terminal/grdev: simplify DRM event parsing

Coverity complained about this code and is partially right. We are not
really protected against integer overflows. Sure, unlikely, but lets just
avoid any overflows and properly protect our parser loop.

1 files changed, 5 insertions, 7 deletions

diff --git a/src/libsystemd-terminal/grdev-drm.c b/src/libsystemd-terminal/grdev-drm.c
index 7a6e1d993..6b130116d 100644
--- a/src/libsystemd-terminal/grdev-drm.c
+++ b/src/libsystemd-terminal/grdev-drm.c
@@ -2195,7 +2195,8 @@ static int grdrm_card_io_fn(sd_event_source *s, int fd, uint32_t revents, void *
         uint32_t id, counter;
         grdrm_object *object;
         char buf[4096];
-        ssize_t l, i;
+        size_t len;
+        ssize_t l;
 
         if (revents & (EPOLLHUP | EPOLLERR)) {
                 /* Immediately close device on HUP; no need to flush pending
@@ -2214,15 +2215,12 @@ static int grdrm_card_io_fn(sd_event_source *s, int fd, uint32_t revents, void *
                         log_debug("grdrm: %s/%s: read error: %m", card->base.session->name, card->base.name);
                         grdrm_card_close(card);
                         return 0;
-                } else if ((size_t)l < sizeof(*event)) {
-                        log_debug("grdrm: %s/%s: short read of %zd bytes", card->base.session->name, card->base.name, l);
-                        return 0;
                 }
 
-                for (i = 0; i < l; i += event->length) {
-                        event = (void*)&buf[i];
+                for (len = l; len > 0; len -= event->length) {
+                        event = (void*)buf;
 
-                        if (i + (ssize_t)sizeof(*event) > l || i + (ssize_t)event->length > l) {
+                        if (len < sizeof(*event) || len < event->length) {
                                 log_debug("grdrm: %s/%s: truncated event", card->base.session->name, card->base.name);
                                 break;
                         }