lld: elf: discard more specific .gnu.linkonce section

Summary:
lld discards .gnu.linonce.* sections work around a bug in glibc.
https://sourceware.org/bugzilla/show_bug.cgi?id=20543

Unfortunately, the Linux kernel uses a section named
.gnu.linkonce.this_module to store infomation about kernel modules. The
kernel reads data from this section when loading kernel modules, and
errors if it fails to find this section. The current behavior of lld
discards this section when kernel modules are linked, so kernel modules
linked with lld are unloadable by the linux kernel.

The Linux kernel should use a comdat section instead of .gnu.linkonce.
The minimum version of binutils supported by the kernel supports comdat
sections. The kernel is also not relying on the old linkonce behavior;
it seems to have chosen a name that contains a deprecated GNU feature.

Changing the section name now in the kernel would require all kernel
modules to be recompiled to make use of the new section name. Instead,
rather than discarding .gnu.linkonce.*, let's discard the more specific
section name to continue working around the glibc issue while supporting
linking Linux kernel modules.

Link: https://github.com/ClangBuiltLinux/linux/issues/329

Reviewers: pcc, espindola

Reviewed By: pcc

Subscribers: nathanchance, emaste, arichardson, void, srhines

Differential Revision: https://reviews.llvm.org/D57294

llvm-svn: 352302

lld/ELF/InputFiles.cpp

@@ -733,7 +733,8 @@ InputSectionBase *ObjFile<ELFT>::createInputSection(const Elf_Shdr &Sec) {
   // sections. Drop those sections to avoid duplicate symbol errors.
   // FIXME: This is glibc PR20543, we should remove this hack once that has been
   // fixed for a while.
-  if (Name.startswith(".gnu.linkonce."))
+  if (Name == ".gnu.linkonce.t.__x86.get_pc_thunk.bx" ||
+      Name == ".gnu.linkonce.t.__i686.get_pc_thunk.bx")
     return &InputSection::Discarded;
 
   // If we are creating a new .build-id section, strip existing .build-id

lld/test/ELF/comdat-linkonce.s

@@ -4,7 +4,12 @@
 // RUN: ld.lld -shared %t.o %t2.o -o %t
 // RUN: ld.lld -shared %t2.o %t.o -o %t
 
-.section .gnu.linkonce.t.zed
+.section .gnu.linkonce.t.__x86.get_pc_thunk.bx
 .globl abc
 abc:
 nop
+
+.section .gnu.linkonce.t.__i686.get_pc_thunk.bx
+.globl def
+def:
+nop

lld/test/ELF/no-discard-this_module.s

@@ -0,0 +1,41 @@
+// REQUIRES: x86
+// RUN: llvm-mc -filetype=obj -triple=x86_64-linux-gnu -save-temp-labels %s -o %t
+// RUN: ld.lld %t -o %t2
+// RUN: llvm-readobj -s -sd -t %t2 | FileCheck %s
+
+.global _start
+_start:
+
+// This section and symbol is used by Linux kernel modules. Ensure it's not
+// accidentally discarded.
+.section .gnu.linkonce.this_module:
+__this_module:
+.byte 0x00
+
+// CHECK: Section {
+// CHECK:    Index:
+// CHECK:    Name: .gnu.linkonce.this_module
+// CHECK-NEXT:    Type: SHT_PROGBITS
+// CHECK-NEXT:    Flags [
+// CHECK-NEXT:    ]
+// CHECK-NEXT:    Address:
+// CHECK-NEXT:    Offset:
+// CHECK-NEXT:    Size:
+// CHECK-NEXT:    Link:
+// CHECK-NEXT:    Info:
+// CHECK-NEXT:    AddressAlignment:
+// CHECK-NEXT:    EntrySize:
+// CHECK-NEXT:    SectionData (
+// CHECK-NEXT:      0000: 00                                   |.|
+// CHECK-NEXT:    )
+// CHECK-NEXT:  }
+
+// CHECK:  Symbol {
+// CHECK:    Name: __this_module
+// CHECK-NEXT:    Value:
+// CHECK-NEXT:    Size:
+// CHECK-NEXT:    Binding: Local
+// CHECK-NEXT:    Type: None
+// CHECK-NEXT:    Other:
+// CHECK-NEXT:    Section: .gnu.linkonce.this_module:
+// CHECK-NEXT:  }