Re: [PATCH v2] Fix exec migration on Windows (w32+w64).

[Daniel P . Berrangé]

On Mon, Jan 16, 2023 at 11:17:08AM +0400, Marc-André Lureau wrote:
> Hi
> 
> On Mon, Jan 16, 2023 at 5:35 AM John Berberian, Jr <jeb.study@gmail.com> 
> wrote:
> >
> > * Use cmd instead of /bin/sh on Windows.
> >
> > * Try to auto-detect cmd.exe's path, but default to a hard-coded path.
> >
> > Note that this will require that gspawn-win[32|64]-helper.exe and
> > gspawn-win[32|64]-helper-console.exe are included in the Windows binary
> > distributions (cc: Stefan Weil).
> >
> > Signed-off-by: "John Berberian, Jr" <jeb.study@gmail.com>
> > ---
> > Whoops, forgot a header. Here's a revised patch.
> >
> >  migration/exec.c | 24 ++++++++++++++++++++++++
> >  1 file changed, 24 insertions(+)
> >
> > diff --git a/migration/exec.c b/migration/exec.c
> > index 375d2e1b54..38604d73a6 100644
> > --- a/migration/exec.c
> > +++ b/migration/exec.c
> > @@ -23,12 +23,31 @@
> >  #include "migration.h"
> >  #include "io/channel-command.h"
> >  #include "trace.h"
> > +#include "qemu/cutils.h"
> >
> > +#ifdef WIN32
> > +const char *exec_get_cmd_path(void);
> > +const char *exec_get_cmd_path(void)
> > +{
> > +    g_autofree char *detected_path = g_new(char, MAX_PATH);
> > +    if (GetSystemDirectoryA(detected_path, MAX_PATH) == 0) {
> > +        warn_report("Could not detect cmd.exe path, using default.");
> > +        return "C:\\Windows\\System32\\cmd.exe";
> > +    }
> > +    pstrcat(detected_path, MAX_PATH, "\\cmd.exe");
> > +    return g_steal_pointer(&detected_path);
> > +}
> > +#endif
> >
> >  void exec_start_outgoing_migration(MigrationState *s, const char *command, 
> > Error **errp)
> >  {
> >      QIOChannel *ioc;
> > +
> > +#ifdef WIN32
> > +    const char *argv[] = { exec_get_cmd_path(), "/c", command, NULL };
> > +#else
> >      const char *argv[] = { "/bin/sh", "-c", command, NULL };
> > +#endif
> 
> It may be a better idea to use g_shell_parse_argv() instead.

For non-Windows that would not be compatible though.  eg if someone is
currently using shell redirection or pipelined commands:

  migrate  "exec:dd of=/foo 1>/dev/null 2>&1"

When we introduce a new QAPI format for migration args though, I've
suggested we drop support for passing exec via shell, and require an
explicit argv[] array:

  https://lists.gnu.org/archive/html/qemu-devel/2023-01/msg01434.html

For Windows since we don't have back compat to worry about, we
can avoid passing via cmd.exe from the start.

With regards,
Daniel
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