[PATCH v3] xenconsole: adjust pty opening error checking and handling

[Matthew Daley <mattd@bugfuzz.com>]

Currently we check the pty path received from xenstore with access(); if
it indicates that the pty is not accessible, we loop around and wait for
a new path to appear in xenstore.

This has several issues:
* If a path has been written to xenstore, it can be assumed that that
  pty should already be accessible to xenconsole, and hence any error
  that occurs while trying to open it should be fatal and not ignored
* If access() indicates no access to the pty, the memory allocated for
  the path is leaked when going around the loop again
* The accessibility of the pty could change between the access() and
  open() calls, leading to a TOCTOU race (this is what Coverity is
  complaining about).

By removing the explicit access() check and just erroring out whenever
open() fails, we fix all these issues.

Coverity-ID: 1056047
Signed-off-by: Matthew Daley <mattd@bugfuzz.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
v3: Improve commit message to explain what behaviours are fixed (and
hence changed)

 tools/console/client/main.c |    7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/tools/console/client/main.c b/tools/console/client/main.c
index 38c856a..3242008 100644
--- a/tools/console/client/main.c
+++ b/tools/console/client/main.c
@@ -116,12 +116,9 @@ static int get_pty_fd(struct xs_handle *xs, char *path, int seconds)
 			 * disambiguate: just read the pty path */
 			pty_path = xs_read(xs, XBT_NULL, path, &len);
 			if (pty_path != NULL) {
-				if (access(pty_path, R_OK|W_OK) != 0)
-					continue;
 				pty_fd = open(pty_path, O_RDWR | O_NOCTTY);
-				if (pty_fd == -1) 
-					err(errno, "Could not open tty `%s'", 
-					    pty_path);
+				if (pty_fd == -1)
+					err(errno, "Could not open tty `%s'", pty_path);
 				free(pty_path);
 			}
 		}
-- 
1.7.10.4