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

[Greg Kurz]

On Fri, 29 Apr 2022 19:28:39 +0800
Bin Meng <bmeng.cn@gmail.com> 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:
> 20220425142705.2099270-10-bmeng.cn@gmail.com/">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?
> 

The 9p2000.L protocol is tailored for a linux client. As mentioned in
the changelog, the spec explicitly mentions 'numerical linux errno'.
A macOS client should thus convert back errno in a Rlerror message
to a macOS errno.

> > +#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