[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.

0001-fs-Prevent-symlink-destroying.patch

+From 074d8425af1fe39b7315962af5a48585ac600d0e Mon Sep 17 00:00:00 2001
+From: Dmitry Chestnykh <[e-mail]>
+Date: Fri, 6 Jan 2023 20:42:15 +0300
+Subject: [PATCH] [fs] Prevent symlink destroying.
+
+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
+---
+ llvm/lib/Support/raw_ostream.cpp              | 19 ++++++++++++++++---
+ .../tools/llvm-objcopy/keep-dso-symlink.test  |  7 +++++++
+ 2 files changed, 23 insertions(+), 3 deletions(-)
+ create mode 100644 llvm/test/tools/llvm-objcopy/keep-dso-symlink.test
+
+diff --git a/llvm/lib/Support/raw_ostream.cpp b/llvm/lib/Support/raw_ostream.cpp
+index 92b15f14c62f..8cc979b33904 100644
+--- a/llvm/lib/Support/raw_ostream.cpp
++++ b/llvm/lib/Support/raw_ostream.cpp
+@@ -982,17 +982,30 @@ void buffer_ostream::anchor() {}
+ 
+ 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)
+diff --git a/llvm/test/tools/llvm-objcopy/keep-dso-symlink.test b/llvm/test/tools/llvm-objcopy/keep-dso-symlink.test
+new file mode 100644
+index 000000000000..2b854cedbb00
+--- /dev/null
++++ b/llvm/test/tools/llvm-objcopy/keep-dso-symlink.test
+@@ -0,0 +1,7 @@
++# 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
+-- 
+2.34.1
+

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

+# REQUIRES: shell
+# 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