From 430d275a399175c7c0673459738979287ec1fd22 Mon Sep 17 00:00:00 2001 From: Peter Lund Date: Tue, 16 Oct 2007 23:29:35 -0700 Subject: 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 Christoph Lameter Cc: Christoph Hellwig Cc: "Maciej W. Rozycki" Cc: Pekka Enberg Cc: Andreas Schwab Cc: Nick Piggin Cc: WU Fengguang Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- lib/radix-tree.c | 14 ++++++++------ 1 file changed, 8 insertions(+), 6 deletions(-) (limited to 'lib/radix-tree.c') diff --git a/lib/radix-tree.c b/lib/radix-tree.c index 53fd44d78716..48c250fe2233 100644 --- a/lib/radix-tree.c +++ b/lib/radix-tree.c @@ -1049,12 +1049,14 @@ radix_tree_node_ctor(struct kmem_cache *cachep, void *node) static __init 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 __init void radix_tree_init_maxindex(void) -- cgit v1.2.3-59-g8ed1b