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

[Greg Kurz]

On Tue, 19 Apr 2022 13:41:59 +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 Tlerror replies (if 9p2000.L is used that is), which
> fixes a bunch of misbehaviours when running a Linux client on macOS
> host.
> 

This even fixes an actual protocol violation :

lerror -- return error code

size[4] Rlerror tag[2] ecode[4]

lerror replaces the reply message used in a successful call. ecode is a
numerical Linux errno.
^^^^^^^^^^^^^^^^^^^^^

Taken from 
https://github.com/chaos/diod/wiki/protocol#lerror----return-error-code


> 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>
> ---
>  hw/9pfs/9p.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index d953035e1c..becc41cbfd 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -57,6 +57,31 @@ enum {
>  
>  P9ARRAY_DEFINE_TYPE(V9fsPath, v9fs_path_free);
>  
> +/* Translates errno from host -> Linux if needed */
> +static int errno_to_dotl(int err) {
> +#if defined(CONFIG_LINUX)
> +    /* nothing to translate (Linux -> Linux) */
> +#elif defined(CONFIG_DARWIN)
> +    /* translation mandatory for macOS hosts */
> +    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 */
> +    }

I'm assuming you have audited all errnos, right ? Just to be sure
that this won't bite anymore.

> +#else
> +#error Missing errno translation to Linux for this host system
> +#endif
> +    return err;
> +}
> +

As with the other patch, I'd rather move this magic to 9p-util.h .

>  static ssize_t pdu_marshal(V9fsPDU *pdu, size_t offset, const char *fmt, ...)
>  {
>      ssize_t ret;
> @@ -1054,6 +1079,8 @@ 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);