2014-11-14 20:22 GMT+03:00 Joe Perches <joe@xxxxxxxxxxx>:
> On Fri, 2014-11-14 at 15:50 +0300, Andrey Ryabinin wrote:
>> On architectures that have support for efficient unaligned access
>> struct printk_log has 4-byte alignment.
>> Specify alignment attribute in type declaration.
>>
>> The whole point of this patch is to fix deadlock which happening
>> when UBSan detects unaligned access in printk() thus UBSan recursively
>> calls printk() with logbuf_lock held by top printk() call.
> []
>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> []
>> @@ -223,7 +223,11 @@ struct printk_log {
>> u8 facility; /* syslog facility */
>> u8 flags:5; /* internal record flags */
>> u8 level:3; /* syslog level */
>> -};
>> +}
>> +#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
>> +__packed __aligned(4)
>> +#endif
>
> Why is adding __packed useful or __aligned(4) useful?
>
On x86_64, from compiler's point of view, 'struct printk_log' should
be 8-byte aligned.
But it stored in log buffer only with 4-byte alignment (LOG_ALIGN define).
This inconsistency makes UBSAN unhappy. UBSAN uses printk to report errors,
thus any access to 4-byte aligned 'struct printk_log' causing
recursive printk() call from ubsan handler.
And if logbuf_lock was taken we end up with deadlock.
Specifying alignment removes this inconsistency. Now compiler knows
that 'printk_log'
actually aligned on 4, and it makes UBSAN happy and silent.
Attribute 'aligned' without attribute 'packed' can only increase alignment.
So __packed is used here because we need to decrease alignment from 8 to 4.
> The struct is naturally aligned on u64 and should be
Not, always. On 32-bits it will be 4-bytes aligned, on 64-bits - 8-bytes aligned
> the size of 2 u64s.
>
> struct printk_log {
> u64 ts_nsec; /* timestamp in nanoseconds */
> u16 len; /* length of entire record */
> u16 text_len; /* length of text buffer */
> u16 dict_len; /* length of dictionary buffer */
> u8 facility; /* syslog facility */
> u8 flags:5; /* internal record flags */
> u8 level:3; /* syslog level */
> };
>
> Is there any case when it's not sizeof(u64) * 2?
>
I think no. As I said __packed used for decreasing alignment only.
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