[lld] r321986 - [ELF] Compress debug sections after assignAddresses and support custom layout
Rafael Avila de Espindola via llvm-commits
llvm-commits at lists.llvm.org
Tue Jan 16 07:41:55 PST 2018
LGTM to the branch.
Cheers,
Rafael
Hans Wennborg <hans at chromium.org> writes:
> Rafael, is this OK for the branch?
>
> On Mon, Jan 8, 2018 at 3:37 PM, James Henderson
> <jh7370.2008 at my.bristol.ac.uk> wrote:
>> Hi Hans,
>>
>> Could this commit be merged into the 6.0 release branch, as it fixes
>> PR35788.
>>
>> Regards,
>>
>> James Henderson
>>
>> On 8 January 2018 at 10:17, James Henderson via llvm-commits
>> <llvm-commits at lists.llvm.org> wrote:
>>>
>>> Author: jhenderson
>>> Date: Mon Jan 8 02:17:03 2018
>>> New Revision: 321986
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=321986&view=rev
>>> Log:
>>> [ELF] Compress debug sections after assignAddresses and support custom
>>> layout
>>>
>>> Previously, in r320472, I moved the calculation of section offsets and
>>> sizes
>>> for compressed debug sections into maybeCompress, which happens before
>>> assignAddresses, so that the compression had the required information.
>>> However,
>>> I failed to take account of relocations that patch such sections. This had
>>> two
>>> effects:
>>>
>>> 1. A race condition existed when a debug section referred to a different
>>> debug
>>> section (see PR35788).
>>> 2. References to symbols in non-debug sections would be patched
>>> incorrectly.
>>> This is because the addresses of such symbols are not calculated until
>>> after
>>> assignAddresses (this was a partial regression caused by r320472, but they
>>> could still have been broken before, in the event that a custom layout was
>>> used
>>> in a linker script).
>>>
>>> assignAddresses does not need to know about the output section size of
>>> non-allocatable sections, because they do not affect the value of Dot.
>>> This
>>> means that there is no longer a reason not to support custom layout of
>>> compressed debug sections, as far as I'm aware. These two points allow for
>>> delaying when maybeCompress can be called, removing the need for the loop
>>> I
>>> previously added to calculate the section size, and therefore the race
>>> condition. Furthermore, by delaying, we fix the issues of relocations
>>> getting
>>> incorrect symbol values, because they have now all been finalized.
>>>
>>> Added:
>>> lld/trunk/test/ELF/Inputs/compress-debug.s
>>> lld/trunk/test/ELF/compress-debug-sections-reloc.s
>>> lld/trunk/test/ELF/linkerscript/compress-debug-sections-custom.s
>>> Modified:
>>> lld/trunk/ELF/LinkerScript.cpp
>>> lld/trunk/ELF/OutputSections.cpp
>>> lld/trunk/ELF/Writer.cpp
>>>
>>> Modified: lld/trunk/ELF/LinkerScript.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/LinkerScript.cpp?rev=321986&r1=321985&r2=321986&view=diff
>>>
>>> ==============================================================================
>>> --- lld/trunk/ELF/LinkerScript.cpp (original)
>>> +++ lld/trunk/ELF/LinkerScript.cpp Mon Jan 8 02:17:03 2018
>>> @@ -669,11 +669,6 @@ void LinkerScript::assignOffsets(OutputS
>>>
>>> switchTo(Sec);
>>>
>>> - // We do not support custom layout for compressed debug sectons.
>>> - // At this point we already know their size and have compressed
>>> content.
>>> - if (Ctx->OutSec->Flags & SHF_COMPRESSED)
>>> - return;
>>> -
>>> // The Size previously denoted how many InputSections had been added to
>>> this
>>> // section, and was used for sorting SHF_LINK_ORDER sections. Reset it
>>> to
>>> // compute the actual size value.
>>>
>>> Modified: lld/trunk/ELF/OutputSections.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/OutputSections.cpp?rev=321986&r1=321985&r2=321986&view=diff
>>>
>>> ==============================================================================
>>> --- lld/trunk/ELF/OutputSections.cpp (original)
>>> +++ lld/trunk/ELF/OutputSections.cpp Mon Jan 8 02:17:03 2018
>>> @@ -183,15 +183,6 @@ template <class ELFT> void OutputSection
>>> !Name.startswith(".debug_"))
>>> return;
>>>
>>> - // Calculate the section offsets and size pre-compression.
>>> - Size = 0;
>>> - for (BaseCommand *Cmd : SectionCommands)
>>> - if (auto *ISD = dyn_cast<InputSectionDescription>(Cmd))
>>> - for (InputSection *IS : ISD->Sections) {
>>> - IS->OutSecOff = alignTo(Size, IS->Alignment);
>>> - this->Size = IS->OutSecOff + IS->getSize();
>>> - }
>>> -
>>> // Create a section header.
>>> ZDebugHeader.resize(sizeof(Elf_Chdr));
>>> auto *Hdr = reinterpret_cast<Elf_Chdr *>(ZDebugHeader.data());
>>>
>>> Modified: lld/trunk/ELF/Writer.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Writer.cpp?rev=321986&r1=321985&r2=321986&view=diff
>>>
>>> ==============================================================================
>>> --- lld/trunk/ELF/Writer.cpp (original)
>>> +++ lld/trunk/ELF/Writer.cpp Mon Jan 8 02:17:03 2018
>>> @@ -427,13 +427,14 @@ template <class ELFT> void Writer<ELFT>:
>>> if (errorCount())
>>> return;
>>>
>>> + Script->assignAddresses();
>>> +
>>> // If -compressed-debug-sections is specified, we need to compress
>>> // .debug_* sections. Do it right now because it changes the size of
>>> // output sections.
