[libcxx-commits] [libcxx] cdc60a3 - [libcxx] Implement the read_symlink function for windows

[Martin Storsjö via libcxx-commits]

Author: Martin Storsjö
Date: 2021-02-03T09:23:58+02:00
New Revision: cdc60a3b9aa523b49329a7a5e4c1774d3b9e3db9

URL: https://github.com/llvm/llvm-project/commit/cdc60a3b9aa523b49329a7a5e4c1774d3b9e3db9
DIFF: https://github.com/llvm/llvm-project/commit/cdc60a3b9aa523b49329a7a5e4c1774d3b9e3db9.diff

LOG: [libcxx] Implement the read_symlink function for windows

Differential Revision: https://reviews.llvm.org/D91172

Added: 
    

Modified: 
    libcxx/src/filesystem/operations.cpp
    libcxx/src/filesystem/posix_compat.h

Removed: 
    


################################################################################
diff  --git a/libcxx/src/filesystem/operations.cpp b/libcxx/src/filesystem/operations.cpp
index 47cd5e23c092..a5463e48f4d7 100644
--- a/libcxx/src/filesystem/operations.cpp
+++ b/libcxx/src/filesystem/operations.cpp
@@ -1227,21 +1227,25 @@ void __permissions(const path& p, perms prms, perm_options opts,
 path __read_symlink(const path& p, error_code* ec) {
   ErrorHandler<path> err("read_symlink", ec, &p);
 
-#ifdef PATH_MAX
+#if defined(PATH_MAX) || defined(MAX_SYMLINK_SIZE)
   struct NullDeleter { void operator()(void*) const {} };
+#ifdef MAX_SYMLINK_SIZE
+  const size_t size = MAX_SYMLINK_SIZE + 1;
+#else
   const size_t size = PATH_MAX + 1;
-  char stack_buff[size];
-  auto buff = std::unique_ptr<char[], NullDeleter>(stack_buff);
+#endif
+  path::value_type stack_buff[size];
+  auto buff = std::unique_ptr<path::value_type[], NullDeleter>(stack_buff);
 #else
   StatT sb;
   if (detail::lstat(p.c_str(), &sb) == -1) {
     return err.report(capture_errno());
   }
   const size_t size = sb.st_size + 1;
-  auto buff = unique_ptr<char[]>(new char[size]);
+  auto buff = unique_ptr<path::value_type[]>(new path::value_type[size]);
 #endif
-  ::ssize_t ret;
-  if ((ret = ::readlink(p.c_str(), buff.get(), size)) == -1)
+  detail::SSizeT ret;
+  if ((ret = detail::readlink(p.c_str(), buff.get(), size)) == -1)
     return err.report(capture_errno());
   _LIBCPP_ASSERT(ret > 0, "TODO");
   if (static_cast<size_t>(ret) >= size)

diff  --git a/libcxx/src/filesystem/posix_compat.h b/libcxx/src/filesystem/posix_compat.h
index af4a3691db33..8062bd65ce00 100644
--- a/libcxx/src/filesystem/posix_compat.h
+++ b/libcxx/src/filesystem/posix_compat.h
@@ -32,6 +32,7 @@
 # define NOMINMAX
 # include <windows.h>
 # include <io.h>
+# include <winioctl.h>
 #else
 # include <unistd.h>
 # include <sys/stat.h>
@@ -39,6 +40,36 @@
 #endif
 #include <time.h>
 
+#if defined(_LIBCPP_WIN32API)
+// This struct isn't defined in the normal Windows SDK, but only in the
+// Windows Driver Kit.
+struct LIBCPP_REPARSE_DATA_BUFFER {
+  unsigned long  ReparseTag;
+  unsigned short ReparseDataLength;
+  unsigned short Reserved;
+  union {
+    struct {
+      unsigned short SubstituteNameOffset;
+      unsigned short SubstituteNameLength;
+      unsigned short PrintNameOffset;
+      unsigned short PrintNameLength;
+      unsigned long  Flags;
+      wchar_t        PathBuffer[1];
+    } SymbolicLinkReparseBuffer;
+    struct {
+      unsigned short SubstituteNameOffset;
+      unsigned short SubstituteNameLength;
+      unsigned short PrintNameOffset;
+      unsigned short PrintNameLength;
+      wchar_t        PathBuffer[1];
+    } MountPointReparseBuffer;
+    struct {
+      unsigned char DataBuffer[1];
+    } GenericReparseBuffer;
+  };
+};
+#endif
+
 _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM
 
 namespace detail {
@@ -400,6 +431,52 @@ int fchmod(int fd, int perms) {
   return fchmod_handle(h, perms);
 }
 
+#define MAX_SYMLINK_SIZE MAXIMUM_REPARSE_DATA_BUFFER_SIZE
+using SSizeT = ::int64_t;
+
+SSizeT readlink(const wchar_t *path, wchar_t *ret_buf, size_t bufsize) {
+  uint8_t buf[MAXIMUM_REPARSE_DATA_BUFFER_SIZE];
+  detail::WinHandle h(path, FILE_READ_ATTRIBUTES, FILE_FLAG_OPEN_REPARSE_POINT);
+  if (!h)
+    return set_errno();
+  DWORD out;
+  if (!DeviceIoControl(h, FSCTL_GET_REPARSE_POINT, nullptr, 0, buf, sizeof(buf),
+                       &out, 0))
+    return set_errno();
+  const auto *reparse = reinterpret_cast<LIBCPP_REPARSE_DATA_BUFFER *>(buf);
+  size_t path_buf_offset = offsetof(LIBCPP_REPARSE_DATA_BUFFER,
+                                    SymbolicLinkReparseBuffer.PathBuffer[0]);
+  if (out < path_buf_offset) {
+    errno = EINVAL;
+    return -1;
+  }
+  if (reparse->ReparseTag != IO_REPARSE_TAG_SYMLINK) {
+    errno = EINVAL;
+    return -1;
+  }
+  const auto &symlink = reparse->SymbolicLinkReparseBuffer;
+  unsigned short name_offset, name_length;
+  if (symlink.PrintNameLength == 0) {
+    name_offset = symlink.SubstituteNameOffset;
+    name_length = symlink.SubstituteNameLength;
+  } else {
+    name_offset = symlink.PrintNameOffset;
+    name_length = symlink.PrintNameLength;
+  }
+  // name_offset/length are expressed in bytes, not in wchar_t
+  if (path_buf_offset + name_offset + name_length > out) {
+    errno = EINVAL;
+    return -1;
+  }
+  if (name_length / sizeof(wchar_t) > bufsize) {
+    errno = ENOMEM;
+    return -1;
+  }
+  memcpy(ret_buf, &symlink.PathBuffer[name_offset / sizeof(wchar_t)],
+         name_length);
+  return name_length / sizeof(wchar_t);
+}
+
 #else
 int symlink_file(const char *oldname, const char *newname) {
   return ::symlink(oldname, newname);
@@ -418,6 +495,7 @@ using ::link;
 using ::lstat;
 using ::mkdir;
 using ::open;
+using ::readlink;
 using ::realpath;
 using ::remove;
 using ::rename;
@@ -429,6 +507,7 @@ using ::truncate;
 
 using StatVFS = struct statvfs;
 using ModeT = ::mode_t;
+using SSizeT = ::ssize_t;
 
 #endif