From 9272c0cd5a465450edfe26a46825e568382c3fd2 Mon Sep 17 00:00:00 2001
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Mon, 4 Apr 2016 14:39:16 -0400
Subject: [PATCH] Fix: use BUG_ON instead of accessing array subscript above
 array bounds
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Building lttng-modules lttng-filter-interpreter.c on a 4.6-rc kernel
triggers the following gcc warning:

In file included from /home/compudj/git/lttng-modules/lttng-filter-interpreter.c:25:0:
/home/compudj/git/lttng-modules/lttng-filter-interpreter.c: In function ‘lttng_filter_interpret_bytecode’:
/home/compudj/git/lttng-modules/lttng-filter.h:144:14: warning: array subscript is above array bounds [-Warray-bounds]
   &(stack)->e[top];    \
              ^
/home/compudj/git/lttng-modules/lttng-filter-interpreter.c:714:4: note: in expansion of macro ‘estack_ax’
    estack_ax(stack, top)->u.s.str = insn->data;
    ^
/home/compudj/git/lttng-modules/lttng-filter.h:144:14: warning: array subscript is above array bounds [-Warray-bounds]
   &(stack)->e[top];    \
              ^
/home/compudj/git/lttng-modules/lttng-filter-interpreter.c:715:4: note: in expansion of macro ‘estack_ax’
    estack_ax(stack, top)->u.s.seq_len = UINT_MAX;

This is because the bound checking is performed in a prior validation
phase (which the compiler does not know about), and we only
WARN_ON_ONCE() if the interpreter sees values that don't fit in array
range.

Use BUG_ON() in the interpreter instead, which ensures we never, ever
reach the out-of-bound condition from a compiler perspective.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 lttng-filter.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/lttng-filter.h b/lttng-filter.h
index 7eea1487..7ac794d6 100644
--- a/lttng-filter.h
+++ b/lttng-filter.h
@@ -139,19 +139,19 @@ struct estack {
 
 #define estack_ax(stack, top)					\
 	({							\
-		WARN_ON_ONCE((top) <= FILTER_STACK_EMPTY);	\
+		BUG_ON((top) <= FILTER_STACK_EMPTY);		\
 		&(stack)->e[top];				\
 	})
 
 #define estack_bx(stack, top)					\
 	({							\
-		WARN_ON_ONCE((top) <= FILTER_STACK_EMPTY + 1);	\
+		BUG_ON((top) <= FILTER_STACK_EMPTY + 1);	\
 		&(stack)->e[(top) - 1];				\
 	})
 
 #define estack_push(stack, top, ax, bx)				\
 	do {							\
-		WARN_ON_ONCE((top) >= FILTER_STACK_LEN - 1);	\
+		BUG_ON((top) >= FILTER_STACK_LEN - 1);		\
 		(stack)->e[(top) - 1].u.v = (bx);		\
 		(bx) = (ax);					\
 		++(top);					\
@@ -159,7 +159,7 @@ struct estack {
 
 #define estack_pop(stack, top, ax, bx)				\
 	do {							\
-		WARN_ON_ONCE((top) <= FILTER_STACK_EMPTY);	\
+		BUG_ON((top) <= FILTER_STACK_EMPTY);		\
 		(ax) = (bx);					\
 		(bx) = (stack)->e[(top) - 2].u.v;		\
 		(top)--;					\
-- 
2.34.1