>>> parallelForEach(OutputSections,
>>> [](OutputSection *Sec) { Sec->maybeCompress<ELFT>();
>>> });
>>>
>>> - Script->assignAddresses();
>>> Script->allocateHeaders(Phdrs);
>>>
>>> // Remove empty PT_LOAD to avoid causing the dynamic linker to try to
>>> mmap a
>>>
>>> Added: lld/trunk/test/ELF/Inputs/compress-debug.s
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/Inputs/compress-debug.s?rev=321986&view=auto
>>>
>>> ==============================================================================
>>> --- lld/trunk/test/ELF/Inputs/compress-debug.s (added)
>>> +++ lld/trunk/test/ELF/Inputs/compress-debug.s Mon Jan 8 02:17:03 2018
>>> @@ -0,0 +1,5 @@
>>> +.text
>>> +.fill 0x44
>>> +
>>> +.section .debug_info,"", at progbits
>>> +.fill 0x43
>>>
>>> Added: lld/trunk/test/ELF/compress-debug-sections-reloc.s
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/compress-debug-sections-reloc.s?rev=321986&view=auto
>>>
>>> ==============================================================================
>>> --- lld/trunk/test/ELF/compress-debug-sections-reloc.s (added)
>>> +++ lld/trunk/test/ELF/compress-debug-sections-reloc.s Mon Jan 8 02:17:03
>>> 2018
>>> @@ -0,0 +1,26 @@
>>> +# REQUIRES: x86, zlib
>>> +
>>> +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
>>> +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux
>>> %S/Inputs/compress-debug.s -o %t2.o
>>> +# RUN: ld.lld %t2.o %t.o -o %t1 --compress-debug-sections=zlib -Ttext=0
>>> +# RUN: llvm-dwarfdump %t1 -debug-str | FileCheck %s
>>> +# These two checks correspond to the patched values of a_sym and
>>> a_debug_sym.
>>> +# D = 0x44 - address of .text input section for this file (the start
>>> address of
>>> +# .text is 0 as requested on the command line, and the size of the
>>> +# preceding .text in the other input file is 0x44).
>>> +# C = 0x43 - offset of .debug_info section for this file (the size of
>>> +# the preceding .debug_info from the other input file is 0x43).
>>> +# CHECK: 0x00000000: "D"
>>> +# CHECK: 0x00000004: "C"
>>> +
>>> +.text
>>> +a_sym:
>>> +nop
>>> +
>>> +.section .debug_str,"", at progbits
>>> +.long a_sym
>>> +.long a_debug_sym
>>> +
>>> +.section .debug_info,"", at progbits
>>> +a_debug_sym:
>>> +.long 0x88776655
>>>
>>> Added: lld/trunk/test/ELF/linkerscript/compress-debug-sections-custom.s
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/linkerscript/compress-debug-sections-custom.s?rev=321986&view=auto
>>>
>>> ==============================================================================
>>> --- lld/trunk/test/ELF/linkerscript/compress-debug-sections-custom.s
>>> (added)
>>> +++ lld/trunk/test/ELF/linkerscript/compress-debug-sections-custom.s Mon
>>> Jan 8 02:17:03 2018
>>> @@ -0,0 +1,35 @@
>>> +# REQUIRES: x86, zlib
>>> +
>>> +# RUN: echo "SECTIONS { \
>>> +# RUN: .text : { . += 0x10; *(.text) } \
>>> +# RUN: .debug_str : { . += 0x10; *(.debug_str) } \
>>> +# RUN: .debug_info : { . += 0x10; *(.debug_info) } \
>>> +# RUN: }" > %t.script
>>> +
>>> +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
>>> +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux
>>> %S/../Inputs/compress-debug.s -o %t2.o
>>> +# RUN: ld.lld %t2.o %t.o -o %t1 --compress-debug-sections=zlib -T
>>> %t.script
>>> +# RUN: llvm-dwarfdump %t1 -debug-str | FileCheck %s
>>> +# These two checks correspond to the patched values of a_sym and
>>> a_debug_sym.
>>> +# T = 0x54 - address of .text input section for this file (the start
>>> address of
>>> +# .text is 0 by default, the size of the preceding .text in the other
>>> input
>>> +# file is 0x44, and the linker script adds an additional 0x10).
>>> +# S = 0x53 - offset of .debug_info section for this file (the size of
>>> +# the preceding .debug_info from the other input file is 0x43, and
>>> the
>>> +# linker script adds an additional 0x10).
>>> +# Also note that the .debug_str offsets are also offset by 0x10, as
>>> directed by
>>> +# the linker script.
>>> +# CHECK: 0x00000010: "T"
>>> +# CHECK: 0x00000014: "S"
>>> +
>>> +.text
>>> +a_sym:
>>> +nop
>>> +
>>> +.section .debug_str,"", at progbits
>>> +.long a_sym
>>> +.long a_debug_sym
>>> +
>>> +.section .debug_info,"", at progbits
>>> +a_debug_sym:
>>> +.long 0x88776655
>>>
>>>
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