Re: [PATCH] doc:kmsg: explictly state the return value in case of SEEK_CUR

[Petr Mladek <pmladek@suse.com>]

On Thu 2020-07-09 12:54:15, Bruno Meneguele wrote:
> The commit 625d3449788f ("Revert "kernel/printk: add kmsg SEEK_CUR
> handling"") reverted a change done to the return value in case a SEEK_CUR
> operation was performed for kmsg buffer based on the fact that different
> userspace apps were handling the new return value (-ESPIPE) in different
> ways, breaking them.
> 
> At the same time -ESPIPE was the wrong decision because kmsg /does support/
> seek() but doesn't follow the "normal" behavior userspace is used to.
> Because of that and also considering the time -EINVAL has been used, it was
> decided to keep this way to avoid more userspace breakage.
> 
> This patch adds an official statement to the kmsg documentation pointing to
> the current return value for SEEK_CUR, -EINVAL, thus userspace libraries and
> apps can refer to it for a definitive guide on what to expected.
> 
> --- a/Documentation/ABI/testing/dev-kmsg
> +++ b/Documentation/ABI/testing/dev-kmsg
> @@ -56,6 +56,11 @@ Description:	The /dev/kmsg character device node provides userspace access
>  		  seek after the last record available at the time
>  		  the last SYSLOG_ACTION_CLEAR was issued.
>  
> +		Considering that the seek operation is supported but has 
> +		special meaning to the device, any attempt to seek specific
> +		positions on the buffer (i.e.  using SEEK_CUR) results in an
> +		-EINVAL error returned to userspace.

Sigh, I see that devkmsg_llseek() returns -ESPIPE when offset is not
zero. This is a real mess.

I wonder if we could afford to switch this one to -EINVAL and reduce
the mess.

Anyway, for a random reader, it might be pretty unclear what is
exactly special about /dev/kmsg. I wonder if the following might
be more explanatory and strightforward:

		Other seek operations or offsets are not supported because of
		the special behavior. The device allows to read or write
		only whole variable lenght messages that are stored in
		a ring buffer.

		Because of the non-standard behavior also the error values
		are non-standardand. -ESPIPE is returned for non-zero offset.
		-EINVAL is returned for other operations, e.g. SEEK_CUR.
		This behavior is historical and could not be modified
		wihtout the risk of breaking userspace.


Finally, only few people read documentation. We should add some
warning also to the code. I think about a something like:

/*
 * Be careful when modifying this function!!!
 *
 * Only few operations are supported because the device works only with
 * the entire variable length messages. Non-standard error values are
 * returned in the other cases. User space applications might depend
 * on this behavior.
 */

Best Regards,
Petr