[PATCH] D13763: [lld][elf2] Sort allocated TLS sections at the start of the R/W PT_LOAD.

[Michael Spencer via llvm-commits]

Bigcheese created this revision.
Bigcheese added reviewers: rafael, ruiu.
Bigcheese added a subscriber: llvm-commits.

http://reviews.llvm.org/D13763

Files:
  ELF/Writer.cpp
  test/elf2/section-layout.s

Index: test/elf2/section-layout.s
===================================================================
--- test/elf2/section-layout.s
+++ test/elf2/section-layout.s
@@ -9,10 +9,25 @@
 .text
 _start:
 
-.section h,""
-.section g,"", at nobits
-.section f,"aw", at nobits
-.section e,"aw"
+.section v,"T", at nobits
+.section u,"T"
+.section t,"x", at nobits
+.section s,"x"
+.section r,"w", at nobits
+.section q,"w"
+.section p,"wx", at nobits
+.section o,"wx"
+.section n,"", at nobits
+.section m,""
+
+.section l,"awx", at nobits
+.section k,"awx"
+.section j,"aw", at nobits
+.section i,"aw"
+.section h,"aT", at nobits
+.section g,"awT", at nobits
+.section f,"aT"
+.section e,"awT"
 .section d,"ax", at nobits
 .section c,"ax"
 .section b,"a", at nobits
@@ -22,7 +37,26 @@
 // CHECK: Name: b
 // CHECK: Name: c
 // CHECK: Name: d
-// CHECK: Name: e
+
+// TLS sections are only sorted on NOBITS.
 // CHECK: Name: f
+// CHECK: Name: e
 // CHECK: Name: h
 // CHECK: Name: g
+
+// CHECK: Name: i
+// CHECK: Name: j
+// CHECK: Name: k
+// CHECK: Name: l
+
+// Non allocated sections are in input order.
+// CHECK: Name: v
+// CHECK: Name: u
+// CHECK: Name: t
+// CHECK: Name: s
+// CHECK: Name: r
+// CHECK: Name: q
+// CHECK: Name: p
+// CHECK: Name: o
+// CHECK: Name: n
+// CHECK: Name: m
Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -297,6 +297,14 @@
   uintX_t AFlags = A->getFlags();
   uintX_t BFlags = B->getFlags();
 
+  // The TLS spec says that the linker can assume that TLS sections are R/W. Do
+  // this so we can generate a minimal PT_TLS Phdr. Non-allocated TLS sections
+  // are ignored because they shouldn't be in the middle of allocated sections.
+  if (AFlags & SHF_TLS && AFlags & SHF_ALLOC)
+    AFlags |= SHF_WRITE;
+  if (BFlags & SHF_TLS && BFlags & SHF_ALLOC)
+    BFlags |= SHF_WRITE;
+
   // Allocatable sections go first to reduce the total PT_LOAD size and
   // so debug info doesn't change addresses in actual code.
   bool AIsAlloc = AFlags & SHF_ALLOC;
@@ -316,6 +324,15 @@
   if (AIsWritable != BIsWritable)
     return BIsWritable;
 
+  // The TLS initialization block needs to be a single contiguous block in a R/W
+  // PT_LOAD, so stick TLS sections directly before R/W sections. The TLS NOBITS
+  // sections are placed here as they don't take up virtual address space in the
+  // PT_LOAD.
+  bool AIsTLS = AFlags & SHF_TLS;
+  bool BIsTLS = BFlags & SHF_TLS;
+  if (AIsTLS != BIsTLS)
+    return AIsTLS;
+
   // For a corresponding reason, put non exec sections first (the program
   // header PT_LOAD is not executable).
   bool AIsExec = AFlags & SHF_EXECINSTR;


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