Re: [PATCH v2 1/2] kernel: printk: specify alignment for struct printk_log

[Joe Perches <joe@perches.com>]

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?

The struct is naturally aligned on u64 and should be
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?