Instead of special casing the permission handling and watch event
firing for the special watch paths "@introduceDomain" and
"@releaseDomain", use static dummy nodes added to the data base when
starting Xenstore.

The node accounting needs to reflect that change by adding the special
nodes in the domain_entry_fix() call in setup_structure().

Note that this requires to rework the calls of fire_watches() for the
special events in order to avoid leaking memory.

Move the check for a valid node name from get_node() to
get_node_canonicalized(), as it allows to use get_node() for the
special nodes, too.

In order to avoid read and write accesses to the special nodes use a
special variant for obtaining the current node data for the permission
handling.

This allows to simplify quite some code. In future sub-nodes of the
special nodes will be possible due to this change, allowing more fine
grained permission control of special events for specific domains.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
V2:
- add get_spec_node()
- expand commit message (Julien Grall)
---
 tools/xenstore/xenstored_control.c |   3 -
 tools/xenstore/xenstored_core.c    |  92 +++++++++-------
 tools/xenstore/xenstored_domain.c  | 162 ++++-------------------------
 tools/xenstore/xenstored_domain.h  |   6 --
 tools/xenstore/xenstored_watch.c   |  17 +--
 5 files changed, 80 insertions(+), 200 deletions(-)

diff --git a/tools/xenstore/xenstored_control.c b/tools/xenstore/xenstored_control.c
index d1aaa00bf4..41e6992591 100644
--- a/tools/xenstore/xenstored_control.c
+++ b/tools/xenstore/xenstored_control.c
@@ -676,9 +676,6 @@ static const char *lu_dump_state(const void *ctx, struct connection *conn)
 	if (ret)
 		goto out;
 	ret = dump_state_connections(fp);
-	if (ret)
-		goto out;
-	ret = dump_state_special_nodes(fp);
 	if (ret)
 		goto out;
 	ret = dump_state_nodes(fp, ctx);
diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
index 1650821922..f96714e1b8 100644
--- a/tools/xenstore/xenstored_core.c
+++ b/tools/xenstore/xenstored_core.c
@@ -616,7 +616,8 @@ static void get_acc_data(TDB_DATA *key, struct node_account_data *acc)
 static unsigned int get_acc_domid(struct connection *conn, TDB_DATA *key,
 				  unsigned int domid)
 {
-	return (!conn || key->dptr[0] == '/') ? domid : conn->id;
+	return (!conn || key->dptr[0] == '/' || key->dptr[0] == '@')
+	       ? domid : conn->id;
 }
 
 int do_tdb_write(struct connection *conn, TDB_DATA *key, TDB_DATA *data,
@@ -958,10 +959,6 @@ static struct node *get_node(struct connection *conn,
 {
 	struct node *node;
 
-	if (!name || !is_valid_nodename(name)) {
-		errno = EINVAL;
-		return NULL;
-	}
 	node = read_node(conn, ctx, name);
 	/* If we don't have permission, we don't have node. */
 	if (node) {
@@ -1250,9 +1247,23 @@ static struct node *get_node_canonicalized(struct connection *conn,
 	*canonical_name = canonicalize(conn, ctx, name);
 	if (!*canonical_name)
 		return NULL;
+	if (!is_valid_nodename(*canonical_name)) {
+		errno = EINVAL;
+		return NULL;
+	}
 	return get_node(conn, ctx, *canonical_name, perm);
 }
 
+static struct node *get_spec_node(struct connection *conn, const void *ctx,
+				  const char *name, char **canonical_name,
+				  unsigned int perm)
+{
+	if (name[0] == '@')
+		return get_node(conn, ctx, name, perm);
+
+	return get_node_canonicalized(conn, ctx, name, canonical_name, perm);
+}
+
 static int send_directory(const void *ctx, struct connection *conn,
 			  struct buffered_data *in)
 {
@@ -1737,8 +1748,7 @@ static int do_get_perms(const void *ctx, struct connection *conn,
 	char *strings;
 	unsigned int len;
 
-	node = get_node_canonicalized(conn, ctx, onearg(in), NULL,
-				      XS_PERM_READ);
+	node = get_spec_node(conn, ctx, onearg(in), NULL, XS_PERM_READ);
 	if (!node)
 		return errno;
 
