Re: [PATCH v2 4/5] 9pfs: fix wrong errno being sent to Linux client on macOS host

[Greg Kurz]

On Thu, 21 Apr 2022 17:07:49 +0200
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> Linux and macOS only share some errno definitions with equal macro
> name and value. In fact most mappings for errno are completely
> different on the two systems.
> 
> This patch converts some important errno values from macOS host to
> corresponding Linux errno values before eventually sending such error
> codes along with 'Rlerror' replies (if 9p2000.L is used that is). Not
> having translated errnos before violated the 9p2000.L protocol spec,
> which says:
> 
>   "
>   size[4] Rlerror tag[2] ecode[4]
> 
>   ... ecode is a numerical Linux errno.
>   "
> 
>   https://github.com/chaos/diod/wiki/protocol#lerror----return-error-code
> 
> This patch fixes a bunch of misbehaviours when running a Linux client
> on macOS host. For instance this patch fixes:
> 
>   mount -t 9p -o posixacl ...
> 
> on Linux guest if security_mode=mapped was used for 9p server, which
> refused to mount successfully, because macOS returned ENOATTR==93
> when client tried to retrieve POSIX ACL xattrs, because errno 93
> is defined as EPROTONOSUPPORT==93 on Linux, so Linux client believed
> that xattrs were not supported by filesystem on host in general.
> 
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> Link: https://lore.kernel.org/qemu-devel/20220421124835.3e664669@bahia/
> ---

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

>  hw/9pfs/9p-util.h | 30 ++++++++++++++++++++++++++++++
>  hw/9pfs/9p.c      |  2 ++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h
> index 2cc9a5dbfb..c3526144c9 100644
> --- a/hw/9pfs/9p-util.h
> +++ b/hw/9pfs/9p-util.h
> @@ -58,6 +58,36 @@ static inline uint64_t host_dev_to_dotl_dev(dev_t dev)
>  #endif
>  }
>  
> +/* Translates errno from host -> Linux if needed */
> +static inline int errno_to_dotl(int err) {
> +#if defined(CONFIG_LINUX)
> +    /* nothing to translate (Linux -> Linux) */
> +#elif defined(CONFIG_DARWIN)
> +    /*
> +     * translation mandatory for macOS hosts
> +     *
> +     * FIXME: Only most important errnos translated here yet, this should be
> +     * extended to as many errnos being translated as possible in future.
> +     */
> +    if (err == ENAMETOOLONG) {
> +        err = 36; /* ==ENAMETOOLONG on Linux */
> +    } else if (err == ENOTEMPTY) {
> +        err = 39; /* ==ENOTEMPTY on Linux */
> +    } else if (err == ELOOP) {
> +        err = 40; /* ==ELOOP on Linux */
> +    } else if (err == ENOATTR) {
> +        err = 61; /* ==ENODATA on Linux */
> +    } else if (err == ENOTSUP) {
> +        err = 95; /* ==EOPNOTSUPP on Linux */
> +    } else if (err == EOPNOTSUPP) {
> +        err = 95; /* ==EOPNOTSUPP on Linux */
> +    }
> +#else
> +#error Missing errno translation to Linux for this host system
> +#endif
> +    return err;
> +}
> +
>  #ifdef CONFIG_DARWIN
>  #define qemu_fgetxattr(...) fgetxattr(__VA_ARGS__, 0, 0)
>  #define qemu_lgetxattr(...) getxattr(__VA_ARGS__, 0, XATTR_NOFOLLOW)
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index 4a296a0b94..0cd0c14c2a 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -1054,6 +1054,8 @@ static void coroutine_fn pdu_complete(V9fsPDU *pdu, 
> ssize_t len)
>              }
>              len += ret;
>              id = P9_RERROR;
> +        } else {
> +            err = errno_to_dotl(err);
>          }
>  
>          ret = pdu_marshal(pdu, len, "d", err);