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authorJeff Mahoney <jeffm@suse.com>2011-01-12 17:00:48 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2011-01-13 08:03:19 -0800
commit9ab020cf07e457a8b425bf5af17e704f90f86d8b (patch)
tree8119f741492575b12968f325cb225ec16f437fc7 /kernel/taskstats.c
parent0329326e85aaa30fb8d427828c718d565c287385 (diff)
downloadlinaro-lsk-9ab020cf07e457a8b425bf5af17e704f90f86d8b.tar.gz
taskstats: use better ifdef for alignment
Commit 4be2c95d ("taskstats: pad taskstats netlink response for aligment issues on ia64") added a null field to align the taskstats structure but the discussion centered around ia64. The issue exists on other platforms with inefficient unaligned access and adding them piecemeal would be an unmaintainable mess. This patch uses Dave Miller's suggestion of using a combination of CONFIG_64BIT && !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS to determine whether alignment is needed. Note that this will cause breakage on those platforms with applications like iotop which had hard-coded offsets into the packet to access the taskstats structure. The message seen on systems without the alignment fixes looks like: kernel unaligned access to 0xe000023879dca9bc, ip=0xa000000100133d10 The addresses may vary but resolve to locations inside __delayacct_add_tsk. iotop makes what I'd call unreasonable assumptions about the contents of a netlink genetlink packet containing generic attributes. They're typed and have headers that specify value lengths, so the client can (should) identify and skip the ones the client doesn't understand. The kernel, as of version 2.6.36, presented a packet like so: +--------------------------------+ | genlmsghdr - 4 bytes | +--------------------------------+ | NLA header - 4 bytes | /* Aggregate header */ +-+------------------------------+ | | NLA header - 4 bytes | /* PID header */ | +------------------------------+ | | pid/tgid - 4 bytes | | +------------------------------+ | | NLA header - 4 bytes | /* stats header */ | + -----------------------------+ <- oops. aligned on 4 byte boundary | | struct taskstats - 328 bytes | +-+------------------------------+ The iotop code expects that the kernel will behave as it did then, assuming that the packet format is set in stone. The format is set in stone, but the packet offsets are not. There's nothing in the packet format that guarantees that the packet will be sent in exactly the same way. The attribute contents are set (or versioned) and the aggregate contents are set but they can be anywhere in the packet. The issue here isn't that an unaligned structure gets passed to userspace, it's that the NLA infrastructure has something of a weakness: The 4 byte attribute header may force the payload to be unaligned. The taskstats structure is created at an unaligned location and then 64-bit values are operated on inside the kernel, so the unaligned access warnings gets spewed everywhere. It's possible to use the unaligned access API to operate on the structure in the kernel but it seems like a wasted effort to work around userspace code that isn't following the packet format. Any new additions would also need the be worked around. It's a maintenance nightmare. The conclusion of the earlier discussion seemed to be "ok fine, if we have to break it, don't break it on arches that don't have the problem." Dave pointed out that the unaligned access problem doesn't only exist on ia64, but also on other 64-bit arches that don't have efficient unaligned access and it should be fixed there as well. The committed version of the patch and this addition keep with the conclusion of that discussion not to break it unnecessarily, which the pid padding and the packet padding fixes did do. x86_64 and powerpc don't suffer this problem so they shouldn't suffer the solution. Other 64-bit architectures do and will, though. Signed-off-by: Jeff Mahoney <jeffm@suse.com> Reported-by: David S. Miller <davem@davemloft.net> Acked-by: David S. Miller <davem@davemloft.net> Cc: Dan Carpenter <error27@gmail.com> Cc: Balbir Singh <balbir@in.ibm.com> Cc: Florian Mickler <florian@mickler.org> Cc: Guillaume Chazarain <guichaz@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel/taskstats.c')
-rw-r--r--kernel/taskstats.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/kernel/taskstats.c b/kernel/taskstats.c
index 69691eb4b71..3971c6b9d58 100644
--- a/kernel/taskstats.c
+++ b/kernel/taskstats.c
@@ -348,7 +348,7 @@ static int parse(struct nlattr *na, struct cpumask *mask)
return ret;
}
-#ifdef CONFIG_IA64
+#if defined(CONFIG_64BIT) && !defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
#define TASKSTATS_NEEDS_PADDING 1
#endif