PR5916: Exploit kretprobe data storage area

Since 2.6.25, kretprobes can carry a data packet to be filled in an
entry_handler.  This patch lets us store our implicitly-saved $target
variables in .return probes in that data area.

* tapset/kretprobe.stp: New get/set functions for kretprobe data.
* translate.cxx (c_unparser::emit_common_header): Add context->pi_longs.
* tapsets.cxx
  (dwarf_var_expanding_visitor::visit_target_symbol_saved_return): Switch
  between the old and new methods of saving $vars in .return probes.
  (dwarf_var_expanding_visitor::gen_mapped_saved_return): The old way.
  (dwarf_var_expanding_visitor::gen_kretprobe_saved_return): The new way.
  (dwarf_derived_probe::join_group): Don't register paired entry-handlers.
  (dwarf_derived_probe::dwarf_derived_probe): Remember saved-var details.
  (dwarf_derived_probe_group::emit_module_decls): Output saved-var details.
  Also split the kretprobe handler whether we're on entry or return.
  (dwarf_derived_probe_group::emit_module_init): Prepare the entry handler.
* testsuite/systemtap.base/kretprobe-vars.stp: Test implicit $var saving.

2 files changed, 74 insertions, 0 deletions

diff --git a/testsuite/systemtap.base/kretprobe-vars.exp b/testsuite/systemtap.base/kretprobe-vars.exp
new file mode 100644
index 00000000..5955b64f
--- /dev/null
+++ b/testsuite/systemtap.base/kretprobe-vars.exp
@@ -0,0 +1,4 @@
+# Check that implicitly-saved $target variables have the same
+# value as those manually saved on entry.
+set test "kretprobe-vars"
+stap_run $srcdir/$subdir/$test.stp no_load $all_pass_string -c "cat /dev/null"
diff --git a/testsuite/systemtap.base/kretprobe-vars.stp b/testsuite/systemtap.base/kretprobe-vars.stp
new file mode 100644
index 00000000..b932e202
--- /dev/null
+++ b/testsuite/systemtap.base/kretprobe-vars.stp
@@ -0,0 +1,70 @@
+# Check that implicitly-saved $target variables have the same
+# value as those manually saved on entry.
+
+
+# saved individual parameters
+global read_file
+global read_file_mode
+global read_buf
+global read_count
+global read_pos
+
+# the saved string $$parms
+global read_parms
+
+# error count
+global errors
+
+probe begin
+{
+  println("systemtap starting probe")
+}
+
+function check_num(name, ent, ret)
+{
+  if (ent != ret) {
+    printf("%s mismatch, entry:%#x vs. return:%#x\n", name, ent, ret)
+    errors++
+  }
+}
+function check_str(name, ent, ret)
+{
+  if (ent != ret) {
+    printf("%s mismatch, entry:'%s' vs. return:'%s'\n", name, ent, ret)
+    errors++
+  }
+}
+
+probe kernel.function("vfs_read").call
+{
+  if (tid() != target())
+    next
+
+  read_file = $file
+  read_file_mode = $file->f_mode
+  read_buf = $buf
+  read_count = $count
+  read_pos = $pos
+
+  read_parms = $$parms
+}
+
+probe kernel.function("vfs_read").return
+{
+  if (tid() != target())
+    next
+
+  println("systemtap ending probe")
+
+  check_num("file", read_file, $file)
+  check_num("file->f_mode", read_file_mode, $file->f_mode)
+  check_num("buf", read_buf, $buf)
+  check_num("count", read_count, $count)
+  check_num("pos", read_pos, $pos)
+
+  check_str("parms", read_parms, $$parms)
+
+  if (!errors)
+    println("systemtap test success")
+  exit()
+}