screencap: Fix parsing of display IDs

The underlying type of DisplayId is uint64_t, but screencap parsed the
IDs as signed 64-bit, invoking undefined behavior for out-of-range IDs.
In practice, out-of-range IDs were clamped, so SF failed to find them.

Fixes: 302580952
Test: screencap works for virtual display on Felix
Change-Id: I09a863d0c68dbb857b6f756b51159e5e3d853f5d

1 files changed, 12 insertions, 3 deletions

diff --git a/cmds/screencap/screencap.cpp b/cmds/screencap/screencap.cpp
index ee9c464219d9..2d235331a672 100644
--- a/cmds/screencap/screencap.cpp
+++ b/cmds/screencap/screencap.cpp
@@ -142,13 +142,22 @@ int main(int argc, char** argv)
             case 'p':
                 png = true;
                 break;
-            case 'd':
-                displayIdOpt = DisplayId::fromValue(atoll(optarg));
+            case 'd': {
+                errno = 0;
+                char* end = nullptr;
+                const uint64_t id = strtoull(optarg, &end, 10);
+                if (!end || *end != '\0' || errno == ERANGE) {
+                    fprintf(stderr, "Invalid display ID: Out of range [0, 2^64).\n");
+                    return 1;
+                }
+
+                displayIdOpt = DisplayId::fromValue(id);
                 if (!displayIdOpt) {
-                    fprintf(stderr, "Invalid display ID: %s\n", optarg);
+                    fprintf(stderr, "Invalid display ID: Incorrect encoding.\n");
                     return 1;
                 }
                 break;
+            }
             case '?':
             case 'h':
                 if (ids.size() == 1) {