[llvm-objcopy] preserve file ownership when overwritten by root

As of binutils 2.36, GNU strip calls chown(2) for "sudo strip foo" and
"sudo strip foo -o foo", but no "sudo strip foo -o bar" or "sudo strip
foo -o ./foo". In other words, while "sudo strip foo -o bar" creates a
new file bar with root access, "sudo strip foo" will keep the owner and
group of foo unchanged. Currently llvm-objcopy and llvm-strip behave
differently, always changing the owner and gropu to root. The
discrepancy prevents Chrome OS from migrating to llvm-objcopy and
llvm-strip as they change file ownership and cause intended users/groups
to lose access when invoked by sudo with the following sequence
(recommended in man page of GNU strip).

1.<Link the executable as normal.>
1.<Copy "foo" to "foo.full">
1.<Run "strip --strip-debug foo">
1.<Run "objcopy --add-gnu-debuglink=foo.full foo">

This patch makes llvm-objcopy and llvm-strip follow GNU's behavior.

Link: crbug.com/1108880

1 files changed, 13 insertions, 3 deletions

diff --git a/llvm/lib/Support/FileOutputBuffer.cpp b/llvm/lib/Support/FileOutputBuffer.cpp
index 3342682..7b2a512 100644
--- a/llvm/lib/Support/FileOutputBuffer.cpp
+++ b/llvm/lib/Support/FileOutputBuffer.cpp
@@ -125,7 +125,8 @@ createInMemoryBuffer(StringRef Path, size_t Size, unsigned Mode) {
 }
 
 static Expected<std::unique_ptr<FileOutputBuffer>>
-createOnDiskBuffer(StringRef Path, size_t Size, unsigned Mode) {
+createOnDiskBuffer(StringRef Path, size_t Size, unsigned Mode,
+                   bool KeepOwnership, unsigned UserID, unsigned GroupID) {
   Expected<fs::TempFile> FileOrErr =
       fs::TempFile::create(Path + ".tmp%%%%%%%", Mode);
   if (!FileOrErr)
@@ -133,6 +134,13 @@ createOnDiskBuffer(StringRef Path, size_t Size, unsigned Mode) {
   fs::TempFile File = std::move(*FileOrErr);
 
 #ifndef _WIN32
+  // Try to preserve file ownership if requested.
+  if (KeepOwnership) {
+    fs::file_status Stat;
+    if (!fs::status(File.FD, Stat) && Stat.getUser() == 0)
+      fs::changeFileOwnership(File.FD, UserID, GroupID);
+  }
+
   // On Windows, CreateFileMapping (the mmap function on Windows)
   // automatically extends the underlying file. We don't need to
   // extend the file beforehand. _chsize (ftruncate on Windows) is
@@ -163,7 +171,8 @@ createOnDiskBuffer(StringRef Path, size_t Size, unsigned Mode) {
 
 // Create an instance of FileOutputBuffer.
 Expected<std::unique_ptr<FileOutputBuffer>>
-FileOutputBuffer::create(StringRef Path, size_t Size, unsigned Flags) {
+FileOutputBuffer::create(StringRef Path, size_t Size, unsigned Flags,
+                         unsigned UserID, unsigned GroupID) {
   // Handle "-" as stdout just like llvm::raw_ostream does.
   if (Path == "-")
     return createInMemoryBuffer("-", Size, /*Mode=*/0);
@@ -196,7 +205,8 @@ FileOutputBuffer::create(StringRef Path, size_t Size, unsigned Flags) {
     if (Flags & F_no_mmap)
       return createInMemoryBuffer(Path, Size, Mode);
     else
-      return createOnDiskBuffer(Path, Size, Mode);
+      return createOnDiskBuffer(Path, Size, Mode, Flags & F_keep_ownership,
+                                UserID, GroupID);
   default:
     return createInMemoryBuffer(Path, Size, Mode);
   }