Commit 430d275a authored by Peter Lund's avatar Peter Lund Committed by Linus Torvalds

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: default avatarPeter Lund <firefly@vax64.dk>
Christoph Lameter <clameter@sgi.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: "Maciej W. Rozycki" <macro@linux-mips.org>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Andreas Schwab <schwab@suse.de>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: WU Fengguang <wfg@mail.ustc.edu.cn>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 22e48eaf
...@@ -1049,12 +1049,14 @@ radix_tree_node_ctor(struct kmem_cache *cachep, void *node) ...@@ -1049,12 +1049,14 @@ radix_tree_node_ctor(struct kmem_cache *cachep, void *node)
static __init unsigned long __maxindex(unsigned int height) static __init unsigned long __maxindex(unsigned int height)
{ {
unsigned int tmp = height * RADIX_TREE_MAP_SHIFT; unsigned int width = height * RADIX_TREE_MAP_SHIFT;
unsigned long index = (~0UL >> (RADIX_TREE_INDEX_BITS - tmp - 1)) >> 1; int shift = RADIX_TREE_INDEX_BITS - width;
if (tmp >= RADIX_TREE_INDEX_BITS) if (shift < 0)
index = ~0UL; return ~0UL;
return index; if (shift >= BITS_PER_LONG)
return 0UL;
return ~0UL >> shift;
} }
static __init void radix_tree_init_maxindex(void) static __init void radix_tree_init_maxindex(void)
......
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