Add stub library that just feeds files to the fuzzing target.

This will allow the fuzzers to be run as part of "make check".

lib/fuzzstub/fuzzstub.c

@@ -0,0 +1,106 @@
+/*
+ * SPDX-License-Identifier: ISC
+ *
+ * Copyright (c) 2021 Todd C. Miller <Todd.Miller@sudo.ws>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/*
+ * This is an open source non-commercial project. Dear PVS-Studio, please check it.
+ * PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com
+ */
+
+#include <config.h>
+
+#include <sys/stat.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <limits.h>
+#include <fcntl.h>
+#include <unistd.h>
+#if defined(HAVE_STDINT_H)
+# include <stdint.h>
+#elif defined(HAVE_INTTYPES_H)
+# include <inttypes.h>
+#endif
+
+#include "sudo_compat.h"
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
+
+/*
+ * Simple driver for fuzzers built for LLVM libfuzzer.
+ * This stub library allows fuzz targets to be built and run without
+ * libfuzzer.  No actual fuzzing will occur but the provided inputs
+ * will be tested.
+ */
+int
+main(int argc, char *argv[])
+{
+    size_t bufsize = 0;
+    ssize_t nread;
+    struct stat sb;
+    uint8_t *buf = NULL;
+    int fd, i, errors = 0;
+
+    /* Test provided input files. */
+    for (i = 1; i < argc; i++) {
+	fd = open(argv[i], O_RDONLY);
+	if (fd == -1 || fstat(fd, &sb) != 0) {
+	    fprintf(stderr, "open %s: %s\n", argv[i], strerror(errno));
+	    if (fd != -1)
+		close(fd);
+	    errors++;
+	    continue;
+	}
+	if (sb.st_size > SSIZE_MAX) {
+	    errno = E2BIG;
+	    fprintf(stderr, "%s: %s\n", argv[i], strerror(errno));
+	    close(fd);
+	    errors++;
+	    continue;
+	}
+	if (bufsize < (size_t)sb.st_size) {
+	    void *tmp = realloc(buf, sb.st_size);
+	    if (tmp == NULL) {
+		fprintf(stderr, "realloc: %s\n", strerror(errno));
+		close(fd);
+		errors++;
+		continue;
+	    }
+	    buf = tmp;
+	    bufsize = sb.st_size;
+	}
+	nread = read(fd, buf, sb.st_size);
+	if (nread != sb.st_size) {
+	    if (nread == -1)
+		fprintf(stderr, "read %s: %s\n", argv[i], strerror(errno));
+	    else
+		fprintf(stderr, "read %s: short read\n", argv[i]);
+	    close(fd);
+	    errors++;
+	    continue;
+	}
+	close(fd);
+
+	/* NOTE: doesn't support LLVMFuzzerInitialize() (but we don't use it) */
+	LLVMFuzzerTestOneInput(buf, nread);
+    }
+    free(buf);
+
+    return errors;
+}