Re: [RFC PATCH] linux-user/syscall: Use g_file_open_tmp()

[Ján Tomko]

On a Thursday in 2020, Daniel P. Berrangé wrote:
> On Thu, Feb 27, 2020 at 11:06:21AM +0100, Philippe Mathieu-Daudé wrote:
> > Use GLib g_file_open_tmp() instead of getenv + snprintf + mkstemp.
> >
> > Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
> > ---
> > RFC because I'm not sure g_autoptr(GError) works this way.
>
> It does work. Any struct that's defined in GLib has support for
> g_autoptr(). If you aren't suyre though, just check for a
> G_DEFINE_AUTOPTR_CLEANUP_FUNC() macro usage that refers to the
> struct in question
>
> $ grep -r 'G_DEFINE_AUTOPTR_CLEANUP_FUNC(GError' /usr/include/glib-2.0
> /usr/include/glib-2.0/glib/glib-autocleanups.h:G_DEFINE_AUTOPTR_CLEANUP_FUNC(GError,
>  g_error_free)
>
> >  linux-user/syscall.c | 11 ++++-------
> >  1 file changed, 4 insertions(+), 7 deletions(-)
> >
> > diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> > index 8d27d10807..0e44969e16 100644
> > --- a/linux-user/syscall.c
> > +++ b/linux-user/syscall.c
> > @@ -7282,17 +7282,14 @@ static int do_openat(void *cpu_env, int dirfd, const 
> > char *pathname, int flags,
> >      }
> >
> >      if (fake_open->filename) {
> > -        const char *tmpdir;
> > -        char filename[PATH_MAX];
> > +        g_autoptr(GError) gerr = NULL;
> > +        g_autofree gchar *filename = NULL;
> >          int fd, r;
> >
> >          /* create temporary file to map stat to */
> > -        tmpdir = getenv("TMPDIR");
> > -        if (!tmpdir)
> > -            tmpdir = "/tmp";
> > -        snprintf(filename, sizeof(filename), "%s/qemu-open.XXXXXX", tmpdir);
> > -        fd = mkstemp(filename);
> > +        fd = g_file_open_tmp("qemu-open.XXXXXX", &filename, &gerr);
>
> g_file_open_tmp, calls g_get_tmp_name, which calls
> g_get_tmp_dir, which defaults to $TMPDIR, falling back
> to /tmp. So we're using the same dir as before.
>
> >          if (fd < 0) {
> > +            fprintf(stderr, "Error opening %s: %s\n", filename, gerr->message);
>
> This is wrong - the returned "filename" is only valid when
> g_file_open_tmp succeeds. So the use of "filename" here
> is likely a NULL. Given that the only place you use "filename"
> is in the error path, and that's not valid, we can simply
> eliminate it entirely, and pass NULL into g_file_open_tmp

'filename' is used right below to unlink it.
But it can be dropped from the error message here since it's included in the
error reported by GLib.

Jano

> >              return fd;
> >          }
> >          unlink(filename);
>
> Regards,
> Daniel
> --
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