Re: [RFT 1/2] printk: make dynamic kernel ring buffer alignemnt explicit

[Stephen Warren <swarren@wwwdotorg.org>]

On 06/18/2014 01:33 PM, Luis R. Rodriguez wrote:
> On Wed, Jun 18, 2014 at 09:56:03AM -0600, Stephen Warren wrote:
>> On 06/18/2014 05:14 AM, Luis R. Rodriguez wrote:
>>> From: "Luis R. Rodriguez" <mcgrof@suse.com>
>>>
>>> We have to consider alignment for the ring buffer both for the
>>> default static size, and then also for when an dynamic allocation
>>> is made when the log_buf_len=n kernel parameter is passed to set
>>> the size specifically to a size larger than the default size set
>>> by the architecture through CONFIG_LOG_BUF_SHIFT.
>>>
>>> The default static kernel ring buffer can be aligned properly if
>>> architectures set CONFIG_LOG_BUF_SHIFT properly, we provide ranges
>>> for the size though so even if CONFIG_LOG_BUF_SHIFT has a sensible
>>> aligned value it can be reduced to a non aligned value. Commit
>>> 6ebb017de9 by Andrew ensures the static buffer is always aligned
>>> and the decision of alignment is done by the compiler by using
>>> __alignof__(struct log) (curious what value caused the crash?).
>>
>> IIRC the issue was that __log_buf's type is char[] so without the
>> __aligned it could have any alignment at all, e.g. 1 or 2. However,
>> struct printk_log is stored in the buffer rather than just char*, and so
>> if __log_buf isn't aligned to the required alignment for that structure,
>> that can caused unaligned accesses to fields in the structure, which
>> isn't supported on ARM in at least some cases.
>>
>> As such, I think the change to setup_log_buf() in this patch makes sense
>> (although I suppose in practice memblock_virt_alloc() probably has some
>> minimum internal alignment that dwards LOG_ALIGN, but that's an
>> implementation detail we shouldn't rely on).
> 
> Thanks for the feedback.
> 
> memblock_virt_alloc() will by default align to L1 cache, so if that satisfies
> the architecture alignment it should be safe, but perhaps not optimal for
> saving a few bytes. Still curious if without this patch a crash can be
> triggered somehow with some log_buf_len=n, if so this can go to stable.

If memblock_virt_alloc() aligns to L1 cache, then I believe that the
crash would never trigger.