@@ -1780,17 +1790,9 @@ static int do_set_perms(const void *ctx, struct connection *conn,
 	if (perms.p[0].perms & XS_PERM_IGNORE)
 		return ENOENT;
 
-	/* First arg is node name. */
-	if (strstarts(in->buffer, "@")) {
-		if (set_perms_special(conn, in->buffer, &perms))
-			return errno;
-		send_ack(conn, XS_SET_PERMS);
-		return 0;
-	}
-
 	/* We must own node to do this (tools can do this too). */
-	node = get_node_canonicalized(conn, ctx, in->buffer, &name,
-				      XS_PERM_WRITE | XS_PERM_OWNER);
+	node = get_spec_node(conn, ctx, in->buffer, &name,
+			     XS_PERM_WRITE | XS_PERM_OWNER);
 	if (!node)
 		return errno;
 
@@ -2388,7 +2390,9 @@ void setup_structure(bool live_update)
 		manual_node("/", "tool");
 		manual_node("/tool", "xenstored");
 		manual_node("/tool/xenstored", NULL);
-		domain_entry_fix(dom0_domid, 3, true);
+		manual_node("@releaseDomain", NULL);
+		manual_node("@introduceDomain", NULL);
+		domain_entry_fix(dom0_domid, 5, true);
 	}
 
 	check_store();
@@ -3170,6 +3174,23 @@ static int dump_state_node(const void *ctx, struct connection *conn,
 	return WALK_TREE_OK;
 }
 
+static int dump_state_special_node(FILE *fp, const void *ctx,
+				   struct dump_node_data *data,
+				   const char *name)
+{
+	struct node *node;
+	int ret;
+
+	node = read_node(NULL, ctx, name);
+	if (!node)
+		return dump_state_node_err(data, "Dump node read node error");
+
+	ret = dump_state_node(ctx, NULL, node, data);
+	talloc_free(node);
+
+	return ret;
+}
+
 const char *dump_state_nodes(FILE *fp, const void *ctx)
 {
 	struct dump_node_data data = {
@@ -3181,6 +3202,11 @@ const char *dump_state_nodes(FILE *fp, const void *ctx)
 	if (walk_node_tree(ctx, NULL, "/", &walkfuncs, &data))
 		return data.err;
 
+	if (dump_state_special_node(fp, ctx, &data, "@releaseDomain"))
+		return data.err;
+	if (dump_state_special_node(fp, ctx, &data, "@introduceDomain"))
+		return data.err;
+
 	return NULL;
 }
 
@@ -3354,25 +3380,21 @@ void read_state_node(const void *ctx, const void *state)
 		node->perms.p[i].id = sn->perms[i].domid;
 	}
 
-	if (strstarts(name, "@")) {
-		set_perms_special(&conn, name, &node->perms);
-		talloc_free(node);
-		return;
-	}
-
-	parentname = get_parent(node, name);
-	if (!parentname)
-		barf("allocation error restoring node");
-	parent = read_node(NULL, node, parentname);
-	if (!parent)
-		barf("read parent error restoring node");
+	if (!strstarts(name, "@")) {
+		parentname = get_parent(node, name);
+		if (!parentname)
+			barf("allocation error restoring node");
+		parent = read_node(NULL, node, parentname);
+		if (!parent)
+			barf("read parent error restoring node");
 
-	if (add_child(node, parent, name))
-		barf("allocation error restoring node");
+		if (add_child(node, parent, name))
+			barf("allocation error restoring node");
 
-	set_tdb_key(parentname, &key);
-	if (write_node_raw(NULL, &key, parent, true))
-		barf("write parent error restoring node");
+		set_tdb_key(parentname, &key);
+		if (write_node_raw(NULL, &key, parent, true))
+			barf("write parent error restoring node");
+	}
 
 	set_tdb_key(name, &key);
 	if (write_node_raw(NULL, &key, node, true))
diff --git a/tools/xenstore/xenstored_domain.c b/tools/xenstore/xenstored_domain.c
index 3ad1028edb..c881c93d1d 100644
--- a/tools/xenstore/xenstored_domain.c
+++ b/tools/xenstore/xenstored_domain.c
@@ -43,9 +43,6 @@ static evtchn_port_t virq_port;
 
 xenevtchn_handle *xce_handle = NULL;
 
