Bug 23930

Summary:	Problems linking objectfiles optimized for size (-Os)
Product:	binutils	Reporter:	Artem Savkov <asavkov>
Component:	ld	Assignee:	Not yet assigned to anyone <unassigned>
Status:	RESOLVED FIXED
Severity:	normal	CC:	hjl.tools
Priority:	P2
Version:	2.29
Target Milestone:	2.32
Host:		Target:
Build:		Last reconfirmed:
Attachments:	Reproducer A workaround

Description Artem Savkov 2018-11-28 16:06:25 UTC

Created attachment 11416 [details]
Reproducer

The issue was found while building linux kernel with CONFIG_CC_OPTIMIZE_FOR_SIZE (adds -Os to cflags) and KCFLAGS="-ffunction-sections", while linking vmlinux I get the fillowing warnings and error:

  ld: warning: dot moved backwards before `.text.__startup_secondary_64'
  ld: warning: dot moved backwards before `.text.__startup_secondary_64'
  ld: final link failed: File truncated
  make: *** [Makefile:1032: vmlinux] Error 1                                       

.iplt section seem to cause issues during linking. This is only reproducible when vmas and lmas are different, failing section sh_addralign is 1(caused by -Os) and .iplt is before failing section and the section offset needs to be just right. I am attaching a small reproducer that I managed to put together based on linux kernel linker script [1].

This is what I see happening:
.iplt does not have ignored flag set on first pass of one_lang_size_sections_pass() but has it set on subsequent passes, this results in dot of the following section moving backwards, resulting in a warning and section lma not being set properly which later results in s_start being set to a negative value in assign_file_positions_for_load_sections() yielding a "File truncated" error.

I have bisected this issue to the following commit: 38b123494b "x86-64: Improve PLT generation and synthetic PLT symbols"

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/vmlinux.lds.S

Comment 1 H.J. Lu 2018-11-28 23:38:45 UTC

Created attachment 11418 [details]
A workaround

The bug is in ldlang.c.  In the meantime, please try this workaround in
x86 backend.

Comment 2 Artem Savkov 2018-11-29 08:51:31 UTC

The workaround fixes both the reproducer and original kernel build for me.

Comment 3 H.J. Lu 2018-11-30 13:57:39 UTC

This is the patch:

https://sourceware.org/ml/binutils/2018-11/msg00279.html

I will check in.

Comment 4 Sourceware Commits 2018-12-01 14:39:31 UTC

The master branch has been updated by H.J. Lu <hjl@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=b44ee3a8cf21294eeb079ffbada7eeb559a9c6b4

commit b44ee3a8cf21294eeb079ffbada7eeb559a9c6b4
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Sat Dec 1 05:42:33 2018 -0800

    x86: Delay setting the iplt section alignment
    
    Delay setting its alignment until we know it is non-empty.  Otherwise an
    empty iplt section may change vma and lma of the following sections, which
    triggers moving dot of the following section backwards, resulting in a
    warning and section lma not being set properly.  It later leads to a
    "File truncated" error.
    
    bfd/
    
    	PR ld/23930
    	* elfxx-x86.c (_bfd_x86_elf_size_dynamic_sections): Update
    	the iplt section alignment if it is non-empty.
    	(_bfd_x86_elf_link_setup_gnu_properties): Set plt.iplt_alignment
    	and delay setting the iplt section alignment.
    	* elfxx-x86.h (elf_x86_plt_layout): Add iplt_alignment.
    
    ld/
    
    	PR ld/23930
    	* testsuite/ld-i386/i386.exp: Run pr23930.
    	* testsuite/ld-i386/pr23930.d: New file.
    	* testsuite/ld-x86-64/pr23930-32.t: Likewise.
    	* testsuite/ld-x86-64/pr23930-x32.d: Likewise.
    	* testsuite/ld-x86-64/pr23930.d: Likewise.
    	* testsuite/ld-x86-64/pr23930.t: Likewise.
    	* testsuite/ld-x86-64/pr23930a.s: Likewise.
    	* testsuite/ld-x86-64/pr23930b.s: Likewise.
    	* testsuite/ld-x86-64/x86-64.exp: Run pr23930 and pr23930-x32.

Comment 5 H.J. Lu 2018-12-01 14:41:55 UTC

Fixed for 2.32.