From: Linus Torvalds Date: Wed, 19 Dec 2012 15:18:35 +0000 (-0800) Subject: blk: avoid divide-by-zero with zero discard granularity X-Git-Tag: v3.8-rc1~53 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=59771079c18c44e39106f0f30054025acafadb41;p=pandora-kernel.git blk: avoid divide-by-zero with zero discard granularity Commit 8dd2cb7e880d ("block: discard granularity might not be power of 2") changed a couple of 'binary and' operations into modulus operations. Which turned the harmless case of a zero discard_granularity into a possible divide-by-zero. The code also had a much more subtle bug: it was doing the modulus of a value in bytes using 'sector_t'. That was always conceptually wrong, but didn't actually matter back when the code assumed a power-of-two granularity: we only looked at the low bits anyway. But with potentially arbitrary sector numbers, using a 'sector_t' to express bytes is very very wrong: depending on configuration it limits the starting offset of the device to just 32 bits, and any overflow would result in a wrong value if the modulus wasn't a power-of-two. So re-write the code to not only protect against the divide-by-zero, but to do the starting sector arithmetic in sectors, and using the proper types. [ For any mathematicians out there: it also looks monumentally stupid to do the 'modulo granularity' operation *twice*, never mind having a "+ granularity" in the second modulus op. But that's the easiest way to avoid negative values or overflow, and it is how the original code was done. ] Reported-by: Ingo Molnar Reported-by: Doug Anderson Cc: Neil Brown Cc: Shaohua Li Acked-by: Jens Axboe Signed-off-by: Linus Torvalds --- Reading git-diff-tree failed