Re: [PATCH] drivers: w1: make w1_slave::flags long to avoid casts

Michal Nazarewicz
2013-10-31T09:12:06Z

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On Wed, Oct 30 2013, Andrew Morton wrote:
> On Sat, 26 Oct 2013 12:56:11 +0100 Michal Nazarewicz <mpn@google.com> wrote:
>
>> From: Michal Nazarewicz <mina86@mina86.com>
>>
>> Changing flags field of the w1_slave to unsigned long may on
>> some architectures increase the size of the structure, but
>> otherwise makes the code more kosher as casting is avoided
>> and *_bit family of calls do not attempt to operate on an
>> entity of bigger size than realy is available.
>>
>> The current behaviour does not introduce any bugs (since any
>> bytes past flags field are preserved)
>
> hm, what does this mean....
>
>> --- a/drivers/w1/w1.c
>> +++ b/drivers/w1/w1.c
>> @@ -709,7 +709,7 @@ static int w1_attach_slave_device(struct w1_master *dev, struct w1_reg_num *rn)
>>
>> sl->owner = THIS_MODULE;
>> sl->master = dev;
>> - set_bit(W1_SLAVE_ACTIVE, (long *)&sl->flags);
>> + set_bit(W1_SLAVE_ACTIVE, &sl->flags);
>
> ... I'd have though that running this code on little-endian 64-bit
> would result in a scribble over ...
>
>> --- a/drivers/w1/w1.h
>> +++ b/drivers/w1/w1.h
>> @@ -67,8 +67,8 @@ struct w1_slave
>> struct w1_reg_num reg_num;
>> atomic_t refcnt;
>> u8 rom[9];
>> - u32 flags;
>> int ttl;
>
> ... w1_slave.ttl?

Now that I look at documentation, I think you are correct, but the
problem is on big-endian 64-bit architectures. The fix is still
valid, but the commit message not so much. Something along the
lines of the following would be better:

-------- >8 --------------------------------------------------------
drivers: w1: make w1_slave::flags long to avoid memory corruption

On architectures where long is more then 32 bits, modifying a 32-bit
field with set_bit (and other atomic bit operations) may cause bytes
following the field to by modified.

Because the endianness of the bits within a field is the native
endianness of the CPU[1], on big-endian machines, bit number zero is
in the last byte of the field.

Therefore, “set_bit(0, ptr)” on a 64-bit big-endian machine is
roughly equivalent to “((char *)ptr)[7] |= 1”, and since w1 driver
uses a 32-bit field for holding the flags, this causes bytes beyond
the field to be modified.

[1] From Documentation/atomic_ops.txt:

Native atomic bit operations are defined to operate on objects
aligned to the size of an "unsigned long" C data type, and are
least of that size. The endianness of the bits within each
"unsigned long" are the native endianness of the cpu.
-------- >8 --------------------------------------------------------

>> + unsigned long flags;
>>
>> struct w1_master *master;
>> struct w1_family *family;

--
Best regards, _ _
.o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o
..o | Computer Science, Michał “mina86” Nazarewicz (o o)
ooo +--<mpn@google.com>--<xmpp:mina86@jabber.org>--ooO--(_)--Ooo--

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