[PATCH] D45788: Mitigate relocation overflow

[Han Shen via Phabricator via llvm-commits]

shenhan updated this revision to Diff 143133.
shenhan marked an inline comment as done.

Repository:
  rLLD LLVM Linker

https://reviews.llvm.org/D45788

Files:
  ELF/Writer.cpp


Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -681,8 +681,9 @@
   RF_NOT_INTERP = 1 << 17,
   RF_NOT_ALLOC = 1 << 16,
   RF_WRITE = 1 << 15,
-  RF_EXEC_WRITE = 1 << 13,
-  RF_EXEC = 1 << 12,
+  RF_EXEC_WRITE = 1 << 14,
+  RF_EXEC = 1 << 13,
+  RF_PROGBITS = 1 << 12,
   RF_NON_TLS_BSS = 1 << 11,
   RF_NON_TLS_BSS_RO = 1 << 10,
   RF_NOT_TLS = 1 << 9,
@@ -717,6 +718,12 @@
   if (!(Sec->Flags & SHF_ALLOC))
     return Rank | RF_NOT_ALLOC;
 
+  // Place sections with PROGBITS closer. We want to put progbits
+  // sections closer as an effort to prevent relocation overflow
+  // between these sections.
+  if (Sec->Type == SHT_PROGBITS && !isRelroSection(Sec))
+    Rank |= RF_PROGBITS;
+
   // Sort sections based on their access permission in the following
   // order: R, RX, RWX, RW.  This order is based on the following
   // considerations:
@@ -1674,17 +1681,20 @@
 // The linker is expected to define SECNAME_start and SECNAME_end
 // symbols for a few sections. This function defines them.
 template <class ELFT> void Writer<ELFT>::addStartEndSymbols() {
+  // These symbols resolve to the image base or ".text" if the section
+  // does not exist. Set symbol value to ".text" mitigates the possibilities
+  // that an relocation from .text section to these symbols overflows.
+  // A special value -1 indicates end of the section.
+  OutputSection *DefaultOutSec = findSection(".text");
+  if (!DefaultOutSec && Config->Pic)
+    DefaultOutSec = Out::ElfHeader;
   auto Define = [&](StringRef Start, StringRef End, OutputSection *OS) {
-    // These symbols resolve to the image base if the section does not exist.
-    // A special value -1 indicates end of the section.
     if (OS) {
       addOptionalRegular(Start, OS, 0);
       addOptionalRegular(End, OS, -1);
     } else {
-      if (Config->Pic)
-        OS = Out::ElfHeader;
-      addOptionalRegular(Start, OS, 0);
-      addOptionalRegular(End, OS, 0);
+      addOptionalRegular(Start, DefaultOutSec, 0);
+      addOptionalRegular(End, DefaultOutSec, 0);
     }
   };
 


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