[PATCH 1/4] libxfs: avoid negative (and full-width) shifts in radix-tree.c

Eric Sandeen
2015-10-09T00:25:02Z

Pull this commit over from the kernel, as libubsan spotted a
bad shift at runtime:

commit 430d275a399175c7c0673459738979287ec1fd22
Author: Peter Lund <firefly@vax64.dk>
Date: Tue Oct 16 23:29:35 2007 -0700

avoid negative (and full-width) shifts in radix-tree.c

Negative shifts are not allowed in C (the result is undefined). Same thing
with full-width shifts.

It works on most platforms but not on the VAX with gcc 4.0.1 (it results in an
"operand reserved" fault).

Shifting by more than the width of the value on the left is also not
allowed. I think the extra '>> 1' tacked on at the end in the original
code was an attempt to work around that. Getting rid of that is an extra
feature of this patch.

Here's the chapter and verse, taken from the final draft of the C99
standard ("6.5.7 Bitwise shift operators", paragraph 3):

"The integer promotions are performed on each of the operands. The
type of the result is that of the promoted left operand. If the
value of the right operand is negative or is greater than or equal
to the width of the promoted left operand, the behavior is
undefined."

Thank you to Jan-Benedict Glaw, Christoph Hellwig, Maciej Rozycki, Pekka
Enberg, Andreas Schwab, and Christoph Lameter for review. Special thanks
to Andreas for spotting that my fix only removed half the undefined
behaviour.

Signed-off-by: Peter Lund <firefly@vax64.dk>

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---
libxfs/radix-tree.c | 14 ++++++++------
1 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/libxfs/radix-tree.c b/libxfs/radix-tree.c
index 4d44ab4..eef9c36 100644
--- a/libxfs/radix-tree.c
+++ b/libxfs/radix-tree.c
@@ -784,12 +784,14 @@ int radix_tree_tagged(struct radix_tree_root *root, unsigned int tag)

static unsigned long __maxindex(unsigned int height)
{
- unsigned int tmp = height * RADIX_TREE_MAP_SHIFT;
- unsigned long index = (~0UL >> (RADIX_TREE_INDEX_BITS - tmp - 1)) >> 1;
-
- if (tmp >= RADIX_TREE_INDEX_BITS)
- index = ~0UL;
- return index;
+ unsigned int width = height * RADIX_TREE_MAP_SHIFT;
+ int shift = RADIX_TREE_INDEX_BITS - width;
+
+ if (shift < 0)
+ return ~0UL;
+ if (shift >= BITS_PER_LONG)
+ return 0UL;
+ return ~0UL >> shift;
}

static void radix_tree_init_maxindex(void)
--
1.7.1