Date: Tue, 22 Mar 2005 22:21:07 +0100 (MET)
From: Jan Engelhardt <jengelh@linux01.gwdg.de>
To: linux-os <linux-os@analogic.com>
cc: Linux kernel <linux-kernel@vger.kernel.org>
Subject: Re: lseek on /proc/kmsg
In-Reply-To: <Pine.LNX.4.61.0503221423560.6369@chaos.analogic.com>
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 <Pine.LNX.4.61.0503222020470.32461@yvahk01.tjqt.qr>
 <Pine.LNX.4.61.0503221423560.6369@chaos.analogic.com>
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> Gawd, you are a hacker. I already have to suck on pipes
> because I can't seek them. Now, I can't even seek a
> file-system???!!

Here goodie goodie...

diff -pdru linux-2.6.11.4/fs/proc/kmsg.c linux-2.6.11-AS9/fs/proc/kmsg.c
--- linux-2.6.11.4/fs/proc/kmsg.c       2005-03-21 20:14:58.000000000 +0100
+++ linux-2.6.11-AS9/fs/proc/kmsg.c     2005-03-22 21:28:40.000000000 +0100
@@ -46,10 +46,15 @@ static unsigned int kmsg_poll(struct fil
        return 0;
 }

+static loff_t kmsg_seek(struct file *filp, loff_t offset, int origin) {
+    if(origin != 2 /* SEEK_END */ || offset < 0) { return -ESPIPE; }
+    return do_syslog(5, NULL, 0);
+}

 struct file_operations proc_kmsg_operations = {
        .read           = kmsg_read,
        .poll           = kmsg_poll,
        .open           = kmsg_open,
        .release        = kmsg_release,
+        .llseek         = kmsg_seek,
 };


Works so far that do_syslog is called with the correct parameters --
however, that does not work. (Did I discover a bug?)

# rcsyslog stop;  # so that kmsg is not slurped by someone else
# perl -le 'open X,"</proc/kmsg";seek X,0,2;print read X,$b,3'

the perl command should block, because with the seek(), we've just emptied the 
syslog ring buffer. Obviously, it does not, and read() succeeds - prints 3.
Any hints on what's wrong here?


Jan Engelhardt
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