[PATCH] D27516: [ELF] - Place ".text" to be first section if -Ttext is used.

[George Rimar via Phabricator via llvm-commits]

grimar created this revision.
grimar added reviewers: ruiu, rafael.
grimar added subscribers: llvm-commits, grimar, evgeny777.

Some loaders, for example one from PR31295 use -N -Ttext combination.
Previously our output was in next order:

  .rodata
  .text
  ...

That caused an issue. Imagine .rodata was assigned to 0x10000 and then -Ttext sets .text below that.
That could cause sections addresses intersections.
Both GNU linkers able handles that correctly.

gold just places .text to be first section. In that case
we can start assigning VA from -Ttext value if was specified and do that in a forward loop, just like we 
already do in void Writer<ELFT>::assignAddresses().
That is what this patch implements.


https://reviews.llvm.org/D27516

Files:
  ELF/Writer.cpp
  test/ELF/ttext.s


Index: test/ELF/ttext.s
===================================================================
--- test/ELF/ttext.s
+++ test/ELF/ttext.s
@@ -0,0 +1,27 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+
+# RUN: ld.lld -N %t -o %t1
+# RUN: llvm-objdump -section-headers %t1 | FileCheck %s
+# CHECK:      Sections:
+# CHECK-NEXT: Idx Name         Size      Address         Type
+# CHECK-NEXT:   0              00000000 0000000000000000
+# CHECK-NEXT:   1 .rodata      00000008 00000000002000e8 DATA
+# CHECK-NEXT:   2 .text        00000001 00000000002000f0 TEXT DATA
+
+## Check that sections does not intersect if -Ttext used.
+# RUN: ld.lld -N -Ttext 0x2000e8 %t -o %t1
+# RUN: llvm-objdump -section-headers %t1 | FileCheck %s --check-prefix=TEXT
+# TEXT:      Sections:
+# TEXT-NEXT: Idx Name       Size      Address         Type
+# TEXT-NEXT:   0            00000000 0000000000000000
+# TEXT-NEXT:   1 .text      00000001 00000000002000e8 TEXT DATA
+# TEXT-NEXT:   2 .rodata    00000008 00000000002000e9 DATA
+
+.text
+.globl _start
+_start:
+ nop
+
+.section .rodata,"a"
+ .quad 1
Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -482,7 +482,18 @@
 template <class ELFT>
 static bool compareSectionsNonScript(const OutputSectionBase *A,
                                      const OutputSectionBase *B) {
-  // Put .interp first because some loaders want to see that section
+  // Some loaders use -Ttext option to set .text section address.
+  // In that case we place it at the begining to be able to
+  // assign VA in a simple forward loop later. That is consistent
+  // with what GNU gold do.
+  if (Config->SectionStartMap.count(".text")) {
+    bool AIsText = A->getName() == ".text";
+    bool BIsText = B->getName() == ".text";
+    if (AIsText != BIsText)
+      return AIsText;
+  }
+
+  // Put .interp early because some loaders want to see that section
   // on the first page of the executable file when loaded into memory.
   bool AIsInterp = A->getName() == ".interp";
   bool BIsInterp = B->getName() == ".interp";


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