Re: [PATCH v5 4/6] 9pfs: fix wrong errno being sent to Linux client on macOS host

[Greg Kurz]

On Fri, 29 Apr 2022 14:46:26 +0200
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> On Freitag, 29. April 2022 13:28:39 CEST Bin Meng wrote:
> > On Fri, Apr 29, 2022 at 7:16 PM 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.
> > 
> > This issue looks exact the same issue we were trying to fix when
> > supporting 9p on Windows host,
> > 
> > What we did is like this:
> > http://patchwork.ozlabs.org/project/qemu-devel/patch/20220425142705.2099270->
> >  10-bmeng.cn@gmail.com/
> > 
> > But we had some questions in mind (see the commit message of our
> > patch, and below)
> > 
> > > 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 */
> > > +    }
> > 
> > What happens if a macOS guest is running on QEMU from a macOS host?
> > Here all macOS errnos are translated to the Linux errnos. Will macOS
> > be happy?
> 
> Look at the commit log of this patch: it is a matter of which protocol is 
> used 
> (currently there are 3 [1] protocol versions):
> 
>    * 9p2000: nothing to translate here, as this protocol version does not
>      return a numeric error code, it only returns an error string (and we are 
>      no longer supporting 9p2000 version in QEMU anyway BTW [1]):
>      http://ericvh.github.io/9p-rfc/rfc9p2000.html#anchor27
> 
>    * 9p2000.L: errno *must* be in Linux errno mapping:
>      https://github.com/chaos/diod/wiki/protocol#lerror----return-error-code
> 
>    * 9p2000.u: this one returns both an error code and error string, and it 
>      says the error string should be preferred being interpreted by client:
>      http://ericvh.github.io/9p-rfc/rfc9p2000.u.html#anchor15
> 
> In this patch here I only translated errno for 9p2000.L, whereas you are 
> always translating it, no matter wich protocol version is used. You might 
> argue that there should be a translation for 9p2000.u as well, but in the end 
> we don't know the OS running on guest in this case. It could be Linux or 
> something else.
> 

In the case of 9p2000.u the spec says "to provide a hint of the underlying
UNIX error number which caused the error on the server" and even mentions
"consistency problems of mapping error numbers betweeen different versions
of UNIX"... this basically means that errno in 9p2000.u is undefined since
it depends on the host. It is thus unusable unless the guest runs a compatible
UNIX variant. In any case, there's really nothing to translate.

> [1] https://wiki.qemu.org/Documentation/9p#9p_Protocol
> 
> Best regards,
> Christian Schoenebeck
> 
> > 
> > > +#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);
> > > 
> > > --
> > 
> > Regards,
> > Bin
> 
>