[MCObjectFileInfo] Add getPCSection() helper

[melver]

Summary

Add MCObjectFileInfo::getPCSection() helper to get a section for PCs
requested via MD_pcsections.

[dvyukov]

llvm/lib/MC/MCObjectFileInfo.cpp

1189

Do we actually plan to write to these sections? Does setting SHF_WRITE prevent sharing of pages across multiple processes? Of SHF_WRITE pages still mapped as shared/write-protected first and then subject to copy-on-write?

[melver]

llvm/lib/MC/MCObjectFileInfo.cpp

1189

This was needed for relocations (because linker needs section writable if it contains relocations).
glibc seems to map sections as MAP_COPY (AFAIK), which is MAP_PRIVATE on Linux, so it'll COW.

Now that we have relative relocations, maybe we don't need it anymore, but I'd like to reserve the option to modify the data in-place (whether we do it, or someone else finds a use for it).
Changing it again in future is too painful, and we're likely stuck with whatever we decide now. I don't see a huge downside to it being writable (there's no security concern here).

Cc @MaskRay

[dvyukov]

llvm/lib/MC/MCObjectFileInfo.cpp

1189

If it's shared across processes and COW, then it should be fine.

[MaskRay]

llvm/lib/MC/MCObjectFileInfo.cpp

1189

Note: Relocate relocations usually refer to R_*_RELATIVE. PC-relative static relocations are very different and should not be conflated.

[melver]

Squashed into "[AsmPrinter] Emit PCs into requested PCSections".

llvm/include/llvm/MC/MCObjectFileInfo.h

public:
   MCSection *getStackSizesSection(const MCSection &TextSec) const;
 
   MCSection *getBBAddrMapSection(const MCSection &TextSec) const;
 
   MCSection *getPseudoProbeSection(const MCSection *TextSec) const;
 
   MCSection *getPseudoProbeDescSection(StringRef FuncName) const;
 
+  MCSection *getPCSection(StringRef Name, const MCSection *TextSec) const;
+
   // ELF specific sections.
   MCSection *getDataRelROSection() const { return DataRelROSection; }
   const MCSection *getMergeableConst4Section() const {
     return MergeableConst4Section;
   }
   const MCSection *getMergeableConst8Section() const {
     return MergeableConst8Section;
   }

llvm/lib/MC/MCObjectFileInfo.cpp

if (Ctx->getTargetTriple().supportsCOMDAT() && !FuncName.empty()) {
       return Ctx->getELFSection(S->getName(), S->getType(), Flags,
                                 S->getEntrySize(),
                                 S->getName() + "_" + FuncName,
                                 /*IsComdat=*/true);
     }
   }
   return PseudoProbeDescSection;
 }
+
+MCSection *MCObjectFileInfo::getPCSection(StringRef Name,
+                                          const MCSection *TextSec) const {
+  if (Ctx->getObjectFileType() != MCContext::IsELF)
+    return nullptr;
+
+  // SHF_WRITE for relocations, and let user post-process data in-place.
+  unsigned Flags = ELF::SHF_WRITE | ELF::SHF_ALLOC | ELF::SHF_LINK_ORDER;
+
+  if (!TextSec)
+    TextSec = getTextSection();
+
+  StringRef GroupName;
+  const auto &ElfSec = static_cast<const MCSectionELF &>(*TextSec);
+  if (const MCSymbol *Group = ElfSec.getGroup()) {
+    GroupName = Group->getName();
+    Flags |= ELF::SHF_GROUP;
+  }
+  return Ctx->getELFSection(Name, ELF::SHT_PROGBITS, Flags, 0, GroupName, true,
+                            ElfSec.getUniqueID(),
+                            cast<MCSymbolELF>(TextSec->getBeginSymbol()));
+}