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

["Daniel P. Berrangé" <berrange@redhat.com>]

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 <philmd@redhat.com>
> ---
> 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

>              return fd;
>          }
>          unlink(filename);

Regards,
Daniel
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