Re: [PATCH v2.1 1/3] virtiofsd: Add an option to enable/disable posix acls

[Vivek Goyal <vgoyal@redhat.com>]

fuse has an option FUSE_POSIX_ACL which needs to be opted in by fuse
server to enable posix acls. 

Add virtiofsd option "-o posix_acl/no_posix_acl" to let users enable/disable
posix acl support. By default it is disabled as of now.

Currently even if file server has not opted in for FUSE_POSIX_ACL, user can
still query acl and set acl, and system.posix_acl_access and
system.posix_acl_default xattrs show up listxattr response. 

Miklos said this is confusing. So he said lets block and filter
system.posix_acl_access and system.posix_acl_default xattrs in
getxattr/setxattr/listxattr if user has explicitly disabled
posix acls using -o no_posix_acl.

As of now continuing to keeping the existing behavior if user did not
specify any option to disable acl support due to concerns about backward
compatibility. 

v2.1: Fixed lo_removexattr()

v2: block system.posix_acl_access and system.posix_acl_default xattrs
    if user explicitly disabled acls. (Miklos)

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 tools/virtiofsd/passthrough_ll.c |  100 ++++++++++++++++++++++++++++++++++++++-
 1 file changed, 99 insertions(+), 1 deletion(-)

Index: rhvgoyal-qemu/tools/virtiofsd/passthrough_ll.c
===================================================================
--- rhvgoyal-qemu.orig/tools/virtiofsd/passthrough_ll.c	2021-02-18 13:12:26.839770339 -0500
+++ rhvgoyal-qemu/tools/virtiofsd/passthrough_ll.c	2021-02-18 13:38:22.344930118 -0500
@@ -169,6 +169,7 @@ struct lo_data {
     /* An O_PATH file descriptor to /proc/self/fd/ */
     int proc_self_fd;
     int user_killpriv_v2, killpriv_v2;
+    int user_posix_acl;
 };
 
 static const struct fuse_opt lo_opts[] = {
@@ -201,6 +202,8 @@ static const struct fuse_opt lo_opts[] =
     { "announce_submounts", offsetof(struct lo_data, announce_submounts), 1 },
     { "killpriv_v2", offsetof(struct lo_data, user_killpriv_v2), 1 },
     { "no_killpriv_v2", offsetof(struct lo_data, user_killpriv_v2), 0 },
+    { "posix_acl", offsetof(struct lo_data, user_posix_acl), 1 },
+    { "no_posix_acl", offsetof(struct lo_data, user_posix_acl), 0 },
     FUSE_OPT_END
 };
 static bool use_syslog = false;
@@ -661,6 +664,23 @@ static void lo_init(void *userdata, stru
         conn->want &= ~FUSE_CAP_HANDLE_KILLPRIV_V2;
         lo->killpriv_v2 = 0;
     }
+
+    if (lo->user_posix_acl == 1) {
+        /*
+         * User explicitly asked for this option. Enable it unconditionally.
+         * If connection does not have this capability, it should fail
+         * in fuse_lowlevel.c
+         */
+        fuse_log(FUSE_LOG_DEBUG, "lo_init: enabling posix acl\n");
+        conn->want |= FUSE_CAP_POSIX_ACL;
+    } else {
+        /*
+         * Either user specified to disable posix_acl, or did not specify
+         * anything. In both the cases do not enable posix acl.
+         */
+        fuse_log(FUSE_LOG_DEBUG, "lo_init: disabling posix_acl\n");
+        conn->want &= ~FUSE_CAP_POSIX_ACL;
+    }
 }
 
 static void lo_getattr(fuse_req_t req, fuse_ino_t ino,
@@ -2612,6 +2632,63 @@ static int xattr_map_server(const struct
     return -ENODATA;
 }
 
+static bool block_xattr(struct lo_data *lo, const char *name)
+{
+    /*
+     * If user explicitly enabled posix_acl or did not provide any option,
+     * do not block acl. Otherwise block system.posix_acl_access and
+     * system.posix_acl_default xattrs.
+     */
+    if (lo->user_posix_acl) {
+        return false;
+    }
+    if (!strcmp(name, "system.posix_acl_access") ||
+        !strcmp(name, "system.posix_acl_default"))
+            return true;
+
+    return false;
+}
+
+/*
+ * Returns number of bytes in xattr_list after filtering on success. This
+ * could be zero as well if nothing is left after filtering.
+ *
+ * Returns negative error code on failure.
+ * xattr_list is modified in place.
+ */
+static int remove_blocked_xattrs(struct lo_data *lo, char *xattr_list,
+                                 unsigned in_size)
+{
+    size_t out_index, in_index;
+
+    /*
+     * As of now we only filter out acl xattrs. If acls are enabled or
+     * they have not been explicitly disabled, there is nothing to
+     * filter.
+     */
+    if (lo->user_posix_acl) {
+        return in_size;
+    }
+
+    out_index = 0;
+    in_index = 0;
+    while (in_index < in_size) {
+        char *in_ptr = xattr_list + in_index;
+
+        /* Length of current attribute name */
+        size_t in_len = strlen(xattr_list + in_index) + 1;
+
+        if (!block_xattr(lo, in_ptr)) {
+            if (in_index != out_index) {
+                memmove(xattr_list + out_index, xattr_list + in_index, in_len);
+            }
+            out_index += in_len;
+        }
+        in_index += in_len;
+     }
+    return out_index;
+}
+
 static void lo_getxattr(fuse_req_t req, fuse_ino_t ino, const char *in_name,
                         size_t size)
 {
@@ -2625,6 +2702,11 @@ static void lo_getxattr(fuse_req_t req,
     int saverr;
     int fd = -1;
 
+    if (block_xattr(lo, in_name)) {
+        fuse_reply_err(req, EOPNOTSUPP);
+        return;
+    }
+
     mapped_name = NULL;
     name = in_name;
     if (lo->xattrmap) {
@@ -2766,7 +2848,6 @@ static void lo_listxattr(fuse_req_t req,
         if (ret == 0) {
             goto out;
         }
-
         if (lo->xattr_map_list) {
             /*
              * Map the names back, some attributes might be dropped,
@@ -2813,6 +2894,12 @@ static void lo_listxattr(fuse_req_t req,
                 goto out;
             }
         }
+
+        ret = remove_blocked_xattrs(lo, value, ret);
+        if (ret <= 0) {
+            saverr = -ret;
+            goto out;
+        }
         fuse_reply_buf(req, value, ret);
     } else {
         /*
@@ -2851,6 +2938,11 @@ static void lo_setxattr(fuse_req_t req,
     int saverr;
     int fd = -1;
 
+    if (block_xattr(lo, in_name)) {
+        fuse_reply_err(req, EOPNOTSUPP);
+        return;
+    }
+
     mapped_name = NULL;
     name = in_name;
     if (lo->xattrmap) {
@@ -2917,6 +3009,11 @@ static void lo_removexattr(fuse_req_t re
     int saverr;
     int fd = -1;
 
+    if (block_xattr(lo, in_name)) {
+        fuse_reply_err(req, EOPNOTSUPP);
+        return;
+    }
+
     mapped_name = NULL;
     name = in_name;
     if (lo->xattrmap) {
@@ -3604,6 +3701,7 @@ int main(int argc, char *argv[])
         .allow_direct_io = 0,
         .proc_self_fd = -1,
         .user_killpriv_v2 = -1,
+        .user_posix_acl = -1,
     };
     struct lo_map_elem *root_elem;
     struct lo_map_elem *reserve_elem;