Xen devel - Feb 2011 - [PATCH]do not allow multiple xenconsole connect to the same pty

allowing multiple xenconsole connect to the same pty is just insane,
prevent this by acquiring a write flock on the pty fd once we opened it.

Signed-off-by: Li Dongyang <lidongyang@novell.com>

diff -r c64dcc4d2eca -r 08ea56ee1b70 tools/console/client/main.c
--- a/tools/console/client/main.c	Thu Feb 10 17:24:41 2011 +0000
+++ b/tools/console/client/main.c	Tue Feb 15 12:00:21 2011 +0800
@@ -96,6 +96,7 @@
  * Assumes there is already a watch set in the store for this path. */
 {
 	struct timeval tv;
+	struct flock lock;
 	fd_set watch_fdset;
 	int xs_fd = xs_fileno(xs), pty_fd = -1;
 	int start, now;
@@ -122,6 +123,12 @@
 				if (pty_fd == -1) 
 					err(errno, "Could not open tty `%s''", 
 					    pty_path);
+				memset(&lock, 0, sizeof(lock));
+				lock.l_type = F_WRLCK;
+				lock.l_whence = SEEK_SET;
+				if (fcntl(pty_fd, F_SETLK, &lock) != 0)
+					err(errno, "Could not lock tty ''%s''",
+					    pty_path);
 				free(pty_path);
 			}
 		}

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allowing multiple xenconsole connect to the same pty is just insane,
prevent this by acquiring a write flock on the pty fd once we opened it.

Signed-off-by: Li Dongyang <lidongyang@novell.com>

diff -r c64dcc4d2eca -r 08ea56ee1b70 tools/console/client/main.c
--- a/tools/console/client/main.c	Thu Feb 10 17:24:41 2011 +0000
+++ b/tools/console/client/main.c	Tue Feb 15 12:00:21 2011 +0800
@@ -96,6 +96,7 @@
  * Assumes there is already a watch set in the store for this path. */
 {
 	struct timeval tv;
+	struct flock lock;
 	fd_set watch_fdset;
 	int xs_fd = xs_fileno(xs), pty_fd = -1;
 	int start, now;
@@ -122,6 +123,12 @@
 				if (pty_fd == -1) 
 					err(errno, "Could not open tty `%s''", 
 					    pty_path);
+				memset(&lock, 0, sizeof(lock));
+				lock.l_type = F_WRLCK;
+				lock.l_whence = SEEK_SET;
+				if (fcntl(pty_fd, F_SETLK, &lock) != 0)
+					err(errno, "Could not lock tty ''%s''",
+					    pty_path);
 				free(pty_path);
 			}
 		}

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Li Dongyang writes ("[Xen-devel] [PATCH]do not allow multiple xenconsole
connect to the same pty"):
> allowing multiple xenconsole connect to the same pty is just insane,
> prevent this by acquiring a write flock on the pty fd once we opened it.
I don''t think that flocking a pty is likely to be very portable.

I guess what happens at the moment if you do this is that input is
interleaved (fine) but output is distributed randomly amongst the
available clients ?

Ian.

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On Tue, 15 Feb 2011 19:40:04 +0000
Ian Jackson <Ian.Jackson@eu.citrix.com> wrote:
> Li Dongyang writes ("[Xen-devel] [PATCH]do not allow multiple
xenconsole connect to the same pty"):
> > allowing multiple xenconsole connect to the same pty is just insane,
> > prevent this by acquiring a write flock on the pty fd once we opened
it.
> 
> I don''t think that flocking a pty is likely to be very portable.
> 
> I guess what happens at the moment if you do this is that input is
> interleaved (fine) but output is distributed randomly amongst the
> available clients ?
Use POSIX pty interfaces and you don''t get these problems. See
grantpt/unlockpt/posix_openpt.


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Alan Cox writes ("Re: [Xen-devel] [PATCH]do not allow multiple xenconsole
connect to the same pty"):
> Use POSIX pty interfaces and you don''t get these problems. See
> grantpt/unlockpt/posix_openpt.
Thanks for your helpful comment.  Unfortunately it''s not really
relevant in this situation.  What our code is doing is using ptys as
an internal IPC mechanism.  I think this is very wrong and I hope we
can fix it in the Xen 4.2 release cycle.  But for now it''s what we
have.

Ian.

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On Wednesday, February 16, 2011 03:40:04 AM Ian Jackson
wrote:
> Li Dongyang writes ("[Xen-devel] [PATCH]do not allow multiple
xenconsole connect to the same pty"):
> > allowing multiple xenconsole connect to the same pty is just insane,
> > prevent this by acquiring a write flock on the pty fd once we opened
it.
> 
> I don''t think that flocking a pty is likely to be very portable.
Thanks for reviewing, and what do you mean by ''not likely to be very
portable''?
flock(2) is not POSIX, but fcntl(3p) is POSIX, maybe I missed somthing?

Li Dongyang
> 
> I guess what happens at the moment if you do this is that input is
> interleaved (fine) but output is distributed randomly amongst the
> available clients ?
> 
> Ian.
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
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Li Dongyang writes ("Re: [Xen-devel] [PATCH]do not allow multiple
xenconsole connect to the same pty"):
> Thanks for reviewing, and what do you mean by ''not likely to be
very
> portable''?  flock(2) is not POSIX, but fcntl(3p) is POSIX, maybe I
> missed somthing?
I mean that I wouldn''t be surprised if a platform we try to target
for dom0 (or at least, not break) doesn''t support flock (or fcntl
F_SETLK for that matter) on ptys.

Ian.

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