[fs] Prevent symlink destroying.

[chestnykh]

Summary

If we open symlink with llvm-strip then the current behaviour is to destroy it and create
new regular file. This behaviour is different from the behaviour of binutils strip.
The new behaviour is to modify the real file and to keep symlink.
PR#59848

[MaskRay]

I think we have made a decision (for crbug.com/1108880 c2a84771bb63947695ea50b89160c02b36fb634d @manojgupta) that we always create a new file instead of reusing the existing file.
It can be argued that our behavior has stronger security characteristics since it doesn't follow a symlink (see https://sourceware.org/bugzilla/show_bug.cgi?id=26945 ).
Perhaps we should just accept this incompatibility with GNU objcopy.

llvm/lib/Support/raw_ostream.cpp

 
 void buffer_unique_ostream::anchor() {}
 
-Error llvm::writeToOutput(StringRef OutputFileName,
+Error llvm::writeToOutput(StringRef OutputCandidateFileName,
                           std::function<Error(raw_ostream &)> Write) {
-  if (OutputFileName == "-")
+  if (OutputCandidateFileName == "-")
     return Write(outs());
 
-  if (OutputFileName == "/dev/null") {
+  if (OutputCandidateFileName == "/dev/null") {
     raw_null_ostream Out;
     return Write(Out);
   }
 
   unsigned Mode = sys::fs::all_read | sys::fs::all_write | sys::fs::all_exe;
+
+  llvm::SmallString<128> RealPathOfSymlink;
+  if (sys::fs::is_symlink_file(OutputCandidateFileName)) {
+    std::error_code EC =
+        sys::fs::real_path(OutputCandidateFileName, RealPathOfSymlink);
+    if (EC)
+      return errorCodeToError(EC);
+  }
+
+  Twine OutputFileName = RealPathOfSymlink.str().empty()
+                             ? OutputCandidateFileName
+                             : Twine(RealPathOfSymlink);
+
   Expected<sys::fs::TempFile> Temp =
       sys::fs::TempFile::create(OutputFileName + ".temp-stream-%%%%%%", Mode);
   if (!Temp)

llvm/test/tools/llvm-objcopy/keep-dso-symlink.test

+# RUN: touch empty.c
+# RUN: clang empty.c -shared -o empty.so.1
+# RUN: ln -sf empty.so.1 empty.so
+# RUN: llvm-strip --strip-unneeded empty.so
+# RUN: [ -L empty.so ] && echo "Still symlink" | FileCheck %s
+
+; CHECK: Still symlink