[mutter/gnome-3-22] xwayland: Fix lockfile size confusion

[Florian Müllner <fmuellner src gnome org>]

commit 5b232a80cdde00fe8606766538c8b80b3d6a54ee
Author: Daniel Stone <daniels collabora com>
Date:   Thu Nov 17 11:00:25 2016 +0000

    xwayland: Fix lockfile size confusion
    
    Similarly to Weston (where this code originated), there were two errors
    in the X11 lockfile handling.
    
    Firstly, after reading 11 characters from the lock file (which could
    have been placed by any process), there was no guarantee of
    NUL-termination, meaning strtol could've theoretically run off the end
    of the string.
    
    Secondly, whilst writing the new lock, the trailing NUL byte was not
    correctly accounted for. The size passed as an input to snprintf takes
    the maximum size of the string including the trailing NUL, whilst the
    return (and the input to write) gives the actual size of the string
    without the trailing NUL.
    
    The code did attempt to check the return value, however snprintf returns
    the size of the _potential_ string written, before snprintf culls it, so
    this was off by one, and the LF was not being written.
    
    Signed-off-by: Daniel Stone <daniels collabora com>
    
    https://bugzilla.gnome.org/show_bug.cgi?id=774613

 src/wayland/meta-xwayland.c |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)
---
diff --git a/src/wayland/meta-xwayland.c b/src/wayland/meta-xwayland.c
index e3ab632..50cfc7c 100644
--- a/src/wayland/meta-xwayland.c
+++ b/src/wayland/meta-xwayland.c
@@ -224,6 +224,7 @@ try_display (int    display,
       close (fd);
       fd = -1;
 
+      pid[10] = '\0';
       other = strtol (pid, &end, 0);
       if (end != pid + 10)
         {
@@ -277,7 +278,7 @@ create_lock_file (int display, int *display_out)
   char *filename;
   int fd;
 
-  char pid[11];
+  char pid[12];
   int size;
   int number_of_tries = 0;
 
@@ -293,8 +294,10 @@ create_lock_file (int display, int *display_out)
     }
 
   /* Subtle detail: we use the pid of the wayland compositor, not the xserver
-   * in the lock file. */
-  size = snprintf (pid, 11, "%10d\n", getpid ());
+   * in the lock file. Another subtlety: snprintf returns the number of bytes
+   * it _would've_ written without either the NUL or the size clamping, hence
+   * the disparity in size. */
+  size = snprintf (pid, 12, "%10d\n", getpid ());
   if (size != 11 || write (fd, pid, 11) != 11)
     {
       unlink (filename);