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[COFF] Don't write long section names for sections that will be mapped at runtime 

Sections that will be mapped at runtime will only have the short
section name available, since the string table it points into isn't
mapped. Therefore prefer truncating those names over writing a
long name that is unavailable at runtime.

This allows libunwind to find the .eh_frame section at runtime even
if the module was built with debug info enabled.

Differential Revision: https://reviews.llvm.org/D40025

llvm-svn: 318391
This commit is contained in:
Martin Storsjo 2017-11-16 12:06:42 +00:00
parent 43fa5911a1
commit 46304e03ec
2 changed files with 27 additions and 2 deletions
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10
lld/COFF/Writer.cpp
View File

@ -211,7 +211,8 @@ void OutputSection::writeHeaderTo(uint8_t *Buf) {
// If name is too long, write offset into the string table as a name.
sprintf(Hdr->Name, "/%d", StringTableOff);
} else {
assert(!Config->Debug || Name.size() <= COFF::NameSize);
assert(!Config->Debug || Name.size() <= COFF::NameSize ||
(Hdr->Characteristics & IMAGE_SCN_MEM_DISCARDABLE) == 0);
strncpy(Hdr->Name, Name.data(),
std::min(Name.size(), (size_t)COFF::NameSize));
}
@ -541,6 +542,13 @@ void Writer::createSymbolAndStringTable() {
StringRef Name = Sec->getName();
if (Name.size() <= COFF::NameSize)
continue;
// If a section isn't discardable (i.e. will be mapped at runtime),
// prefer a truncated section name over a long section name in
// the string table that is unavailable at runtime. This is different from
// what link.exe does, but finding ".eh_fram" instead of "/4" is useful
// to libunwind.
if ((Sec->getPermissions() & IMAGE_SCN_MEM_DISCARDABLE) == 0)
continue;
Sec->setStringTableOff(addEntryToStringTable(Name));
}

19
lld/test/COFF/long-section-name.test
View File

@ -2,6 +2,7 @@
# RUN: lld-link /debug /out:%t.exe /entry:main %t.obj
# RUN: llvm-readobj -sections %t.exe | FileCheck %s
# CHECK: Name: .eh_fram (
# CHECK: Name: .data_long_section_name
# CHECK: Name: .text_long_section_name
@ -11,10 +12,14 @@ header:
Characteristics: [ ]
sections:
- Name: .text_long_section_name
Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_DISCARDABLE ]
Alignment: 4
SectionData: B82A000000C3
- Name: .data_long_section_name
Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE, IMAGE_SCN_MEM_DISCARDABLE ]
Alignment: 4
SectionData: "00"
- Name: .eh_frame
Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
Alignment: 4
SectionData: "00"
@ -49,6 +54,18 @@ symbols:
NumberOfLinenumbers: 0
CheckSum: 0
Number: 0
- Name: .eh_frame
Value: 0
SectionNumber: 3
SimpleType: IMAGE_SYM_TYPE_NULL
ComplexType: IMAGE_SYM_DTYPE_NULL
StorageClass: IMAGE_SYM_CLASS_STATIC
SectionDefinition:
Length: 0
NumberOfRelocations: 0
NumberOfLinenumbers: 0
CheckSum: 0
Number: 0
- Name: main
Value: 0
SectionNumber: 1