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Disrecommend the use of display socket name wayland-0

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Connecting by default to wayland-0 when environment variables
WAYLAND_DISPLAY/WAYLAND_SOCKET are not set can cause unexpected
connections when more than one session is being used. Specifically, if you
run a program from a non-Wayland context (over ssh, on a VT, in an X11
session, from a job scheduler), while some unrelated compositor is
listening on wayland-0, the program may connect to the compositor and
leave the user wondering what has happened.

This commit adjusts the documentation to clarify that the wayland-0
fallback is kept "for compatibility reasons", implying that in typical
use programs do not rely on it. The changes to the wayland-server
documentation (+ publican docs) briefly explain why compositors should
avoid providing wayland-0.

To avoid breaking existing software, the fallback to wayland-0 has not
been removed (and perhaps never will be).

As part of this change, the wl_display_add_socket_auto function has been
documented to describe its action as well as the recommended alternative
of connecting to the first available socket in the sequence wayland-1,
wayland-2, etc.

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>
This commit is contained in:
Manuel Stoeckl 2021-02-07 14:35:28 -05:00
parent ddf21afa6e
commit 8f518f958a
3 changed files with 50 additions and 5 deletions
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9
doc/publican/sources/Protocol.xml
View file

@ -92,9 +92,12 @@
<title>Wire Format</title>
<para>
The protocol is sent over a UNIX domain stream socket, where the endpoint
usually is named <systemitem class="service">wayland-0</systemitem>
(although it can be changed via <emphasis>WAYLAND_DISPLAY</emphasis>
in the environment). Beginning in Wayland 1.15, implementations can
is specified by the environment variable <emphasis>WAYLAND_DISPLAY</emphasis>.
(For compatibility reasons, if this environment variable is not set,
clients may try to connect to socket <systemitem class="service">wayland-0</systemitem>.
As this behavior may lead to accidental connections,
compositors are recommended never to set <emphasis>WAYLAND_DISPLAY</emphasis>
to this value.) Beginning in Wayland 1.15, implementations can
optionally support server socket endpoints located at arbitrary
locations in the filesystem by setting <emphasis>WAYLAND_DISPLAY</emphasis>
to the absolute path at which the server endpoint listens.

3
src/wayland-client.c
View file

@ -1111,7 +1111,8 @@ wl_display_connect_to_fd(int fd)
*
* Connect to the Wayland display named \c name. If \c name is \c NULL,
* its value will be replaced with the WAYLAND_DISPLAY environment
* variable if it is set, otherwise display "wayland-0" will be used.
* variable if it is set. For compatibility reasons, if WAYLAND_DISPLAY
* is not set, the value "wayland-0" will be used.
*
* If WAYLAND_SOCKET is set, it's interpreted as a file descriptor number
* referring to an already opened socket. In this case, the socket is used

43
src/wayland-server.c
View file

@ -1549,6 +1549,41 @@ _wl_display_add_socket(struct wl_display *display, struct wl_socket *s)
return 0;
}
/** Automatically pick a Wayland display socket for the clients to connect to.
*
* \param display Wayland display to which the socket should be added.
* \return The socket name if success. NULL if failed.
*
* This function is equivalent to calling, for %d ranging from 0 to 32,
* the function:
*
* \code{.c}
* wl_display_add_socket(display, "wayland-%d")
* \endcode
*
* and returning the name of the new socket as soon as a call succeeds.
* The string returned should not be modified or freed.
*
* Because this function first tries to register the Wayland display
* socket name "wayland-0", and clients (for compatibility reasons) try
* to connect to "wayland-0" if the environment variable WAYLAND_DISPLAY
* is not set, clients may connect by accident. As a result, this function
* is NOT recommended. Instead, compositors should explicitly
* try socket names starting at e.g. wayland-1; for example:
*
* \code{.c}
* for (int i = 1; i <= 32; i++) {
* char display_name[16] = "";
* snprintf(display_name, sizeof display_name, "wayland-%d", i);
* if (wl_display_add_socket(display, display_name) == 0) {
* return strdup(display_name);
* }
* }
* return NULL;
* \endcode
*
* \memberof wl_display
*/
WL_EXPORT const char *
wl_display_add_socket_auto(struct wl_display *display)
{
@ -1643,7 +1678,13 @@ wl_display_add_socket_fd(struct wl_display *display, int sock_fd)
*
* If NULL is passed as name, then it would look for WAYLAND_DISPLAY env
* variable for the socket name. If WAYLAND_DISPLAY is not set, then default
* wayland-0 is used.
* wayland-0 is used. This fallback, kept for compatibility reasons,
* has two problems: 1) most clients will by default connect to wayland-0
* if their environment does not contain WAYLAND_DISPLAY, and thus may
* connect to this socket by accident when run outside of a GUI; 2) some
* clients do not by default connect to wayland-0, and always require
* WAYLAND_DISPLAY to be set. Consequently, passing NULL in as an argument
* to this function is not recommended.
*
* If the socket name is a relative path, the Unix socket will be created in
* the directory pointed to by environment variable XDG_RUNTIME_DIR. If