[PATCH] Fix searching of kernel_source_tree for kernel built with O option

[Hou Tao <houtao1 at huawei dot com>]

When generating kernel module for a systemtap script that uses trace-point
probe, if the vanilla kernel is built by using O=build_path option and
r=build_path option is passed to stap, stap will not be able to find
kernel_source_tree and will fail on pass-2.

Linux kernel will create a symlink named source to the source tree
for out-of-source build since (399b835be30e "kbuild: add a symlink
to the source for separate objdirs"), so fix the problem by checking
whether or not the symlink exists and using it as the kernel_source_tree.

Also using a new helper dir_exists() instead of file_exists() to
ensure the existence of the directory of source tree.

Signed-off-by: Hou Tao <houtao1@huawei.com>
---
 tapsets.cxx | 34 ++++++++++++++++++++++++++--------
 util.cxx    | 13 +++++++++++++
 util.h      |  1 +
 3 files changed, 40 insertions(+), 8 deletions(-)

diff --git a/tapsets.cxx b/tapsets.cxx
index ca94be4ce..8617796d8 100644
--- a/tapsets.cxx
+++ b/tapsets.cxx
@@ -12391,16 +12391,34 @@ tracepoint_builder::init_dw(systemtap_session& s)
     }
 
   // find kernel_source_tree from a source link, when different from build
-  if (s.kernel_source_tree == "" && endswith(s.kernel_build_tree, "/build"))
+  if (s.kernel_source_tree == "")
     {
-      string source_tree = s.kernel_build_tree;
-      source_tree.replace(source_tree.length() - 5, 5, "source");
-      if (file_exists(source_tree) &&
-          resolve_path(source_tree) != resolve_path(s.kernel_build_tree))
+      vector<string> source_trees;
+
+      // vendor kernel (e.g. Fedora): the source link is in the same dir
+      // as the build tree
+      if (endswith(s.kernel_build_tree, "/build"))
+        {
+          string source_tree = s.kernel_build_tree;
+          source_tree.replace(source_tree.length() - 5, 5, "source");
+          source_trees.push_back(source_tree);
+        }
+
+      // vanilla kernel: the source link is in the build tree
+      source_trees.push_back(s.kernel_build_tree + "/source");
+
+      for (unsigned i = 0; i < source_trees.size(); i++)
         {
-          if (s.verbose > 2)
-            clog << _F("Located kernel source tree at '%s'", source_tree.c_str()) << endl;
-          s.kernel_source_tree = source_tree;
+          string source_tree = source_trees[i];
+
+          if (dir_exists(source_tree) &&
+              resolve_path(source_tree) != resolve_path(s.kernel_build_tree))
+            {
+              if (s.verbose > 2)
+                clog << _F("Located kernel source tree at '%s'", source_tree.c_str()) << endl;
+              s.kernel_source_tree = source_tree;
+              break;
+            }
         }
     }
 
diff --git a/util.cxx b/util.cxx
index 1b1127995..b4f1e9c25 100644
--- a/util.cxx
+++ b/util.cxx
@@ -103,6 +103,19 @@ file_exists (const string &path)
   return false;
 }
 
+// Check that a dir is present
+bool
+dir_exists(const string &path)
+{
+  struct stat info;
+
+  if (stat(path.c_str(), &info) == 0 &&
+      S_ISDIR(info.st_mode))
+    return true;
+
+  return false;
+}
+
 // Copy a file.  The copy is done via a temporary file and atomic
 // rename.
 bool
diff --git a/util.h b/util.h
index b4c4e40b7..4e30d4237 100644
--- a/util.h
+++ b/util.h
@@ -72,6 +72,7 @@ const char *get_home_directory(void);
 size_t get_file_size(const std::string &path);
 size_t get_file_size(int fd);
 bool file_exists (const std::string &path);
+bool dir_exists(const std::string &path);
 bool copy_file(const std::string& src, const std::string& dest,
                bool verbose=false);
 int create_dir(const char *dir, int mode = 0777);
-- 
2.16.2.dirty