Truncate eventlog event for large payload (#20221)

RTS eventlog events for postCapsetVecEvent are truncated if payload
is larger than EVENT_PAYLOAD_SIZE_MAX
Previously, postCapsetVecEvent records eventlog event with payload
of variable size larger than EVENT_PAYLOAD_SIZE_MAX (2^16) without
any validation, resulting in corrupted data.
For example, this happens when a Haskell binary is invoked with very
long command line arguments exceeding 2^16 bytes (see #20221).
Now we check the size of accumulated payload messages incrementally,
and truncate the message just before the payload size exceeds
EVENT_PAYLOAD_SIZE_MAX. RTS will warn the user with a message showing
how many arguments are truncated.

1 files changed, 11 insertions, 1 deletions

diff --git a/rts/eventlog/EventLog.c b/rts/eventlog/EventLog.c
index 53d5e28a61..dfa9ecc0fb 100644
--- a/rts/eventlog/EventLog.c
+++ b/rts/eventlog/EventLog.c
@@ -754,7 +754,17 @@ void postCapsetVecEvent (EventTypeNum tag,
 
     for (int i = 0; i < argc; i++) {
         // 1 + strlen to account for the trailing \0, used as separator
-        size += 1 + strlen(argv[i]);
+        int increment = 1 + strlen(argv[i]);
+        if (size + increment > EVENT_PAYLOAD_SIZE_MAX) {
+            errorBelch("Event size exceeds EVENT_PAYLOAD_SIZE_MAX, record only %"
+                       FMT_Int " out of %" FMT_Int " args",
+                       (long long) i,
+                       (long long) argc);
+            argc = i;
+            break;
+        } else {
+            size += increment;
+        }
     }
 
     ACQUIRE_LOCK(&eventBufMutex);