-static struct node_perms dom_release_perms;
-static struct node_perms dom_introduce_perms;
-
 struct domain
 {
 	/* The id of this domain */
@@ -225,27 +222,6 @@ static void unmap_interface(void *interface)
 	xengnttab_unmap(*xgt_handle, interface, 1);
 }
 
-static void remove_domid_from_perm(struct node_perms *perms,
-				   struct domain *domain)
-{
-	unsigned int cur, new;
-
-	if (perms->p[0].id == domain->domid)
-		perms->p[0].id = priv_domid;
-
-	for (cur = new = 1; cur < perms->num; cur++) {
-		if (perms->p[cur].id == domain->domid)
-			continue;
-
-		if (new != cur)
-			perms->p[new] = perms->p[cur];
-
-		new++;
-	}
-
-	perms->num = new;
-}
-
 static int domain_tree_remove_sub(const void *ctx, struct connection *conn,
 				  struct node *node, void *arg)
 {
@@ -297,8 +273,24 @@ static void domain_tree_remove(struct domain *domain)
 			       "error when looking for orphaned nodes\n");
 	}
 
-	remove_domid_from_perm(&dom_release_perms, domain);
-	remove_domid_from_perm(&dom_introduce_perms, domain);
+	walk_node_tree(domain, NULL, "@releaseDomain", &walkfuncs, domain);
+	walk_node_tree(domain, NULL, "@introduceDomain", &walkfuncs, domain);
+}
+
+static void fire_special_watches(const char *name)
+{
+	void *ctx = talloc_new(NULL);
+	struct node *node;
+
+	if (!ctx)
+		return;
+
+	node = read_node(NULL, ctx, name);
+
+	if (node)
+		fire_watches(NULL, ctx, name, node, true, NULL);
+
+	talloc_free(ctx);
 }
 
 static int destroy_domain(void *_domain)
@@ -326,7 +318,7 @@ static int destroy_domain(void *_domain)
 			unmap_interface(domain->interface);
 	}
 
-	fire_watches(NULL, domain, "@releaseDomain", NULL, true, NULL);
+	fire_special_watches("@releaseDomain");
 
 	wrl_domain_destroy(domain);
 
@@ -384,7 +376,7 @@ void check_domains(void)
 		;
 
 	if (notify)
-		fire_watches(NULL, NULL, "@releaseDomain", NULL, true, NULL);
+		fire_special_watches("@releaseDomain");
 }
 
 /* We scan all domains rather than use the information given here. */
@@ -633,8 +625,7 @@ static struct domain *introduce_domain(const void *ctx,
 		}
 
 		if (!is_master_domain && !restore)
-			fire_watches(NULL, ctx, "@introduceDomain", NULL,
-				     true, NULL);
+			fire_special_watches("@introduceDomain");
 	} else {
 		/* Use XS_INTRODUCE for recreating the xenbus event-channel. */
 		if (domain->port)
@@ -840,59 +831,6 @@ const char *get_implicit_path(const struct connection *conn)
 	return conn->domain->path;
 }
 
-static int set_dom_perms_default(struct node_perms *perms)
-{
-	perms->num = 1;
-	perms->p = talloc_array(NULL, struct xs_permissions, perms->num);
-	if (!perms->p)
-		return -1;
-	perms->p->id = 0;
-	perms->p->perms = XS_PERM_NONE;
-
-	return 0;
-}
-
-static struct node_perms *get_perms_special(const char *name)
-{
-	if (!strcmp(name, "@releaseDomain"))
-		return &dom_release_perms;
-	if (!strcmp(name, "@introduceDomain"))
-		return &dom_introduce_perms;
-	return NULL;
-}
-
-int set_perms_special(struct connection *conn, const char *name,
-		      struct node_perms *perms)
-{
-	struct node_perms *p;
-
-	p = get_perms_special(name);
-	if (!p)
-		return EINVAL;
-
-	if ((perm_for_conn(conn, p) & (XS_PERM_WRITE | XS_PERM_OWNER)) !=
-	    (XS_PERM_WRITE | XS_PERM_OWNER))
-		return EACCES;
-
-	p->num = perms->num;
-	talloc_free(p->p);
-	p->p = perms->p;
-	talloc_steal(NULL, perms->p);
-
-	return 0;
-}
-
-bool check_perms_special(const char *name, struct connection *conn)
-{
-	struct node_perms *p;
-
-	p = get_perms_special(name);
-	if (!p)
-		return false;
-
-	return perm_for_conn(conn, p) & XS_PERM_READ;
-}
-
 void dom0_init(void)
 {
 	evtchn_port_t port;
@@ -962,10 +900,6 @@ void domain_init(int evtfd)
 	if (xce_handle == NULL)
 		barf_perror("Failed to open evtchn device");
 
