Re: [PATCH 5/5] 9pfs: fix removing non-existent POSIX ACL xattr on macOS host

[Greg Kurz <groug@kaod.org>]

On Thu, 21 Apr 2022 12:55:24 +0200
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> On Donnerstag, 21. April 2022 10:26:11 CEST Greg Kurz wrote:
> > On Tue, 19 Apr 2022 13:43:30 +0200
> > 
> > Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:
> > > When mapped POSIX ACL is used, we are ignoring errors when trying
> > > to remove a POSIX ACL xattr that does not exist. On Linux hosts we
> > > would get ENODATA in such cases, on macOS hosts however we get
> > > ENOATTR instead, so ignore ENOATTR errors as well.
> > > 
> > > This patch fixes e.g. a command on Linux guest like:
> > >   cp --preserve=mode old new
> > > 
> > > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > > ---
> > > 
> > >  hw/9pfs/9p-posix-acl.c | 8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/hw/9pfs/9p-posix-acl.c b/hw/9pfs/9p-posix-acl.c
> > > index eadae270dd..2bf155f941 100644
> > > --- a/hw/9pfs/9p-posix-acl.c
> > > +++ b/hw/9pfs/9p-posix-acl.c
> > > @@ -65,7 +65,13 @@ static int mp_pacl_removexattr(FsContext *ctx,
> > > 
> > >      int ret;
> > >      
> > >      ret = local_removexattr_nofollow(ctx, path, MAP_ACL_ACCESS);
> > > 
> > > -    if (ret == -1 && errno == ENODATA) {
> > > +    if (ret == -1 &&
> > > +          (errno == ENODATA
> > > +#ifdef ENOATTR
> > > +          || errno == ENOATTR
> > > +#endif
> > > +          )
> > 
> > We already have this in <qemu/xattr.h> which is included by
> > 9p-posix-acl.c :
> > 
> > /*
> >  * Modern distributions (e.g. Fedora 15), have no libattr.so, place attr.h
> >  * in /usr/include/sys, and don't have ENOATTR.
> >  */
> > 
> > 
> > #ifdef CONFIG_LIBATTR
> > #  include <attr/xattr.h>
> > #else
> > #  if !defined(ENOATTR)
> > #    define ENOATTR ENODATA
> > #  endif
> > #  include <sys/xattr.h>
> > #endif
> > 
> > I guess this patch could just s/ENODATA/ENOATTR/ to avoid the
> > extra ifdefery.
> 
> Not viable, because macOS does have both ENODATA==96 and ENOATTR==93. On Linux 
> the two macros were historically defined to the same numeric values, that's 
> why it worked there.
> 

I was meaning your current fix could simply do:

-    if (ret == -1 && errno == ENODATA) {
+    if (ret == -1 && errno == ENOATTR) {

since ENOATTR works in all cases, but this is rather a hack.

Another solution would be to ensure that local_removexattr_nofollow() only
reports linux errnos. This could be handled cleanly in the
fremovexattrat_nofollow() implementation in 9p-util-darwin.c.

Since the 9p code base mostly assumes the host is linux, this should
probably be generalized to other places where we check errno.

> Maybe I should define a separate macro like:
> 
> #if ...
> # define P9_ENOATTR ENOATTR
> #else
> # define P9_ENOATTR ENODATA
> #end
> 
> ?
> 
> Actually good that you pointed me at this, because I just realized there is a 
> 2nd place in 9p-posix-acl.c which would require this as well. For some reason 
> the 2nd place just did not trigger while I was testing it on macOS.
> 
> Best regards,
> Christian Schoenebeck
> 
>