[PATCH] D37736: [ELF] - Do not spread specific flags when emiting output sections.

[George Rimar via Phabricator via llvm-commits]

grimar updated this revision to Diff 118325.
grimar added a comment.

- Included summary comment from discussion thread.


https://reviews.llvm.org/D37736

Files:
  ELF/LinkerScript.cpp
  test/ELF/linkerscript/arm-exidx-order.s
  test/ELF/linkerscript/symbol-only-flags.s


Index: test/ELF/linkerscript/symbol-only-flags.s
===================================================================
--- test/ELF/linkerscript/symbol-only-flags.s
+++ test/ELF/linkerscript/symbol-only-flags.s
@@ -0,0 +1,20 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: echo "SECTIONS { . = SIZEOF_HEADERS; \
+# RUN:         .tbss : { *(.tbss) }         \
+# RUN:         .foo : { bar = .; } }" > %t.script
+# RUN: ld.lld -o %t --script %t.script %t.o
+# RUN: llvm-readobj -s %t | FileCheck %s
+
+## Check .foo does not get SHF_TLS flag.
+# CHECK:     Section {
+# CHECK:       Index:
+# CHECK:       Name: .foo
+# CHECK-NEXT:  Type: SHT_PROGBITS
+# CHECK-NEXT:  Flags [
+# CHECK-NEXT:    SHF_ALLOC
+# CHECK-NEXT:    SHF_WRITE
+# CHECK-NEXT:  ]
+
+.section .tbss,"awT", at nobits
+.quad 0
Index: test/ELF/linkerscript/arm-exidx-order.s
===================================================================
--- test/ELF/linkerscript/arm-exidx-order.s
+++ test/ELF/linkerscript/arm-exidx-order.s
@@ -0,0 +1,19 @@
+# REQUIRES: arm
+# RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t.o
+# RUN: echo "SECTIONS { . = SIZEOF_HEADERS;    \
+# RUN:         .ARM.exidx : { *(.ARM.exidx*) } \
+# RUN:         .foo : { _foo = 0; } }" > %t.script
+# RUN: ld.lld -T %t.script %t.o -shared -o %t.so
+# RUN: llvm-readobj -s %t.so | FileCheck %s
+
+# CHECK:      Section {
+# CHECK:        Index: 
+# CHECK:        Name: .foo
+# CHECK-NEXT:   Type: SHT_PROGBITS
+# CHECK-NEXT:   Flags [
+# CHECK-NEXT:     SHF_ALLOC
+# CHECK-NEXT:   ]
+
+.fnstart
+.cantunwind
+.fnend
Index: ELF/LinkerScript.cpp
===================================================================
--- ELF/LinkerScript.cpp
+++ ELF/LinkerScript.cpp
@@ -657,17 +657,36 @@
 
 void LinkerScript::adjustSectionsBeforeSorting() {
   // If the output section contains only symbol assignments, create a
-  // corresponding output section. The bfd linker seems to only create them if
-  // '.' is assigned to, but creating these section should not have any bad
-  // consequeces and gives us a section to put the symbol in.
+  // corresponding output section. The issue is what to do with linker script
+  // like ".foo : { symbol = 42; }". One option would be to convert it to
+  // "symbol = 42;". That is, move the symbol out of the empty section
+  // description. That seems to be what bfd does for this simple case. The
+  // problem is that this is not completely general. bfd will give up and
+  // create a dummy section too if there is a ". = . + 1" inside the section
+  // for example.
+  // Given that we want to create the section, we have to worry what impact
+  // it will have on the link. For example, if we just create a section with
+  // 0 for flags, it would change which PT_LOADs are created.
+  // We could remember that that particular section is dummy and ignore it in
+  // other parts of the linker, but unfortunately there are quite a few places
+  // that would need to change:
+  //   * The program header creation.
+  //   * The orphan section placement.
+  //   * The address assignment.
+  // The other option is to pick flags that minimize the impact the section
+  // will have on the rest of the linker. That is why we copy the flags from
+  // the previous sections. That was simplistic, as only a few flags are needed
+  // to keep the impact low, and copying more esoteric flags like SHF_LINK_ORDER
+  // have the opposite effect: it causes undesirable effects on the rest of the
+  // link.
   uint64_t Flags = SHF_ALLOC;
 
   for (BaseCommand * Cmd : Opt.Commands) {
     auto *Sec = dyn_cast<OutputSection>(Cmd);
     if (!Sec)
       continue;
     if (Sec->Live) {
-      Flags = Sec->Flags;
+      Flags = Sec->Flags & (SHF_ALLOC | SHF_WRITE | SHF_EXECINSTR);
       continue;
     }
 


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