chardev: Allow setting file chardev input file on the command line

Our 'file' chardev backend supports both "output from this chardev
is written to a file" and "input from this chardev should be read
from a file" (except on Windows). However, you can only set up
the input file if you're using the QMP interface -- there is no
command line syntax to do it.

Add command line syntax to allow specifying an input file
as well as an output file, using a new 'input-path' suboption.

The specific use case I have is that I'd like to be able to
feed fuzzer reproducer input into qtest without having to use
'-qtest stdio' and put the input onto stdin. Being able to
use a file chardev like this:
 -chardev file,id=repro,path=/dev/null,input-path=repro.txt -qtest chardev:repro
means that stdio is free for use by gdb.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230413150724.404304-3-peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
[thuth: Replace "input-file=" typo with "input-path="]
Signed-off-by: Thomas Huth <thuth@redhat.com>

chardev/char-file.c

@@ -100,6 +100,7 @@ static void qemu_chr_parse_file_out(QemuOpts *opts, ChardevBackend *backend,
                                     Error **errp)
 {
     const char *path = qemu_opt_get(opts, "path");
+    const char *inpath = qemu_opt_get(opts, "input-path");
     ChardevFile *file;
 
     backend->type = CHARDEV_BACKEND_KIND_FILE;
@@ -107,9 +108,16 @@ static void qemu_chr_parse_file_out(QemuOpts *opts, ChardevBackend *backend,
         error_setg(errp, "chardev: file: no filename given");
         return;
     }
+#ifdef _WIN32
+    if (inpath) {
+        error_setg(errp, "chardev: file: input-path not supported on Windows");
+        return;
+    }
+#endif
     file = backend->u.file.data = g_new0(ChardevFile, 1);
     qemu_chr_parse_common(opts, qapi_ChardevFile_base(file));
     file->out = g_strdup(path);
+    file->in = g_strdup(inpath);
 
     file->has_append = true;
     file->append = qemu_opt_get_bool(opts, "append", false);

chardev/char.c

@@ -805,6 +805,9 @@ QemuOptsList qemu_chardev_opts = {
         },{
             .name = "path",
             .type = QEMU_OPT_STRING,
+        },{
+            .name = "input-path",
+            .type = QEMU_OPT_STRING,
         },{
             .name = "host",
             .type = QEMU_OPT_STRING,

qemu-options.hx

@@ -3360,7 +3360,7 @@ DEF("chardev", HAS_ARG, QEMU_OPTION_chardev,
     "-chardev vc,id=id[[,width=width][,height=height]][[,cols=cols][,rows=rows]]\n"
     "         [,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
     "-chardev ringbuf,id=id[,size=size][,logfile=PATH][,logappend=on|off]\n"
-    "-chardev file,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
+    "-chardev file,id=id,path=path[,input-path=input-file][,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
     "-chardev pipe,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
 #ifdef _WIN32
     "-chardev console,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
@@ -3563,13 +3563,19 @@ The available backends are:
     Create a ring buffer with fixed size ``size``. size must be a power
     of two and defaults to ``64K``.
 
-``-chardev file,id=id,path=path``
+``-chardev file,id=id,path=path[,input-path=input-path]``
     Log all traffic received from the guest to a file.
 
     ``path`` specifies the path of the file to be opened. This file will
     be created if it does not already exist, and overwritten if it does.
     ``path`` is required.
 
+    If ``input-path`` is specified, this is the path of a second file
+    which will be used for input. If ``input-path`` is not specified,
+    no input will be available from the chardev.
+
+    Note that ``input-path`` is not supported on Windows hosts.
+
 ``-chardev pipe,id=id,path=path``
     Create a two-way connection to the guest. The behaviour differs
     slightly between Windows hosts and other hosts: