[PATCH] D75518: [MC][ELF] Allow section directives with empty flags

[Ilie Halip via Phabricator via llvm-commits]

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Initially reported here: https://github.com/ClangBuiltLinux/linux/issues/913. I didn't create an entry in bugs.llvm.org because I think the change is very trivial.

GNU as allows empty flags in assembler section directives. LLVM on the other hand doesn't, if the section already exists with different flags. An example:

  $ cat vdso-note.s
  .pushsection .note.Linux, "a", at note ;
   .long 0x1234 ;
  .popsection ;
  
  .pushsection .note.Linux, "", at note ;
   .long 0x5678 ;
  .popsection ;
  
  $ as -o vdso-note.o vdso-note.s && objdump -D vdso-note.o
  [...]
  0000000000000000 <.note.Linux>:
     0:	34 12                	xor    $0x12,%al
     2:	00 00                	add    %al,(%rax)
     4:	78 56                	js     0x5c
  	...
  
  $ clang-11 -cc1as -triple x86_64-pc-linux-gnu -o vdso-note.o vdso-note.s
  vdso-note.s:5:1: error: changed section flags for .note.Linux, expected: 0x2
  .pushsection .note.Linux, "", at note ;
  ^

Just checking that Flags != 0 seems to do the trick.

The error message could be improved too (the 0x2 is a bit cryptic), I'll create a different review for that later.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D75518

Files:
  llvm/lib/MC/MCParser/ELFAsmParser.cpp
  llvm/test/MC/ELF/section-flags-empty.s


Index: llvm/test/MC/ELF/section-flags-empty.s
===================================================================
--- /dev/null
+++ llvm/test/MC/ELF/section-flags-empty.s
@@ -0,0 +1,25 @@
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t
+# RUN: llvm-readelf -S -x .foo -x .bar %t | FileCheck %s
+
+# CHECK-COUNT-1: .foo        {{.*}} AX
+# CHECK-COUNT-1: .bar        {{.*}} WAX
+
+# CHECK:      Hex dump of section '.foo':
+# CHECK-NEXT: 0x00000000 0103
+
+# CHECK:      Hex dump of section '.bar':
+# CHECK-NEXT: 0x00000000 0204
+
+.section .foo,"ax", at progbits
+.byte 1
+
+.pushsection .bar,"wax", at progbits
+.byte 2
+.popsection
+
+.section .foo,"", at progbits
+.byte 3
+
+.pushsection .bar,"", at progbits
+.byte 4
+.popsection
Index: llvm/lib/MC/MCParser/ELFAsmParser.cpp
===================================================================
--- llvm/lib/MC/MCParser/ELFAsmParser.cpp
+++ llvm/lib/MC/MCParser/ELFAsmParser.cpp
@@ -640,7 +640,7 @@
   if (Section->getType() != Type)
     Error(loc, "changed section type for " + SectionName + ", expected: 0x" +
                    utohexstr(Section->getType()));
-  if (Section->getFlags() != Flags)
+  if (Flags && Section->getFlags() != Flags)
     Error(loc, "changed section flags for " + SectionName + ", expected: 0x" +
                    utohexstr(Section->getFlags()));
   if (Section->getEntrySize() != Size)


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