[Support] Provide linux/magic.h fallback for older kernels

[mgorny]

Summary

The function for distinguishing local and remote files added in r295768
unconditionally uses linux/magic.h header to provide necessary
filesystem magic numbers. However, in kernel headers predating 2.6.18
the magic numbers are spread throughout multiple include files.
Furthermore, LLVM did not require kernel headers being installed so far.

To increase the portability across different versions of Linux kernel
and different Linux systems, add CMake header checks for linux/magic.h
and -- if it is missing -- the linux/nfs_fs.h and linux/smb.h headers
which contained the numbers previously.

Furthermore, since the numbers are static and the feature does not seem
critical enough to make LLVM require kernel headers at all, add fallback
constants for the case when none of the necessary headers is available.

[zturner]

Seems like a good fix, thanks for doing this.

[thakis]

llvm/trunk/lib/Support/Unix/Path.inc

79

The three HAVE_LINUX_ referenced here aren't listed in include/llvm/Config/config.h.cmake, so they never make it across from the cmake world to the C world -- they're never defined here. Since this also adds fallback code defining _SUPER_MAGIC things still work, but the cmake checks above are effectively dead code. Since we already have the fallbacks, maybe we should just delete the cmake checks instead of adding yet more stuff to config.h.cmake?

[labath]

llvm/trunk/lib/Support/Unix/Path.inc

79

sounds good to me. It's not like those numbers are ever going to change.

llvm/trunk/cmake/config-ix.cmake

if(APPLE)
   include(CheckCSourceCompiles)
   CHECK_C_SOURCE_COMPILES("
      static const char *__crashreporter_info__ = 0;
      asm(\".desc ___crashreporter_info__, 0x10\");
      int main() { return 0; }"
     HAVE_CRASHREPORTER_INFO)
 endif()
 
+if(${CMAKE_SYSTEM_NAME} STREQUAL "Linux")
+  check_include_file(linux/magic.h HAVE_LINUX_MAGIC_H)
+  if(NOT HAVE_LINUX_MAGIC_H)
+    # older kernels use split files
+    check_include_file(linux/nfs_fs.h HAVE_LINUX_NFS_FS_H)
+    check_include_file(linux/smb.h HAVE_LINUX_SMB_H)
+  endif()
+endif()
+
 # library checks
 if( NOT PURE_WINDOWS )
   check_library_exists(pthread pthread_create "" HAVE_LIBPTHREAD)
   if (HAVE_LIBPTHREAD)
     check_library_exists(pthread pthread_getspecific "" HAVE_PTHREAD_GETSPECIFIC)
     check_library_exists(pthread pthread_rwlock_init "" HAVE_PTHREAD_RWLOCK_INIT)
     check_library_exists(pthread pthread_mutex_lock "" HAVE_PTHREAD_MUTEX_LOCK)
   else()

llvm/trunk/lib/Support/Unix/Path.inc

 #define STATVFS statvfs
 #define FSTATVFS fstatvfs
 #define STATVFS_F_FRSIZE(vfs) vfs.f_frsize
 #else
 #if defined(__OpenBSD__) || defined(__FreeBSD__)
 #include <sys/param.h>
 #include <sys/mount.h>
 #elif defined(__linux__)
+#if defined(HAVE_LINUX_MAGIC_H)
 #include <linux/magic.h>
+#else
+#if defined(HAVE_LINUX_NFS_FS_H)
+#include <linux/nfs_fs.h>
+#endif
+#if defined(HAVE_LINUX_SMB_H)
+#include <linux/smb.h>
+#endif
+#endif
 #include <sys/vfs.h>
 #else
 #include <sys/mount.h>
 #endif
 #define STATVFS statfs
 #define FSTATVFS fstatfs
 #define STATVFS_F_FRSIZE(vfs) static_cast<uint64_t>(vfs.f_bsize)
 #endif
if (errno != ENOENT || !IgnoreNonExisting)
       return std::error_code(errno, std::generic_category());
   }
 
   return std::error_code();
 }
 
 static bool is_local_impl(struct STATVFS &Vfs) {
 #if defined(__linux__)
+#ifndef NFS_SUPER_MAGIC
+#define NFS_SUPER_MAGIC 0x6969
+#endif
+#ifndef SMB_SUPER_MAGIC
+#define SMB_SUPER_MAGIC 0x517B
+#endif
 #ifndef CIFS_MAGIC_NUMBER
 #define CIFS_MAGIC_NUMBER 0xFF534D42
 #endif
   switch ((uint32_t)Vfs.f_type) {
   case NFS_SUPER_MAGIC:
   case SMB_SUPER_MAGIC:
   case CIFS_MAGIC_NUMBER:
     return false;