Fix alert-exceeds-max feature for files > 2GB and < max-filesize

The --alert-exceeds-max feature should alert for all files larger than
2GB because 2GB is the internal limit for individual files.
This isn't working correctly because the `goto done;` exit condition
after recording the exceeds-max heuristic skips over the logic that
reports the alert.

This fix moves the ">2GB" check up to the location where the
max-filesize engine option is set by clamd or clamscan.
If max-filesize > 2GB - 1 is requested, then max-filesize is set to
2GB - 1.

Additionally, a warning is printed if max-filesize > 2GB is requested
(with an exception for when it's maxed out by setting --max-filesize=0).

Resolves: #1030

libclamav/others.c

@@ -623,7 +623,20 @@ cl_error_t cl_engine_set_num(struct cl_engine *engine, enum cl_engine_field fiel
             engine->maxscansize = num;
             break;
         case CL_ENGINE_MAX_FILESIZE:
-            engine->maxfilesize = num;
+            /* We have a limit of around 2GB (INT_MAX - 2). Enforce it here.
+             *
+             * TODO: Large file support is large-ly untested. Remove this restriction and test with a large set of large files of various types.
+             * libclamav's integer type safety has come a long way since 2014, so it's possible we could lift this restriction, but at least one
+             * of the parsers is bound to behave badly with large files. */
+            if ((uint64_t)num > INT_MAX - 2) {
+                if ((uint64_t)num > (uint64_t)2 * 1024 * 1024 * 1024 && num != LLONG_MAX) {
+                    // If greater than 2GB, warn. If exactly at 2GB, don't hassle the user.
+                    cli_warnmsg("Max file-size was set to %lld bytes. Unfortunately, scanning files greater than 2147483647 bytes (2 GiB - 1) is not supported.\n", num);
+                }
+                engine->maxfilesize = INT_MAX - 2;
+            } else {
+                engine->maxfilesize = num;
+            }
             break;
         case CL_ENGINE_MAX_RECURSION:
             if (!num) {

libclamav/scanners.c

@@ -5497,21 +5497,6 @@ static cl_error_t scan_common(cl_fmap_t *map, const char *filepath, const char *
     cli_logg_setup(&ctx);
     logg_initalized = true;
 
-    /* We have a limit of around 2GB (INT_MAX - 2). Enforce it here. */
-    /* TODO: Large file support is large-ly untested. Remove this restriction
-     * and test with a large set of large files of various types. libclamav's
-     * integer type safety has come a long way since 2014, so it's possible
-     * we could lift this restriction, but at least one of the parsers is
-     * bound to behave badly with large files. */
-    if (map->len > INT_MAX - 2) {
-        if (scanoptions->heuristic & CL_SCAN_HEURISTIC_EXCEEDS_MAX) {
-            status = cli_append_potentially_unwanted(&ctx, "Heuristics.Limits.Exceeded.MaxFileSize");
-        } else {
-            status = CL_CLEAN;
-        }
-        goto done;
-    }
-
     status = cli_magic_scan(&ctx, CL_TYPE_ANY);
 
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