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

[Bruno Meneguele <bmeneg@redhat.com>]

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On Fri, Jul 10, 2020 at 02:19:23PM +0200, Petr Mladek wrote:
> 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.
> 

That's a really good question for which I think the answer is something
close to: "that's impossible to predict". I could try some in-house (at
redhat) automated tests to see if something break, but I doubt it is
enough to catch any breakage.

> 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.
> 

Yes, no doubt it's better!

> 
> 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.
>  */
> 

Agreed.

I'm going to prepare a v2 of the patch adding the comments.

Thanks Petr!

-- 
bmeneg 
PGP Key: http://bmeneg.com/pubkey.txt

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