Coccinelle: flag conditions with no effect

Report code constructs where the if and else branch are functionally
identical. In cases where this is intended it really should be
documented - most reported cases probably are bugs.
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Michal Marek <mmarek@suse.com>

scripts/coccinelle/misc/cond_no_effect.cocci

+///Find conditions where if and else branch are functionally
+// identical.
+//
+// There can be false positives in cases where the positional
+// information is used (as with lockdep) or where the identity
+// is a placeholder for not yet handled cases.
+// Unfortunately there also seems to be a tendency to use
+// the last if else/else as a "default behavior" - which some
+// might consider a legitimate coding pattern. From discussion
+// on kernelnewbies though it seems that this is not really an
+// accepted pattern and if at all it would need to be commented
+//
+// In the Linux kernel it does not seem to actually report
+// false positives except for those that were documented as
+// being intentional.
+// the two known cases are:
+//   arch/sh/kernel/traps_64.c:read_opcode()
+//        } else if ((pc & 1) == 0) {
+//              /* SHcompact */
+//              /* TODO : provide handling for this.  We don't really support
+//                 user-mode SHcompact yet, and for a kernel fault, this would
+//                 have to come from a module built for SHcompact.  */
+//              return -EFAULT;
+//      } else {
+//              /* misaligned */
+//              return -EFAULT;
+//      }
+//   fs/kernfs/file.c:kernfs_fop_open()
+//       * Both paths of the branch look the same.  They're supposed to
+//       * look that way and give @of->mutex different static lockdep keys.
+//       */
+//      if (has_mmap)
+//              mutex_init(&of->mutex);
+//      else
+//              mutex_init(&of->mutex);
+//
+// All other cases look like bugs or at least lack of documentation
+//
+// Confidence: Moderate
+// Copyright: (C) 2016 Nicholas Mc Guire, OSADL.  GPLv2.
+// Comments:
+// Options: --no-includes --include-headers
+
+virtual org
+virtual report
+
+@cond@
+statement S1;
+position p;
+@@
+
+* if@p (...) S1 else S1
+
+@script:python depends on org@
+p << cond.p;
+@@
+
+cocci.print_main("WARNING: possible condition with no effect (if == else)",p)
+
+@script:python depends on report@
+p << cond.p;
+@@
+
+coccilib.report.print_report(p[0],"WARNING: possible condition with no effect (if == else)")