Defer CWD in MCContext lookup as late as possible.

[resistor]

Summary

Currently MCContext tries to lookup CWD on construction. This causes sandboxing violations when using LLVM in a daemon without filesystem access. The solution is defer CWD lookup until absolutely necessary.

[chandlerc]

The goal here makes perfect sense, but I wonder if this is the right fix... It seems almost too surgical, and makes it really easy to change MC in a way that yet again violates things.

I feel like the MCContext shouldn't be looking at the filesystem *at all*, and it should be the client that sets up the MCContext that provides a compilation directory, potentially via the filesystem query, where appropriate. Does that make sense here?

[resistor]

That makes sense to me in principle, but I know nothing of why CWD is needed for (some form of?) DWARF generation at all.

[chandlerc]

It's used to resolve relative paths encoded elsewhere in dwarf. I think all
the clients building an mccontext either have an analog (clang) or can
reasonably directly query their cwd (llc and friends).

[echristo]

Agreed, this can be hoisted out fairly easily.

[aprantl]

In D17065#348837, @chandlerc wrote:

It's used to resolve relative paths encoded elsewhere in dwarf. I think all
the clients building an mccontext either have an analog (clang) or can
reasonably directly query their cwd (llc and friends).

So during an LTO build you will get the CWD of wherever the linker was invoked?
Shouldn't this get passed in by the frontend as metadata?

[chandlerc]

Marking this as needing changes along the lines of what I suggested or a further change along the lines of what Adrian suggested.

include/llvm/MC/MCContext.h

 #define LLVM_MC_MCCONTEXT_H
 
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"

     bool SecureLogUsed;
 
     /// The compilation directory to use for DW_AT_comp_dir.
-    SmallString<128> CompilationDir;
+    mutable Optional<SmallString<128>> CompilationDir;
 
     /// The main file name if passed in explicitly.
     std::string MainFileName;

     /// compilation directory and have it be something other than the current
     /// working directory.
     /// Returns an empty string if the current directory cannot be determined.
-    StringRef getCompilationDir() const { return CompilationDir; }
+    StringRef getCompilationDir() const;
 
     /// \brief Set the compilation directory for DW_AT_comp_dir
     /// Override the default (CWD) compilation directory.
-    void setCompilationDir(StringRef S) { CompilationDir = S.str(); }
+    void setCompilationDir(StringRef S) {
+      CompilationDir = SmallString<128>(S.str());
+    }
 
     /// \brief Get the main file name for use in error messages and debug
     /// info. This can be set to ensure we've got the correct file name

lib/MC/MCContext.cpp

       GenDwarfForAssembly(false), GenDwarfFileNumber(0), DwarfVersion(4),
       AllowTemporaryLabels(true), DwarfCompileUnitID(0),
       AutoReset(DoAutoReset) {
-
-  std::error_code EC = llvm::sys::fs::current_path(CompilationDir);
-  if (EC)
-    CompilationDir.clear();
-
   SecureLogFile = getenv("AS_SECURE_LOG_FILE");
   SecureLog = nullptr;
   SecureLogUsed = false;

   delete (raw_ostream *)SecureLog;
 }
 
+StringRef MCContext::getCompilationDir() const {
+  if (!CompilationDir) {
+    SmallString<128> Cwd;
+    std::error_code EC = llvm::sys::fs::current_path(Cwd);
+    if (EC)
+      Cwd.clear();
+    CompilationDir = Cwd;
+  }
+  return *CompilationDir;
+}
+
 //===----------------------------------------------------------------------===//
 // Module Lifetime Management
 //===----------------------------------------------------------------------===//

   SectionSymbols.clear();
   Allocator.Reset();
   Instances.clear();
-  CompilationDir.clear();
+  CompilationDir.reset();
   MainFileName.clear();
   MCDwarfLineTablesCUMap.clear();
   SectionsForRanges.clear();