-	if (set_dom_perms_default(&dom_release_perms) ||
-	    set_dom_perms_default(&dom_introduce_perms))
-		barf_perror("Failed to set special permissions");
-
 	if ((rc = xenevtchn_bind_virq(xce_handle, VIRQ_DOM_EXC)) == -1)
 		barf_perror("Failed to bind to domain exception virq port");
 	virq_port = rc;
@@ -1535,60 +1469,6 @@ const char *dump_state_connections(FILE *fp)
 	return ret;
 }
 
-static const char *dump_state_special_node(FILE *fp, const char *name,
-					   const struct node_perms *perms)
-{
-	struct xs_state_record_header head;
-	struct xs_state_node sn;
-	unsigned int pathlen;
-	const char *ret;
-
-	pathlen = strlen(name) + 1;
-
-	head.type = XS_STATE_TYPE_NODE;
-	head.length = sizeof(sn);
-
-	sn.conn_id = 0;
-	sn.ta_id = 0;
-	sn.ta_access = 0;
-	sn.perm_n = perms->num;
-	sn.path_len = pathlen;
-	sn.data_len = 0;
-	head.length += perms->num * sizeof(*sn.perms);
-	head.length += pathlen;
-	head.length = ROUNDUP(head.length, 3);
-	if (fwrite(&head, sizeof(head), 1, fp) != 1)
-		return "Dump special node error";
-	if (fwrite(&sn, sizeof(sn), 1, fp) != 1)
-		return "Dump special node error";
-
-	ret = dump_state_node_perms(fp, perms->p, perms->num);
-	if (ret)
-		return ret;
-
-	if (fwrite(name, pathlen, 1, fp) != 1)
-		return "Dump special node path error";
-
-	ret = dump_state_align(fp);
-
-	return ret;
-}
-
-const char *dump_state_special_nodes(FILE *fp)
-{
-	const char *ret;
-
-	ret = dump_state_special_node(fp, "@releaseDomain",
-				      &dom_release_perms);
-	if (ret)
-		return ret;
-
-	ret = dump_state_special_node(fp, "@introduceDomain",
-				      &dom_introduce_perms);
-
-	return ret;
-}
-
 void read_state_connection(const void *ctx, const void *state)
 {
 	const struct xs_state_connection *sc = state;
diff --git a/tools/xenstore/xenstored_domain.h b/tools/xenstore/xenstored_domain.h
index b38c82991d..630641d620 100644
--- a/tools/xenstore/xenstored_domain.h
+++ b/tools/xenstore/xenstored_domain.h
@@ -99,11 +99,6 @@ void domain_outstanding_domid_dec(unsigned int domid);
 int domain_get_quota(const void *ctx, struct connection *conn,
 		     unsigned int domid);
 
-/* Special node permission handling. */
-int set_perms_special(struct connection *conn, const char *name,
-		      struct node_perms *perms);
-bool check_perms_special(const char *name, struct connection *conn);
-
 /* Write rate limiting */
 
 #define WRL_FACTOR   1000 /* for fixed-point arithmetic */
@@ -132,7 +127,6 @@ void wrl_apply_debit_direct(struct connection *conn);
 void wrl_apply_debit_trans_commit(struct connection *conn);
 
 const char *dump_state_connections(FILE *fp);
-const char *dump_state_special_nodes(FILE *fp);
 
 void read_state_connection(const void *ctx, const void *state);
 
diff --git a/tools/xenstore/xenstored_watch.c b/tools/xenstore/xenstored_watch.c
index 316c08b7f7..75748ac109 100644
--- a/tools/xenstore/xenstored_watch.c
+++ b/tools/xenstore/xenstored_watch.c
@@ -46,13 +46,6 @@ struct watch
 	char *node;
 };
 
