linux-kernel - Re: [PATCH v2] kernel/printk: add kmsg SEEK

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Date:   Fri, 20 Mar 2020 11:45:08 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Bruno Meneguele <bmeneg@...hat.com>
Cc:     linux-kernel@...r.kernel.org, pmladek@...e.com,
        sergey.senozhatsky@...il.com, rostedt@...dmis.org,
        David.Laight@...LAB.COM
Subject: Re: [PATCH v2] kernel/printk: add kmsg SEEK_CUR handling

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@...hat.com>

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

	-ss