Revert "kernel/printk: add kmsg SEEK_CUR handling"

This reverts commit 8ece3b3eb5.

This commit broke userspace. Bash uses ESPIPE to determine whether or
not the file should be read using "unbuffered I/O", which means reading
1 byte at a time instead of 128 bytes at a time. I used to use bash to
read through kmsg in a really quite nasty way:

    while read -t 0.1 -r line 2>/dev/null || [[ $? -ne 142 ]]; do
       echo "SARU $line"
    done < /dev/kmsg

This will show all lines that can fit into the 128 byte buffer, and skip
lines that don't. That's pretty awful, but at least it worked.

With this change, bash now tries to do 1-byte reads, which means it
skips all the lines, which is worse than before.

Now, I don't really care very much about this, and I'm already look for
a workaround. But I did just spend an hour trying to figure out why my
scripts were broken. Either way, it makes no difference to me personally
whether this is reverted, but it might be something to consider. If you
declare that "trying to read /dev/kmsg with bash is terminally stupid
anyway," I might be inclined to agree with you. But do note that bash
uses lseek(fd, 0, SEEK_CUR)==>ESPIPE to determine whether or not it's
reading from a pipe.

Cc: Bruno Meneguele <bmeneg@redhat.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Petr Mladek <pmladek@suse.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/ABI/testing/dev-kmsg

@@ -56,11 +56,6 @@ 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.
 
-		Due to the record nature of this interface with a "read all"
-		behavior and the specific positions each seek operation sets,
-		SEEK_CUR is not supported, returning -ESPIPE (invalid seek) to
-		errno whenever requested.
-
 		The output format consists of a prefix carrying the syslog
 		prefix including priority and facility, the 64 bit message
 		sequence number and the monotonic timestamp in microseconds,

kernel/printk/printk.c

@@ -974,16 +974,6 @@ 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:
-		/*
-		 * It isn't supported due to the record nature of this
-		 * interface: _SET _DATA and _END point to very specific
-		 * record positions, while _CUR would be more useful in case
-		 * of a byte-based log. Because of that, return the default
-		 * errno value for invalid seek operation.
-		 */
-		ret = -ESPIPE;
-		break;
 	default:
 		ret = -EINVAL;
 	}