[PATCH 3.16 014/192] tools/lib/lockdep: Reduce MAX_LOCK_DEPTH to avoid overflowing lock_chain/: Depth

[Ben Hutchings]

3.16.49-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Ben Hutchings <ben@xxxxxxxxxxxxxxx>

commit 98dcea0cfd04e083ac74137ceb9a632604740e2d upstream.

liblockdep has been broken since commit 75dd602a5198 ("lockdep: Fix
lock_chain::base size"), as that adds a check that MAX_LOCK_DEPTH is
within the range of lock_chain::depth and in liblockdep it is much
too large.

That should have resulted in a compiler error, but didn't because:

- the check uses ARRAY_SIZE(), which isn't yet defined in liblockdep
  so is assumed to be an (undeclared) function
- putting a function call inside a BUILD_BUG_ON() expression quietly
  turns it into some nonsense involving a variable-length array

It did produce a compiler warning, but I didn't notice because
liblockdep already produces too many warnings if -Wall is enabled
(which I'll fix shortly).

Even before that commit, which reduced lock_chain::depth from 8 bits
to 6, MAX_LOCK_DEPTH was too large.

Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: a.p.zijlstra@xxxxxxxxx
Link: http://lkml.kernel.org/r/20170525130005.5947-3-alexander.levin@xxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 tools/lib/lockdep/uinclude/linux/lockdep.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/tools/lib/lockdep/uinclude/linux/lockdep.h
+++ b/tools/lib/lockdep/uinclude/linux/lockdep.h
@@ -8,7 +8,7 @@
 #include <linux/utsname.h>
 
 
-#define MAX_LOCK_DEPTH 2000UL
+#define MAX_LOCK_DEPTH 255UL
 
 #define asmlinkage
 #define __visible