Re: [PATCH v2 0/2] fuse: Fix clearing SGID when access ACL is set

[Vivek Goyal]

On Wed, Apr 14, 2021 at 01:57:01PM +0200, Miklos Szeredi wrote:
> On Thu, Mar 25, 2021 at 4:19 PM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> >
> >
> > Hi,
> >
> > This is V2 of the patchset. Posted V1 here.
> >
> > https://lore.kernel.org/linux-fsdevel/20210319195547.427371-1-vgoyal@xxxxxxxxxx/
> >
> > Changes since V1:
> >
> > - Dropped the helper to determine if SGID should be cleared and open
> >   coded it instead. I will follow up on helper separately in a different
> >   patch series. There are few places already which open code this, so
> >   for now fuse can do the same. Atleast I can make progress on this
> >   and virtiofs can enable ACL support.
> >
> > Luis reported that xfstests generic/375 fails with virtiofs. Little
> > debugging showed that when posix access acl is set that in some
> > cases SGID needs to be cleared and that does not happen with virtiofs.
> >
> > Setting posix access acl can lead to mode change and it can also lead
> > to clear of SGID. fuse relies on file server taking care of all
> > the mode changes. But file server does not have enough information to
> > determine whether SGID should be cleared or not.
> >
> > Hence this patch series add support to send a flag in SETXATTR message
> > to tell server to clear SGID.
> 
> Changed it to have a single extended structure for the request, which
> is how this has always been handled in the fuse API.
> 
> The ABI is unchanged, but you'll need to update the userspace part
> according to the API change.  Otherwise looks good.

Hi Miklos,

I started looking at ACL patches for virtiofsd again. And realized
that this SETXATTR_EXT patch, changes API. So if I update kernel
headers and recompile virtiofsd, setxattr is broken.

# setfattr -n "user.foo" -v "bar" foo.txt
setfattr: foo.txt: Numerical result out of range

I can fix it using following patch. But I am little concerned that
all the users of fuse will have to apply similar patch if they update
kernel headers (including libfuse) and recompile their app.

Is that a concern, or should we rework this so that kernel header
update does not break users.

Thanks
Vivek


---
 tools/virtiofsd/fuse_common.h   |    5 +++++
 tools/virtiofsd/fuse_lowlevel.c |    7 ++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

Index: rhvgoyal-qemu/tools/virtiofsd/fuse_lowlevel.c
===================================================================
--- rhvgoyal-qemu.orig/tools/virtiofsd/fuse_lowlevel.c	2021-06-16 17:39:16.387405071 -0400
+++ rhvgoyal-qemu/tools/virtiofsd/fuse_lowlevel.c	2021-06-17 10:22:04.879150980 -0400
@@ -1419,8 +1419,13 @@ static void do_setxattr(fuse_req_t req,
     struct fuse_setxattr_in *arg;
     const char *name;
     const char *value;
+    bool setxattr_ext = req->se->conn.want & FUSE_CAP_SETXATTR_EXT;
+
+    if (setxattr_ext)
+        arg = fuse_mbuf_iter_advance(iter, sizeof(*arg));
+    else
+        arg = fuse_mbuf_iter_advance(iter, FUSE_COMPAT_SETXATTR_IN_SIZE);
 
-    arg = fuse_mbuf_iter_advance(iter, sizeof(*arg));
     name = fuse_mbuf_iter_advance_str(iter);
     if (!arg || !name) {
         fuse_reply_err(req, EINVAL);
Index: rhvgoyal-qemu/tools/virtiofsd/fuse_common.h
===================================================================
--- rhvgoyal-qemu.orig/tools/virtiofsd/fuse_common.h	2021-06-16 17:39:16.387405071 -0400
+++ rhvgoyal-qemu/tools/virtiofsd/fuse_common.h	2021-06-17 10:24:20.905937326 -0400
@@ -373,6 +373,11 @@ struct fuse_file_info {
 #define FUSE_CAP_HANDLE_KILLPRIV_V2 (1 << 28)
 
 /**
+ * Indicates that file server supports extended struct fuse_setxattr_in
+ */
+#define FUSE_CAP_SETXATTR_EXT (1 << 29)
+
+/**
  * Ioctl flags
  *
  * FUSE_IOCTL_COMPAT: 32bit compat ioctl on 64bit machine