chardev: add path option for pty backend

Add path option to the pty char backend which will create a symbolic
link to the given path that points to the allocated PTY.

This avoids having to make QMP or HMP monitor queries to find out what
the new PTY device path is.

Based on patch from Paulo Neves:

https://patchew.org/QEMU/1548509635-15776-1-git-send-email-ptsneves@gmail.com/

Tested with the following invocations that the link is created and
removed when qemu stops:

  qemu-system-x86_64 -nodefaults -mon chardev=compat_monitor \
  -chardev pty,path=test,id=compat_monitor0

  qemu-system-x86_64 -nodefaults -monitor pty:test

  # check QMP invocation with path set
  qemu-system-x86_64 -nodefaults -qmp tcp:localhost:4444,server=on,wait=off
  nc localhost 4444
  > {"execute": "qmp_capabilities"}
  > {"execute": "chardev-add", "arguments": {"id": "bar", "backend": {
      "type": "pty", "data": {"path": "test" }}}}

  # check QMP invocation with path not set
  qemu-system-x86_64 -nodefaults -qmp tcp:localhost:4444,server=on,wait=off
  nc localhost 4444
  > {"execute": "qmp_capabilities"}
  > {"execute": "chardev-add", "arguments": {"id": "bar", "backend": {
      "type": "pty", "data": {}}}}

Also tested that when a link path is not passed invocations still work, e.g.:

  qemu-system-x86_64 -monitor pty

Co-authored-by: Paulo Neves <ptsneves@gmail.com>
Signed-off-by: Paulo Neves <ptsneves@gmail.com>
[OP: rebase and address original patch review comments]
Signed-off-by: Octavian Purdila <tavip@google.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20240806010735.2450555-1-tavip@google.com>

diff --git a/chardev/char-pty.c b/chardev/char-pty.c
index cc2f7617fe7f966b3e852a0d39fb8c8c7223ec5e..cbb21b76ae8dee16335aa8e3200ba347d7304ae5 100644 (file)
--- a/chardev/char-pty.c
+++ b/chardev/char-pty.c
@@ -29,6 +29,7 @@
 #include "qemu/sockets.h"
 #include "qemu/error-report.h"
 #include "qemu/module.h"
+#include "qemu/option.h"
 #include "qemu/qemu-print.h"
 
 #include "chardev/char-io.h"
@@ -41,6 +42,7 @@ struct PtyChardev {
 
     int connected;
     GSource *timer_src;
+    char *path;
 };
 typedef struct PtyChardev PtyChardev;
 
@@ -204,6 +206,12 @@ static void char_pty_finalize(Object *obj)
     Chardev *chr = CHARDEV(obj);
     PtyChardev *s = PTY_CHARDEV(obj);
 
+    /* unlink symlink */
+    if (s->path) {
+        unlink(s->path);
+        g_free(s->path);
+    }
+
     pty_chr_state(chr, 0);
     object_unref(OBJECT(s->ioc));
     pty_chr_timer_cancel(s);
@@ -330,6 +338,7 @@ static void char_pty_open(Chardev *chr,
     int master_fd, slave_fd;
     char pty_name[PATH_MAX];
     char *name;
+    char *path = backend->u.pty.data->path;
 
     master_fd = qemu_openpty_raw(&slave_fd, pty_name);
     if (master_fd < 0) {
@@ -354,12 +363,36 @@ static void char_pty_open(Chardev *chr,
     g_free(name);
     s->timer_src = NULL;
     *be_opened = false;
+
+    /* create symbolic link */
+    if (path) {
+        int res = symlink(pty_name, path);
+
+        if (res != 0) {
+            error_setg_errno(errp, errno, "Failed to create PTY symlink");
+        } else {
+            s->path = g_strdup(path);
+        }
+    }
+}
+
+static void char_pty_parse(QemuOpts *opts, ChardevBackend *backend,
+                           Error **errp)
+{
+    const char *path = qemu_opt_get(opts, "path");
+    ChardevPty *pty;
+
+    backend->type = CHARDEV_BACKEND_KIND_PTY;
+    pty = backend->u.pty.data = g_new0(ChardevPty, 1);
+    qemu_chr_parse_common(opts, qapi_ChardevPty_base(pty));
+    pty->path = g_strdup(path);
 }
 
 static void char_pty_class_init(ObjectClass *oc, void *data)
 {
     ChardevClass *cc = CHARDEV_CLASS(oc);
 
