[PATCH wayland 2/2] shm: Defer wl_shm_pool_resize if a pool has external references

[Derek Foreman]

If a compositor is rendering in one thread while dispatching wayland
events in another, a wl_shm_pool_resize() could change the memory
mappings it's rendering from and cause a crash.

Now we defer wl_shm_pool_resize() if the compositor has references on a
pool, and perform the actual resize when it drops those references.

Signed-off-by: Derek Foreman <der...@osg.samsung.com>
---
 src/wayland-shm.c | 47 +++++++++++++++++++++++++++++++++++------------
 1 file changed, 35 insertions(+), 12 deletions(-)

diff --git a/src/wayland-shm.c b/src/wayland-shm.c
index 6fbf34b..6351259 100644
--- a/src/wayland-shm.c
+++ b/src/wayland-shm.c
@@ -56,6 +56,7 @@ struct wl_shm_pool {
        int external_refcount;
        char *data;
        int32_t size;
+       int32_t new_size;
 };
 
 struct wl_shm_buffer {
@@ -74,12 +75,35 @@ struct wl_shm_sigbus_data {
 };
 
 static void
+shm_pool_finish_resize(struct wl_shm_pool *pool)
+{
+       void *data;
+
+       if (pool->size == pool->new_size)
+               return;
+
+       data = mremap(pool->data, pool->size, pool->new_size, MREMAP_MAYMOVE);
+       if (data == MAP_FAILED) {
+               wl_resource_post_error(pool->resource,
+                                      WL_SHM_ERROR_INVALID_FD,
+                                      "failed mremap");
+               return;
+       }
+
+       pool->data = data;
+       pool->size = pool->new_size;
+}
+
+static void
 shm_pool_unref(struct wl_shm_pool *pool, bool external)
 {
-       if (external)
+       if (external) {
                pool->external_refcount--;
-       else
+               if (pool->external_refcount == 0)
+                       shm_pool_finish_resize(pool);
+       } else {
                pool->internal_refcount--;
+       }
 
        if (pool->internal_refcount + pool->external_refcount)
                return;
@@ -202,7 +226,6 @@ shm_pool_resize(struct wl_client *client, struct 
wl_resource *resource,
                int32_t size)
 {
        struct wl_shm_pool *pool = wl_resource_get_user_data(resource);
-       void *data;
 
        if (size < pool->size) {
                wl_resource_post_error(resource,
@@ -211,16 +234,15 @@ shm_pool_resize(struct wl_client *client, struct 
wl_resource *resource,
                return;
        }
 
-       data = mremap(pool->data, pool->size, size, MREMAP_MAYMOVE);
-       if (data == MAP_FAILED) {
-               wl_resource_post_error(resource,
-                                      WL_SHM_ERROR_INVALID_FD,
-                                      "failed mremap");
-               return;
-       }
+       pool->new_size = size;
 
-       pool->data = data;
-       pool->size = size;
+       /* If the compositor has taken references on this pool it
+        * may be caching pointers into it. In that case we
+        * defer the resize (which may move the entire mapping)
+        * until the compositor finishes dereferencing the pool.
+        */
+       if (pool->external_refcount == 0)
+               shm_pool_finish_resize(pool);
 }
 
 struct wl_shm_pool_interface shm_pool_interface = {
@@ -251,6 +273,7 @@ shm_create_pool(struct wl_client *client, struct 
wl_resource *resource,
        pool->internal_refcount = 1;
        pool->external_refcount = 0;
        pool->size = size;
+       pool->new_size = size;
        pool->data = mmap(NULL, size,
                          PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
        if (pool->data == MAP_FAILED) {
-- 
2.7.0

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