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

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Donnerstag, 21. April 2022 14:26:37 CEST Greg Kurz wrote:
> 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.

Got it. I tend to go with the former (checking errno == ENOATTR and defining 
ENOATTR if non existent). I find that a bit cleaner than the latter which 
would have the potential to mask another error (ENODATA).

> > 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