[Virtio-fs] [PATCH] virtiofsd: Disable killpriv_v2 by default

[Vivek Goyal <vgoyal@redhat.com>]

We are having bunch of issues with killpriv_v2 enabled by default. First
of all it relies on clearing suid/sgid bits as needed by dropping
capability CAP_FSETID. This does not work for remote filesystems like
NFS (and possibly others). 

Secondly, we are noticing other issues related to clearing of SGID
which leads to failures for xfstests generic/355 and generic/193.

Thirdly, there are other issues w.r.t caching of metadata (suid/sgid)
bits in fuse client with killpriv_v2 enabled. Guest can cache that
data for sometime even if cleared on server.

Second and Third issue are fixable. Just that it might take a little
while to get it fixed in kernel. First one will probably not see
any movement for a long time.

Given these issues, killpriv_v2 does not seem to be a good candidate
for enabling by default. We have already disabled it by default in
rust version of virtiofsd.

Hence this patch disabled killpriv_v2 by default. User can choose to
enable it by passing option "-o killpriv_v2".

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 tools/virtiofsd/passthrough_ll.c |   13 ++-----------
 1 file changed, 2 insertions(+), 11 deletions(-)

Index: rhvgoyal-qemu/tools/virtiofsd/passthrough_ll.c
===================================================================
--- rhvgoyal-qemu.orig/tools/virtiofsd/passthrough_ll.c	2022-07-29 08:19:05.925119947 -0400
+++ rhvgoyal-qemu/tools/virtiofsd/passthrough_ll.c	2022-07-29 08:27:08.048049096 -0400
@@ -767,19 +767,10 @@ static void lo_init(void *userdata, stru
         fuse_log(FUSE_LOG_DEBUG, "lo_init: enabling killpriv_v2\n");
         conn->want |= FUSE_CAP_HANDLE_KILLPRIV_V2;
         lo->killpriv_v2 = 1;
-    } else if (lo->user_killpriv_v2 == -1 &&
-               conn->capable & FUSE_CAP_HANDLE_KILLPRIV_V2) {
-        /*
-         * User did not specify a value for killpriv_v2. By default enable it
-         * if connection offers this capability
-         */
-        fuse_log(FUSE_LOG_DEBUG, "lo_init: enabling killpriv_v2\n");
-        conn->want |= FUSE_CAP_HANDLE_KILLPRIV_V2;
-        lo->killpriv_v2 = 1;
     } else {
         /*
-         * Either user specified to disable killpriv_v2, or connection does
-         * not offer this capability. Disable killpriv_v2 in both the cases
+         * Either user specified to disable killpriv_v2, or did not
+         * specify anything. Disable killpriv_v2 in both the cases.
          */
         fuse_log(FUSE_LOG_DEBUG, "lo_init: disabling killpriv_v2\n");
         conn->want &= ~FUSE_CAP_HANDLE_KILLPRIV_V2;