overflow at realpath()

Bug: https://bugs.llvm.org/show_bug.cgi?id=38116

File: llvm-mirror/clang/blob/master/lib/Frontend/ModuleDependencyCollector.cpp#L108

i.e
if (!realpath(SrcPath.str().c_str(), CanonicalPath))

According to the documentation of realpath() the output buffer needs to be at least of size PATH_MAX specifying output buffers large enough to handle the maximum-size possible result from path manipulation functions. (In that instance, buf’s size comes from uv__fs_pathmax_size(). That function attempts to use pathconf(path, _PC_PATH_MAX) as noted in the realpath(3) docs)

But over here uv__fs_pathmax_size() nor pathconf(path, _PC_PATH_MAX) is used.

Passing an inadequately-sized output buffer to a path manipulation function can result in a buffer overflow. Such functions include realpath() readlink() PathAppend() and others.