[PATCH] D25014: [ELF] Change the way we compute file offsets

[Eugene Leviant via llvm-commits]

evgeny777 updated this revision to Diff 72896.
evgeny777 added a comment.

Added comment for 'FirstInPtLoad'


https://reviews.llvm.org/D25014

Files:
  ELF/OutputSections.h
  ELF/Writer.cpp
  test/ELF/linkerscript/outsections-addr.s

Index: test/ELF/linkerscript/outsections-addr.s
===================================================================
--- test/ELF/linkerscript/outsections-addr.s
+++ test/ELF/linkerscript/outsections-addr.s
@@ -81,7 +81,7 @@
 #CHECK:      SHF_ALLOC
 #CHECK:    ]
 #CHECK:    Address: 0x4008
-#CHECK:    Offset: 0x2008
+#CHECK:    Offset: 0x3008
 #CHECK:    Size: 8
 #CHECK:    Link: 0
 #CHECK:    Info: 0
@@ -96,7 +96,7 @@
 #CHECK:      SHF_ALLOC
 #CHECK:    ]
 #CHECK:    Address: 0x5010
-#CHECK:    Offset: 0x2010
+#CHECK:    Offset: 0x4010
 #CHECK:    Size: 8
 #CHECK:    Link: 0
 #CHECK:    Info: 0
Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -541,6 +541,8 @@
   if (!First)
     First = Sec;
   H.p_align = std::max<typename ELFT::uint>(H.p_align, Sec->getAlignment());
+  if (H.p_type == PT_LOAD && !Config->OFormatBinary)
+    Sec->FirstInPtLoad = First;
 }
 
 template <class ELFT>
@@ -1189,7 +1191,13 @@
   // and does not need any other offset adjusting.
   if (Config->Relocatable || !(Sec->getFlags() & SHF_ALLOC))
     return Off;
-  return alignTo(Off, Config->MaxPageSize, Sec->getVA());
+
+  OutputSectionBase<ELFT> *First = Sec->FirstInPtLoad;
+  // If two sections share the same PT_LOAD the file offset is calculated using
+  // this formula: Off2 = Off1 + (VA2 - VA1).
+  if (!First || Sec == First)
+    return alignTo(Off, Target->MaxPageSize, Sec->getVA());
+  return First->getFileOffset() + Sec->getVA() - First->getVA();
 }
 
 template <class ELFT, class uintX_t>
Index: ELF/OutputSections.h
===================================================================
--- ELF/OutputSections.h
+++ ELF/OutputSections.h
@@ -77,6 +77,7 @@
   void setVA(uintX_t VA) { Header.sh_addr = VA; }
   uintX_t getVA() const { return Header.sh_addr; }
   void setFileOffset(uintX_t Off) { Header.sh_offset = Off; }
+  uintX_t getFileOffset() { return Header.sh_offset; }
   void setSHName(unsigned Val) { Header.sh_name = Val; }
   void writeHeaderTo(Elf_Shdr *SHdr);
   StringRef getName() { return Name; }
@@ -108,6 +109,14 @@
   // Typically the first section of each PT_LOAD segment has this flag.
   bool PageAlign = false;
 
+  // Pointer to the first section in PT_LOAD segment, which this section
+  // also resides in. This field is used to correctly compute file offset
+  // of a section. When two sections share the same load segment, difference
+  // between their file offsets should equal to difference between their 
+  // virtual addresses. To compute some section offset we use the following
+  // formula: Off = Off_first + VA - VA_first.
+  OutputSectionBase<ELFT> *FirstInPtLoad = nullptr;
+
   virtual void finalize() {}
   virtual void finalizePieces() {}
   virtual void assignOffsets() {}


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