Re: [PATCH v4 2/6] 9pfs: fix qemu_mknodat(S_IFSOCK) on macOS

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Mittwoch, 27. April 2022 22:36:25 CEST Greg Kurz wrote:
> On Wed, 27 Apr 2022 20:54:17 +0200
> 
> Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:
> > mknod() on macOS does not support creating sockets, so divert to
> > call sequence socket(), bind() and fchmodat() respectively if S_IFSOCK
> > was passed with mode argument.
> > 
> > Link: https://lore.kernel.org/qemu-devel/17933734.zYzKuhC07K@silver/
> > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > ---
> > 
> >  hw/9pfs/9p-util-darwin.c | 45 +++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 44 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/9pfs/9p-util-darwin.c b/hw/9pfs/9p-util-darwin.c
> > index e24d09763a..7d00db47a9 100644
> > --- a/hw/9pfs/9p-util-darwin.c
> > +++ b/hw/9pfs/9p-util-darwin.c
> > @@ -74,6 +74,45 @@ int fsetxattrat_nofollow(int dirfd, const char
> > *filename, const char *name,> 
> >   */
> >  
> >  #if defined CONFIG_PTHREAD_FCHDIR_NP
> > 
> > +static int create_socket_file_at_cwd(const char *filename, mode_t mode) {
> > +    int fd, err;
> > +    struct sockaddr_un addr = {
> > +        .sun_family = AF_UNIX
> > +    };
> > +
> > +    /*
> > +     * sun_path is only 104 bytes, explicit filename length check
> > required
> > +     */
> > +    if (sizeof(addr.sun_path) - 1 < strlen(filename) + 2) {
> 
> True but I was a bit puzzled by the math until I realized the '+ 2' was
> for the prepended "./" ;-)

Correct ...

> > +        errno = ENAMETOOLONG;
> > +        return -1;
> > +    }
> > +    fd = socket(PF_UNIX, SOCK_DGRAM, 0);
> > +    if (fd == -1) {
> > +        return fd;
> > +    }
> > +    snprintf(addr.sun_path, sizeof(addr.sun_path), "./%s", filename);
> 
> What about the more generic approach of checking snprintf()'s return
> value ? If it is >= sizeof(addr.sun_path) then truncation occured.

... well, I can send a v5 if you prefer that solution, or you can send a follow-up
patch later on. As you wish.

> > +    err = bind(fd, (struct sockaddr *) &addr, sizeof(addr));
> > +    if (err == -1) {
> > +        goto out;
> > +    }
> > +    /*
> > +     * FIXME: Should rather be using descriptor-based fchmod() on the
> > +     * socket file descriptor above (preferably before bind() call),
> > +     * instead of path-based fchmodat(), to prevent concurrent transient
> > +     * state issues between creating the named FIFO file at bind() and
> > +     * delayed adjustment of permissions at fchmodat(). However currently
> > +     * macOS (12.x) does not support such operations on socket file
> > +     * descriptors yet.
> > +     *
> > +     * Filed report with Apple: FB9997731
> > +     */
> > +    err = fchmodat(AT_FDCWD, filename, mode, AT_SYMLINK_NOFOLLOW_ANY);
> > +out:
> > +    close_preserve_errno(fd);
> 
> You could close(fd) earlier now, but you might want to keep the code
> as is in case FB9997731 gets proper attention.
> 
> Anyway, this should do the job so:

Sounds like Akihiko's previous suggestion. I would keep it that way:
https://lore.kernel.org/qemu-devel/eafd4bbf-dbff-323a-179f-8f29905701e1@gmail.com/

> Reviewed-by: Greg Kurz <groug@kaod.org>

Thanks!

Best regards,
Christian Schoenebeck

> > +    return err;
> > +}
> > +
> > 
> >  int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev)
> >  {
> >  
> >      int preserved_errno, err;
> > 
> > @@ -93,7 +132,11 @@ int qemu_mknodat(int dirfd, const char *filename,
> > mode_t mode, dev_t dev)> 
> >      if (pthread_fchdir_np(dirfd) < 0) {
> >      
> >          return -1;
> >      
> >      }
> > 
> > -    err = mknod(filename, mode, dev);
> > +    if (S_ISSOCK(mode)) {
> > +        err = create_socket_file_at_cwd(filename, mode);
> > +    } else {
> > +        err = mknod(filename, mode, dev);
> > +    }
> > 
> >      preserved_errno = errno;
> >      /* Stop using the thread-local cwd */
> >      pthread_fchdir_np(-1);