-static bool check_special_event(const char *name)
-{
-	assert(name);
-
-	return strstarts(name, "@");
-}
-
 /* Is child a subnode of parent, or equal? */
 static bool is_child(const char *child, const char *parent)
 {
@@ -153,14 +146,8 @@ void fire_watches(struct connection *conn, const void *ctx, const char *name,
 
 	/* Create an event for each watch. */
 	list_for_each_entry(i, &connections, list) {
-		/* introduce/release domain watches */
-		if (check_special_event(name)) {
-			if (!check_perms_special(name, i))
-				continue;
-		} else {
-			if (!watch_permitted(i, ctx, name, node, perms))
-				continue;
-		}
+		if (!watch_permitted(i, ctx, name, node, perms))
+			continue;
 
 		list_for_each_entry(watch, &i->watches, list) {
 			if (exact) {
-- 
2.35.3

Hi Juergen,

On 13/12/2022 16:00, Juergen Gross wrote:
> Instead of special casing the permission handling and watch event
> firing for the special watch paths "@introduceDomain" and
> "@releaseDomain", use static dummy nodes added to the data base when
> starting Xenstore.
> 
> The node accounting needs to reflect that change by adding the special
> nodes in the domain_entry_fix() call in setup_structure().
> 
> Note that this requires to rework the calls of fire_watches() for the
> special events in order to avoid leaking memory.
> 
> Move the check for a valid node name from get_node() to
> get_node_canonicalized(), as it allows to use get_node() for the
> special nodes, too.
> 
> In order to avoid read and write accesses to the special nodes use a
> special variant for obtaining the current node data for the permission
> handling.
> 
> This allows to simplify quite some code. In future sub-nodes of the
> special nodes will be possible due to this change, allowing more fine
> grained permission control of special events for specific domains.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
> V2:
> - add get_spec_node()
> - expand commit message (Julien Grall)
> ---
>   tools/xenstore/xenstored_control.c |   3 -
>   tools/xenstore/xenstored_core.c    |  92 +++++++++-------
>   tools/xenstore/xenstored_domain.c  | 162 ++++-------------------------
>   tools/xenstore/xenstored_domain.h  |   6 --
>   tools/xenstore/xenstored_watch.c   |  17 +--
>   5 files changed, 80 insertions(+), 200 deletions(-)
> 
> diff --git a/tools/xenstore/xenstored_control.c b/tools/xenstore/xenstored_control.c
> index d1aaa00bf4..41e6992591 100644
> --- a/tools/xenstore/xenstored_control.c
> +++ b/tools/xenstore/xenstored_control.c
> @@ -676,9 +676,6 @@ static const char *lu_dump_state(const void *ctx, struct connection *conn)
>   	if (ret)
>   		goto out;
>   	ret = dump_state_connections(fp);
> -	if (ret)
> -		goto out;
> -	ret = dump_state_special_nodes(fp);
>   	if (ret)
>   		goto out;
>   	ret = dump_state_nodes(fp, ctx);
> diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
> index 1650821922..f96714e1b8 100644
> --- a/tools/xenstore/xenstored_core.c
> +++ b/tools/xenstore/xenstored_core.c
> @@ -616,7 +616,8 @@ static void get_acc_data(TDB_DATA *key, struct node_account_data *acc)
>   static unsigned int get_acc_domid(struct connection *conn, TDB_DATA *key,
>   				  unsigned int domid)
>   {
> -	return (!conn || key->dptr[0] == '/') ? domid : conn->id;
> +	return (!conn || key->dptr[0] == '/' || key->dptr[0] == '@')

The comment on top of get_acc_domid() needs to be updated.

> +	       ? domid : conn->id;
>   }
>   

[...]

> +static void fire_special_watches(const char *name)
> +{
> +	void *ctx = talloc_new(NULL);
> +	struct node *node;
> +
> +	if (!ctx)
> +		return;
> +
> +	node = read_node(NULL, ctx, name);
> +
> +	if (node)
> +		fire_watches(NULL, ctx, name, node, true, NULL);

NIT: I would consider to log an error (maybe only once) if 'node' is 
NULL. The purpose is to help debugging Xenstored.

The rest of the code LGTM.

