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

[ruiu]

ELF/Writer.cpp

300–302

It sounds like a compiler bug if it emits .tbss or .tdata without the writable bit. Do we actually care about this case?

303

(AFlags & SHF_TLS) && (AFlags & SHF_ALLOC)

[Bigcheese]

ELF/Writer.cpp

300–302

There's assembly. It also makes layout a bit simpler, as you can assume all TLS sections are together, and there's no good reason to reject it.

[ruiu]

ELF/Writer.cpp

300–302

That's still a user code bug, no? I'm not convinced unless it is very prevalent issue that we had to take care of this specially.

[• rafael]

Also please remove the "don't care" sections from the test:

.section v,"T",@nobits
.section u,"T"
section h,"aT",@nobits
.section f,"aT"

We only really care how we sort tls sections that are aw

ELF/Writer.cpp

327

Move this past the comment

// If we got here we know that both A and B are in the same PT_LOAD.

[• rafael]

In summary, this is what I think we should do.

• t.diff1 KBDownload

ELF/Writer.cpp

 template <class ELFT>
 static bool compareOutputSections(OutputSectionBase<ELFT::Is64Bits> *A,
                                   OutputSectionBase<ELFT::Is64Bits> *B) {
   typedef typename ELFFile<ELFT>::uintX_t uintX_t;
 
   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;
   bool BIsAlloc = BFlags & SHF_ALLOC;
   if (AIsAlloc != BIsAlloc)
     return AIsAlloc;
 
   // We don't have any special requirements for the relative order of
   // two non allocatable sections.
   if (!AIsAlloc)
     return false;
 
   // We want the read only sections first so that they go in the PT_LOAD
   // covering the program headers at the start of the file.
   bool AIsWritable = AFlags & SHF_WRITE;
   bool BIsWritable = BFlags & SHF_WRITE;
   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;
   bool BIsExec = BFlags & SHF_EXECINSTR;
   if (AIsExec != BIsExec)
     return BIsExec;
 
   // If we got here we know that both A and B are in the same PT_LOAD.

test/elf2/section-layout.s

 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 # RUN: ld.lld2 %t -o %tout
 # RUN: llvm-readobj -sections %tout | FileCheck %s
 # REQUIRES: x86
 
 # Check that sections are laid out in the correct order.
 
 .global _start
 .text
 _start:
 
-.section h,""
-.section g,"",@nobits
-.section f,"aw",@nobits
-.section e,"aw"
+.section v,"T",@nobits
+.section u,"T"
+.section t,"x",@nobits
+.section s,"x"
+.section r,"w",@nobits
+.section q,"w"
+.section p,"wx",@nobits
+.section o,"wx"
+.section n,"",@nobits
+.section m,""
+
+.section l,"awx",@nobits
+.section k,"awx"
+.section j,"aw",@nobits
+.section i,"aw"
+.section h,"aT",@nobits
+.section g,"awT",@nobits
+.section f,"aT"
+.section e,"awT"
 .section d,"ax",@nobits
 .section c,"ax"
 .section b,"a",@nobits
 .section a,"a"
 
 // CHECK: Name: a
 // 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