable on the > > originally supported systems. > > > > If ld decreases p_align to reduce wasted memory pages, can you describe how > > the objcopy problem (inferring max-page-size) will be solved? > > Note that max-page-size can be trick to infer in the -z noseparate-code case. > > Maybe something like gcd({p_vaddr - p_offset}). If the gcd is 0, pick p_align This is roughly what my patch does.
The master branch has been updated by H.J. Lu <hjl@sourceware.org>: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=74e315dbfe5200c473b226e937935fb8ce391489 commit 74e315dbfe5200c473b226e937935fb8ce391489 Author: H.J. Lu <hjl.tools@gmail.com> Date: Mon Dec 13 19:46:04 2021 -0800 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.
Fixed for x86 in binutils 2.38.