Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 20 Mar 2020 11:45:08 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To:     Bruno Meneguele <bmeneg@redhat.com>
Cc:     linux-kernel@vger.kernel.org, pmladek@suse.com,
        sergey.senozhatsky@gmail.com, rostedt@goodmis.org,
        David.Laight@ACULAB.COM
Subject: Re: [PATCH v2] kernel/printk: add kmsg SEEK_CUR handling
Message-ID: <20200320024508.GB104374@google.com>
References: <20200317103344.574277-1-bmeneg@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200317103344.574277-1-bmeneg@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On (20/03/17 07:33), Bruno Meneguele wrote:
> Userspace libraries, e.g. glibc's dprintf(), perform a SEEK_CUR operation
> over any file descriptor requested to make sure the current position isn't
> pointing to junk due to previous manipulation of that same fd. And whenever
> that fd doesn't have support for such operation, the userspace code expects
> -ESPIPE to be returned.
> 
> However, when the fd in question references the /dev/kmsg interface, the
> current kernel code state returns -EINVAL instead, causing an unexpected
> behavior in userspace: in the case of glibc, when -ESPIPE is returned it
> gets ignored and the call completes successfully, while returning -EINVAL
> forces dprintf to fail without performing any action over that fd:
> 
>   if (_IO_SEEKOFF (fp, (off64_t)0, _IO_seek_cur, _IOS_INPUT|_IOS_OUTPUT) ==
>   _IO_pos_BAD && errno != ESPIPE)
>     return NULL;
> 
> With this patch we make sure to return the correct value when SEEK_CUR is
> requested over kmsg and also add some kernel doc information to formalize
> this behavior.
> 
> Signed-off-by: Bruno Meneguele <bmeneg@redhat.com>

Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

	-ss