bus: add minimal locking around the memfd cache

We want to allow clients to process an sd_bus_message on a different
thread than it was received on. Since unreffing a bus message might
readd some of its memfds to the memfd cache add some minimal locking
around the cache.

1 files changed, 28 insertions, 7 deletions

diff --git a/src/libsystemd-bus/bus-kernel.c b/src/libsystemd-bus/bus-kernel.c
index ede78d7bef..699d24185e 100644
--- a/src/libsystemd-bus/bus-kernel.c
+++ b/src/libsystemd-bus/bus-kernel.c
@@ -721,6 +721,7 @@ int bus_kernel_create(const char *name, char **s) {
 
 int bus_kernel_pop_memfd(sd_bus *bus, void **address, size_t *size) {
         struct memfd_cache *c;
+        int fd;
 
         assert(address);
         assert(size);
@@ -728,8 +729,12 @@ int bus_kernel_pop_memfd(sd_bus *bus, void **address, size_t *size) {
         if (!bus || !bus->is_kernel)
                 return -ENOTSUP;
 
+        assert_se(pthread_mutex_lock(&bus->memfd_cache_mutex) == 0);
+
         if (bus->n_memfd_cache <= 0) {
-                int fd, r;
+                int r;
+
+                assert_se(pthread_mutex_unlock(&bus->memfd_cache_mutex) == 0);
 
                 r = ioctl(bus->input_fd, KDBUS_CMD_MEMFD_NEW, &fd);
                 if (r < 0)
@@ -747,8 +752,18 @@ int bus_kernel_pop_memfd(sd_bus *bus, void **address, size_t *size) {
 
         *address = c->address;
         *size = c->size;
+        fd = c->fd;
+
+        assert_se(pthread_mutex_unlock(&bus->memfd_cache_mutex) == 0);
 
-        return c->fd;
+        return fd;
+}
+
+static void close_and_munmap(int fd, void *address, size_t size) {
+        if (size > 0)
+                assert_se(munmap(address, PAGE_ALIGN(size)) == 0);
+
+        close_nointr_nofail(fd);
 }
 
 void bus_kernel_push_memfd(sd_bus *bus, int fd, void *address, size_t size) {
@@ -757,13 +772,17 @@ void bus_kernel_push_memfd(sd_bus *bus, int fd, void *address, size_t size) {
         assert(fd >= 0);
         assert(size == 0 || address);
 
-        if (!bus || !bus->is_kernel ||
-            bus->n_memfd_cache >= ELEMENTSOF(bus->memfd_cache)) {
+        if (!bus || !bus->is_kernel) {
+                close_and_munmap(fd, address, size);
+                return;
+        }
+
+        assert_se(pthread_mutex_lock(&bus->memfd_cache_mutex) == 0);
 
-                if (size > 0)
-                        assert_se(munmap(address, PAGE_ALIGN(size)) == 0);
+        if (bus->n_memfd_cache >= ELEMENTSOF(bus->memfd_cache)) {
+                assert_se(pthread_mutex_unlock(&bus->memfd_cache_mutex) == 0);
 
-                close_nointr_nofail(fd);
+                close_and_munmap(fd, address, size);
                 return;
         }
 
@@ -780,6 +799,8 @@ void bus_kernel_push_memfd(sd_bus *bus, int fd, void *address, size_t size) {
                 c->size = MEMFD_CACHE_ITEM_SIZE_MAX;
         } else
                 c->size = size;
+
+        assert_se(pthread_mutex_unlock(&bus->memfd_cache_mutex) == 0);
 }
 
 void bus_kernel_flush_memfd(sd_bus *b) {