[modules][Patch][BugzillaID#20467] Diagnose if -fmodules-cache-path is not writeable

[Aaron Ballman]

On Wed, Sep 17, 2014 at 4:04 AM, Vassil Vassilev
<vasil.georgiev.vasilev at cern.ch> wrote:
> Hi,
>   I am attaching a patch addressing llvm.org/bugs/show_bug.cgi?id=20467
> Vassil

> Index: lib/Frontend/CompilerInstance.cpp
> ===================================================================
> --- lib/Frontend/CompilerInstance.cpp (revision 217432)
> +++ lib/Frontend/CompilerInstance.cpp (working copy)
> @@ -339,6 +339,15 @@
>    if (!getHeaderSearchOpts().DisableModuleHash)
>      llvm::sys::path::append(SpecificModuleCache,
>                              getInvocation().getModuleHash());
> +
> +  // If the path is not writable we can't do anything but diagnose.
> +  if (llvm::sys::fs::exists(SpecificModuleCache.str()) &&
> +       !llvm::sys::fs::can_write(SpecificModuleCache.str())) {
> +    SourceLocation NoLoc;
> +    PP->getDiagnostics().Report(NoLoc, diag::err_module_cache_path_not_writable)
> +      << SpecificModuleCache;
> +  }
> +

This has TOCTOU bugs -- the path could be writable at the point of
your check, but at the point which actually attempts to create a file
on that path can still fail. I think the correct way to handle this is
to handle the failure at the point of using the path, not attempting
to fail early.

~Aaron