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

[Greg Kurz <groug@kaod.org>]

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 "./" ;-)

> +        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.

> +    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:

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

> +    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);