Re: [PATCH 0/3] Fix a misaligned load inside ptrace_attach()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 30, 2015 at 09:19:54PM -0700, Palmer Dabbelt wrote:
> I ran across what I believe is a bug in some asm-generic code while
> working on the RISC-V Linux port.  Essentially the problem is that
> wait_on_bit() takes a void *, but then perfroms long-aligned
> operation.  As far as I can tell, this bug could manifest on any other
> architecture that doesn't support misaligned operations and uses this
> particular asm-generic implementation.
> 
> The patch set is split into three parts:
> 
> * #1 fixes the bug by making task_struct.jobctl an unsigned long,
>    which ensures wait_on_bit() always ends up with a long-aligned
>    argument.
> 
> * #2 changes the prototype of wait_on_bit() and friends to take a
>    "unsigned long *" instead of a "void *", with the intent of
>    ensuring these problems don't happen again.
> 
> * #3 is a bit more intrusive: it goes and changes all uses of
>    task_struct.jobctl from int to long.
> 
> I'm not sure if #3 has gone too far, but I think #1 and #2 are sane.
> The cost is making task_struct larger on machines where
> sizeof(long)>sizeof(int), but since it's so big already this isn't too
> much cost.  I thought about making test_bit() perform byte-aligned
> accesses to avoid this cost, but since there are very similar looking
> atomic functions I thought that would be too odd.

Fair enough. Thanks!