[lld] 4c3b74b - [LLD] [COFF] Order .debug_* sections at the end, to avoid leaving gaps if stripped

[Martin Storsjö via llvm-commits]

Author: Martin Storsjö
Date: 2022-03-03T10:08:51+02:00
New Revision: 4c3b74b7f5d6192d1731c88f1aeb396bc51d4949

URL: https://github.com/llvm/llvm-project/commit/4c3b74b7f5d6192d1731c88f1aeb396bc51d4949
DIFF: https://github.com/llvm/llvm-project/commit/4c3b74b7f5d6192d1731c88f1aeb396bc51d4949.diff

LOG: [LLD] [COFF] Order .debug_* sections at the end, to avoid leaving gaps if stripped

So far, we sort all discardable sections at the end, with only some
extra logic to make sure that the .reloc section is at the start
of that group of sections. But if there are other discardable
sections, other than .reloc, they must also be ordered before
.debug_* sections, to avoid leaving gaps if the executable is
stripped.

(Stripping executables doesn't remove all discardable sections,
only the ones named .debug_*).

Rust binaries seem to include a .rmeta section, which is marked
discardable. This fixes stripping such binaries if built with
dwarf debug info included.

This fixes issues observed in MSYS2 in
https://github.com/msys2/MINGW-packages/pull/10555.

Differential Revision: https://reviews.llvm.org/D120805

Added: 
    

Modified: 
    lld/COFF/Writer.cpp
    lld/test/COFF/sort-debug.test

Removed: 
    


################################################################################
diff  --git a/lld/COFF/Writer.cpp b/lld/COFF/Writer.cpp
index 94466e7adbd76..1b9a870d4630c 100644
--- a/lld/COFF/Writer.cpp
+++ b/lld/COFF/Writer.cpp
@@ -928,8 +928,14 @@ void Writer::createSections() {
     // Move DISCARDABLE (or non-memory-mapped) sections to the end of file
     // because the loader cannot handle holes. Stripping can remove other
     // discardable ones than .reloc, which is first of them (created early).
-    if (s->header.Characteristics & IMAGE_SCN_MEM_DISCARDABLE)
+    if (s->header.Characteristics & IMAGE_SCN_MEM_DISCARDABLE) {
+      // Move discardable sections named .debug_ to the end, after other
+      // discardable sections. Stripping only removes the sections named
+      // .debug_* - thus try to avoid leaving holes after stripping.
+      if (s->name.startswith(".debug_"))
+        return 3;
       return 2;
+    }
     // .rsrc should come at the end of the non-discardable sections because its
     // size may change by the Win32 UpdateResources() function, causing
     // subsequent sections to move (see https://crbug.com/827082).

diff  --git a/lld/test/COFF/sort-debug.test b/lld/test/COFF/sort-debug.test
index d8fd7ae605645..bbe2ecd0efd8d 100644
--- a/lld/test/COFF/sort-debug.test
+++ b/lld/test/COFF/sort-debug.test
@@ -10,6 +10,7 @@
 
 # CHECK: Name: .text
 # CHECK: Name: .reloc
+# CHECK: Name: .rmeta
 # CHECK: Name: .debug_abbrev
 # CHECK: Name: .debug_info
 # CHECK: Name: .debug_line
@@ -18,6 +19,7 @@
 
 # NODEBUG: Name: .text
 # NODEBUG: Name: .reloc
+# NODEBUG: Name: .rmeta
 # NODEBUG-NOT: Name: .debug_abbrev
 # NODEBUG-NOT: Name: .debug_info
 # NODEBUG-NOT: Name: .debug_line
@@ -183,6 +185,10 @@ sections:
       - VirtualAddress:  43
         SymbolName:      .text
         Type:            IMAGE_REL_I386_DIR32
+  - Name:            .rmeta
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_DISCARDABLE, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
+    SectionData:     00112233
 symbols:
   - Name:            .text
     Value:           0