Re: kmsg: lseek errors confuse glibc's dprintf

[Mike Crowe]

On Thursday 21 March 2019 at 10:47:26 +0100, Alexander Sverdlin wrote:
> Hello Mike and all,
> 
> On Thu, 15 Jan 2015 17:31:32 +0000
> Mike Crowe <mac@xxxxxxxxxx> wrote:
> 
> > glibc's dprintf implementation does not work correctly with /dev/kmsg file
> > descriptors because glibc treats receiving EBADF and EINVAL from lseek when
> > trying to determine the current file position as errors. See
> > https://sourceware.org/bugzilla/show_bug.cgi?id=17830
> 
> we need to conclude on this issue. This is a real bug which is ignored
> for 4 years now. Mike, would you like to re-send a formal patch?
> I can do it as well, preserving a link to your original patch/report.
> In case you'd like to post it yourself, I can be a tester and/or
> provide a reproducer.

Hi Alexander,

I'm sorry for dropping the ball on this. I think I was travelling at the
time. :( Thank you for picking it up.

I don't mind rebasing my patch once I know which of my two solutions is
preferred. It seems that the -ESPIPE one is the current favourite.

Thanks.

> > >>From what I can tell prior to Kay Sievers printk record commit
> > e11fea92e13fb91c50bacca799a6131c81929986, calling lseek(fd, 0, SEEK_CUR)
> > with such a file descriptor would not return an error.
> > 
> > Prior to Kay's change, Arnd Bergmann's commit
> > 6038f373a3dc1f1c26496e60b6c40b164716f07e seemed to go to some lengths to
> > preserve the successful return code rather than returning (the perhaps more
> > logical) -ESPIPE.
> > 
> > glibc is happy with either a successful return or -ESPIPE.
> > 
> > For maximum compatibility it seems that success should be returned but
> > given Kay's new seek interface perhaps this isn't helpful.
> > 
> > This patch ensures that such a seek succeeds:
> > 
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index 02d6b6d..b3ff6f0 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -693,7 +693,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
> >  	loff_t ret = 0;
> >  
> >  	if (!user)
> > -		return -EBADF;
> > +		return (whence == SEEK_CUR) ? 0 : -EBADF;
> >  	if (offset)
> >  		return -ESPIPE;
> >  
> > @@ -718,6 +718,11 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
> >  		user->idx = log_next_idx;
> >  		user->seq = log_next_seq;
> >  		break;
> > +	case SEEK_CUR:
> > +		/* For compatibility with userspace requesting the
> > +		 * current file position. */
> > +		ret = 0;
> > +		break;
> >  	default:
> >  		ret = -EINVAL;
> >  	}
> > 
> > (although it could be argued that the !user case should return -ESPIPE
> > rather than EBADF since the file descriptor _is_ valid.)
> > 
> > and this patch causes a failure that glibc is prepared to accept:
> > 
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index 02d6b6d..f6b0c93 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -693,7 +693,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
> >  	loff_t ret = 0;
> >  
> >  	if (!user)
> > -		return -EBADF;
> > +		return -ESPIPE;
> >  	if (offset)
> >  		return -ESPIPE;
> >  
> > @@ -718,6 +718,11 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
> >  		user->idx = log_next_idx;
> >  		user->seq = log_next_seq;
> >  		break;
> > +	case SEEK_CUR:
> > +		/* For compatibility with userspace expecting SEEK_CUR
> > +		 * to not yield EINVAL. */
> > +		ret = -ESPIPE;
> > +		break;
> >  	default:
> >  		ret = -EINVAL;
> >  	}
> > 
> > Either makes dprintf work, but is either the right solution?

Mike.