linux-kernel - Re: kmsg: lseek errors confuse glibc's dprintf

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Date:   Thu, 21 Mar 2019 10:47:26 +0100
From:   Alexander Sverdlin <alexander.sverdlin@...il.com>
To:     Mike Crowe <mac@...owe.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, Kay Sievers <kay@...y.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: kmsg: lseek errors confuse glibc's dprintf

Hello Mike and all,

On Thu, 15 Jan 2015 17:31:32 +0000
Mike Crowe <mac@...owe.com> 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.

> >>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?
> 
> Thanks.
> 
> Mike.


-- 
Alexander Sverdlin.