[lld] r321986 - [ELF] Compress debug sections after assignAddresses and support custom layout
Hans Wennborg via llvm-commits
llvm-commits at lists.llvm.org
Tue Jan 16 07:39:33 PST 2018
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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