+    cc->parse = char_pty_parse;
     cc->open = char_pty_open;
     cc->chr_write = char_pty_chr_write;
     cc->chr_update_read_handler = pty_chr_update_read_handler;

diff --git a/chardev/char.c b/chardev/char.c
index 35623c78a39bface8457dcbb40f0d8253e613f7b..c0cc52824b4828879ee08a0b3ef11745956dec0e 100644 (file)
--- a/chardev/char.c
+++ b/chardev/char.c
@@ -428,6 +428,11 @@ QemuOpts *qemu_chr_parse_compat(const char *label, const char *filename,
         qemu_opt_set(opts, "path", p, &error_abort);
         return opts;
     }
+    if (strstart(filename, "pty:", &p)) {
+        qemu_opt_set(opts, "backend", "pty", &error_abort);
+        qemu_opt_set(opts, "path", p, &error_abort);
+        return opts;
+    }
     if (strstart(filename, "tcp:", &p) ||
         strstart(filename, "telnet:", &p) ||
         strstart(filename, "tn3270:", &p) ||

diff --git a/qapi/char.json b/qapi/char.json
index 42eda4f98b42957921486000c8ea7c54a24a9a90..e045354350342b53965783474dbd686900acf37b 100644 (file)
--- a/qapi/char.json
+++ b/qapi/char.json
@@ -444,6 +444,20 @@
   'base': 'ChardevCommon',
   'if': 'CONFIG_SPICE_PROTOCOL' }
 
+##
+# @ChardevPty:
+#
+# Configuration info for pty implementation.
+#
+# @path: optional path to create a symbolic link that points to the
+#     allocated PTY
+#
+# Since: 9.2
+##
+{ 'struct': 'ChardevPty',
+  'data': { '*path': 'str' },
+  'base': 'ChardevCommon' }
+
 ##
 # @ChardevBackendKind:
 #
@@ -655,6 +669,17 @@
 { 'struct': 'ChardevRingbufWrapper',
   'data': { 'data': 'ChardevRingbuf' } }
 
+
+##
+# @ChardevPtyWrapper:
+#
+# @data: Configuration info for pty chardevs
+#
+# Since: 9.2
+##
+{ 'struct': 'ChardevPtyWrapper',
+  'data': { 'data': 'ChardevPty' } }
+
 ##
 # @ChardevBackend:
 #
@@ -675,7 +700,7 @@
             'pipe': 'ChardevHostdevWrapper',
             'socket': 'ChardevSocketWrapper',
             'udp': 'ChardevUdpWrapper',
-            'pty': 'ChardevCommonWrapper',
+            'pty': 'ChardevPtyWrapper',
             'null': 'ChardevCommonWrapper',
             'mux': 'ChardevMuxWrapper',
             'msmouse': 'ChardevCommonWrapper',

diff --git a/qemu-options.hx b/qemu-options.hx
index 20a1ce0d43b09b4bae8fc2e2725b29f4b7a2a51b..d5afefe5b63c47408999995a938f0470389a92e5 100644 (file)
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -3712,7 +3712,7 @@ DEF("chardev", HAS_ARG, QEMU_OPTION_chardev,
     "-chardev console,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
     "-chardev serial,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
 #else
-    "-chardev pty,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
+    "-chardev pty,id=id[,path=path][,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
     "-chardev stdio,id=id[,mux=on|off][,signal=on|off][,logfile=PATH][,logappend=on|off]\n"
 #endif
 #ifdef CONFIG_BRLAPI
@@ -3951,12 +3951,22 @@ The available backends are:
 
     ``path`` specifies the name of the serial device to open.
 
-``-chardev pty,id=id``
-    Create a new pseudo-terminal on the host and connect to it. ``pty``
-    does not take any options.
+``-chardev pty,id=id[,path=path]``
+    Create a new pseudo-terminal on the host and connect to it.
 
     ``pty`` is not available on Windows hosts.
 
+    If ``path`` is specified, QEMU will create a symbolic link at
+    that location which points to the new PTY device.
+
+    This avoids having to make QMP or HMP monitor queries to find out
+    what the new PTY device path is.
+
+    Note that while QEMU will remove the symlink when it exits
+    gracefully, it will not do so in case of crashes or on certain
+    startup errors. It is recommended that the user checks and removes
+    the symlink after QEMU terminates to account for this.
+
 ``-chardev stdio,id=id[,signal=on|off]``
     Connect to standard input and standard output of the QEMU process.
 
@@ -4314,8 +4324,19 @@ SRST
 
             vc:80Cx24C
 
-    ``pty``
-        [Linux only] Pseudo TTY (a new PTY is automatically allocated)
+    ``pty[:path]``
+        [Linux only] Pseudo TTY (a new PTY is automatically allocated).
+
+        If ``path`` is specified, QEMU will create a symbolic link at
+        that location which points to the new PTY device.
+
+        This avoids having to make QMP or HMP monitor queries to find
+        out what the new PTY device path is.
+
+        Note that while QEMU will remove the symlink when it exits
+        gracefully, it will not do so in case of crashes or on certain
+        startup errors. It is recommended that the user checks and
+        removes the symlink after QEMU terminates to account for this.
 
     ``none``
         No device is allocated. Note that for machine types which