Cheers,

-- 
Julien Grall

On 20.12.22 20:39, Julien Grall wrote:
> Hi Juergen,
> 
> On 13/12/2022 16:00, Juergen Gross wrote:
>> Instead of special casing the permission handling and watch event
>> firing for the special watch paths "@introduceDomain" and
>> "@releaseDomain", use static dummy nodes added to the data base when
>> starting Xenstore.
>>
>> The node accounting needs to reflect that change by adding the special
>> nodes in the domain_entry_fix() call in setup_structure().
>>
>> Note that this requires to rework the calls of fire_watches() for the
>> special events in order to avoid leaking memory.
>>
>> Move the check for a valid node name from get_node() to
>> get_node_canonicalized(), as it allows to use get_node() for the
>> special nodes, too.
>>
>> In order to avoid read and write accesses to the special nodes use a
>> special variant for obtaining the current node data for the permission
>> handling.
>>
>> This allows to simplify quite some code. In future sub-nodes of the
>> special nodes will be possible due to this change, allowing more fine
>> grained permission control of special events for specific domains.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>> V2:
>> - add get_spec_node()
>> - expand commit message (Julien Grall)
>> ---
>>   tools/xenstore/xenstored_control.c |   3 -
>>   tools/xenstore/xenstored_core.c    |  92 +++++++++-------
>>   tools/xenstore/xenstored_domain.c  | 162 ++++-------------------------
>>   tools/xenstore/xenstored_domain.h  |   6 --
>>   tools/xenstore/xenstored_watch.c   |  17 +--
>>   5 files changed, 80 insertions(+), 200 deletions(-)
>>
>> diff --git a/tools/xenstore/xenstored_control.c 
>> b/tools/xenstore/xenstored_control.c
>> index d1aaa00bf4..41e6992591 100644
>> --- a/tools/xenstore/xenstored_control.c
>> +++ b/tools/xenstore/xenstored_control.c
>> @@ -676,9 +676,6 @@ static const char *lu_dump_state(const void *ctx, struct 
>> connection *conn)
>>       if (ret)
>>           goto out;
>>       ret = dump_state_connections(fp);
>> -    if (ret)
>> -        goto out;
>> -    ret = dump_state_special_nodes(fp);
>>       if (ret)
>>           goto out;
>>       ret = dump_state_nodes(fp, ctx);
>> diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
>> index 1650821922..f96714e1b8 100644
>> --- a/tools/xenstore/xenstored_core.c
>> +++ b/tools/xenstore/xenstored_core.c
>> @@ -616,7 +616,8 @@ static void get_acc_data(TDB_DATA *key, struct 
>> node_account_data *acc)
>>   static unsigned int get_acc_domid(struct connection *conn, TDB_DATA *key,
>>                     unsigned int domid)
>>   {
>> -    return (!conn || key->dptr[0] == '/') ? domid : conn->id;
>> +    return (!conn || key->dptr[0] == '/' || key->dptr[0] == '@')
> 
> The comment on top of get_acc_domid() needs to be updated.

Oh, yes.

> 
>> +           ? domid : conn->id;
>>   }
> 
> [...]
> 
>> +static void fire_special_watches(const char *name)
>> +{
>> +    void *ctx = talloc_new(NULL);
>> +    struct node *node;
>> +
>> +    if (!ctx)
>> +        return;
>> +
>> +    node = read_node(NULL, ctx, name);
>> +
>> +    if (node)
>> +        fire_watches(NULL, ctx, name, node, true, NULL);
> 
> NIT: I would consider to log an error (maybe only once) if 'node' is NULL. The 
> purpose is to help debugging Xenstored.

I think we can log it always, as this is really a bad problem.


Juergen

Hi Juergen,

On 11/01/2023 06:41, Juergen Gross wrote:
> On 20.12.22 20:39, Julien Grall wrote:
>>> +static void fire_special_watches(const char *name)
>>> +{
>>> +    void *ctx = talloc_new(NULL);
>>> +    struct node *node;
>>> +
>>> +    if (!ctx)
>>> +        return;
>>> +
>>> +    node = read_node(NULL, ctx, name);
>>> +
>>> +    if (node)
>>> +        fire_watches(NULL, ctx, name, node, true, NULL);
>>
>> NIT: I would consider to log an error (maybe only once) if 'node' is 
>> NULL. The purpose is to help debugging Xenstored.
> 
> I think we can log it always, as this is really a bad problem.

That works for me. If it is always logged, then I guess this would need 
to be a syslog message as well.

Cheers,

-- 
Julien Grall