[systemd-devel] [PATCH 2/7] libsystemd-bus: query minimum buffer size for KDBUS_CMD_NAME_QUERY

[Daniel Mack]

When starting in kernel mode, determine the minimum buffer size for
subsequent KDBUS_CMD_NAME_QUERY calls. The value returned by kdbus is
dependant on kernel config settings and may vary from system to system
and also in the future, but it is fix for a bus lifetime.
---
 src/libsystemd-bus/bus-internal.h |  3 +++
 src/libsystemd-bus/bus-kernel.c   | 11 +++++++++++
 2 files changed, 14 insertions(+)

diff --git a/src/libsystemd-bus/bus-internal.h b/src/libsystemd-bus/bus-internal.h
index 4f9d941..088bdad 100644
--- a/src/libsystemd-bus/bus-internal.h
+++ b/src/libsystemd-bus/bus-internal.h
@@ -226,6 +226,9 @@ struct sd_bus {
 
         void *kdbus_buffer;
 
+        /* Minimum buffer size for KDBUS_CMD_NAME_QUERY calls */
+        size_t name_query_min_size;
+
         /* We do locking around the memfd cache, since we want to
          * allow people to process a sd_bus_message in a different
          * thread then it was generated on and free it there. Since
diff --git a/src/libsystemd-bus/bus-kernel.c b/src/libsystemd-bus/bus-kernel.c
index 3ea85d4..94eabfc 100644
--- a/src/libsystemd-bus/bus-kernel.c
+++ b/src/libsystemd-bus/bus-kernel.c
@@ -314,6 +314,7 @@ fail:
 
 int bus_kernel_take_fd(sd_bus *b) {
         struct kdbus_cmd_hello hello;
+        struct kdbus_cmd_name_info dummy_info;
         int r;
 
         assert(b);
@@ -356,6 +357,16 @@ int bus_kernel_take_fd(sd_bus *b) {
         b->bus_client = true;
         b->can_fds = !!(hello.conn_flags & KDBUS_HELLO_ACCEPT_FD);
 
+        /* Send a NAME_QUERY with an empty info struct. That will tell us the
+         * size of the return buffer that we have to provide for future name
+         * queries.  */
+        zero(dummy_info);
+        r = ioctl(b->input_fd, KDBUS_CMD_NAME_QUERY, &dummy_info);
+        if (r < 0 && errno != EMSGSIZE)
+                return -errno;
+
+        b->name_query_min_size = dummy_info.size;
+
         r = bus_start_running(b);
         if (r < 0)
                 return r;
-- 
1.8.4.2