fuzz: support for fork-based fuzzing.

fork() is a simple way to ensure that state does not leak in between
fuzzing runs. Unfortunately, the fuzzer mutation engine relies on
bitmaps which contain coverage information for each fuzzing run, and
these bitmaps should be copied from the child to the parent(where the
mutation occurs). These bitmaps are created through compile-time
instrumentation and they are not shared with fork()-ed processes, by
default. To address this, we create a shared memory region, adjust its
size and map it _over_ the counter region. Furthermore, libfuzzer
doesn't generally expose the globals that specify the location of the
counters/coverage bitmap. As a workaround, we rely on a custom linker
script which forces all of the bitmaps we care about to be placed in a
contiguous region, which is easy to locate and mmap over.

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Message-id: 20200220041118.23264-16-alxndr@bu.edu
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

tests/qtest/fuzz/Makefile.include

@@ -2,5 +2,10 @@ QEMU_PROG_FUZZ=qemu-fuzz-$(TARGET_NAME)$(EXESUF)
 
 fuzz-obj-y += tests/qtest/libqtest.o
 fuzz-obj-y += tests/qtest/fuzz/fuzz.o # Fuzzer skeleton
+fuzz-obj-y += tests/qtest/fuzz/fork_fuzz.o
 
 FUZZ_CFLAGS += -I$(SRC_PATH)/tests -I$(SRC_PATH)/tests/qtest
+
+# Linker Script to force coverage-counters into known regions which we can mark
+# shared
+FUZZ_LDFLAGS += -Xlinker -T$(SRC_PATH)/tests/qtest/fuzz/fork_fuzz.ld

tests/qtest/fuzz/fork_fuzz.c

@@ -0,0 +1,55 @@
+/*
+ * Fork-based fuzzing helpers
+ *
+ * Copyright Red Hat Inc., 2019
+ *
+ * Authors:
+ *  Alexander Bulekov   <alxndr@bu.edu>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+
+#include "qemu/osdep.h"
+#include "fork_fuzz.h"
+
+
+void counter_shm_init(void)
+{
+    char *shm_path = g_strdup_printf("/qemu-fuzz-cntrs.%d", getpid());
+    int fd = shm_open(shm_path, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
+    g_free(shm_path);
+
+    if (fd == -1) {
+        perror("Error: ");
+        exit(1);
+    }
+    if (ftruncate(fd, &__FUZZ_COUNTERS_END - &__FUZZ_COUNTERS_START) == -1) {
+        perror("Error: ");
+        exit(1);
+    }
+    /* Copy what's in the counter region to the shm.. */
+    void *rptr = mmap(NULL ,
+            &__FUZZ_COUNTERS_END - &__FUZZ_COUNTERS_START,
+            PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+    memcpy(rptr,
+           &__FUZZ_COUNTERS_START,
+           &__FUZZ_COUNTERS_END - &__FUZZ_COUNTERS_START);
+
+    munmap(rptr, &__FUZZ_COUNTERS_END - &__FUZZ_COUNTERS_START);
+
+    /* And map the shm over the counter region */
+    rptr = mmap(&__FUZZ_COUNTERS_START,
+            &__FUZZ_COUNTERS_END - &__FUZZ_COUNTERS_START,
+            PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED, fd, 0);
+
+    close(fd);
+
+    if (!rptr) {
+        perror("Error: ");
+        exit(1);
+    }
+}
+
+

tests/qtest/fuzz/fork_fuzz.h

@@ -0,0 +1,23 @@
+/*
+ * Fork-based fuzzing helpers
+ *
+ * Copyright Red Hat Inc., 2019
+ *
+ * Authors:
+ *  Alexander Bulekov   <alxndr@bu.edu>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+
+#ifndef FORK_FUZZ_H
+#define FORK_FUZZ_H
+
+extern uint8_t __FUZZ_COUNTERS_START;
+extern uint8_t __FUZZ_COUNTERS_END;
+
+void counter_shm_init(void);
+
+#endif
+

tests/qtest/fuzz/fork_fuzz.ld

@@ -0,0 +1,37 @@
+/* We adjust linker script modification to place all of the stuff that needs to
+ * persist across fuzzing runs into a contiguous seciton of memory. Then, it is
+ * easy to re-map the counter-related memory as shared.
+*/
+
+SECTIONS
+{
+  .data.fuzz_start : ALIGN(4K)
+  {
+      __FUZZ_COUNTERS_START = .;
+      __start___sancov_cntrs = .;
+      *(_*sancov_cntrs);
+      __stop___sancov_cntrs = .;
+
+      /* Lowest stack counter */
+      *(__sancov_lowest_stack);
+  }
+  .data.fuzz_ordered :
+  {
+      /* Coverage counters. They're not necessary for fuzzing, but are useful
+       * for analyzing the fuzzing performance
+       */
+      __start___llvm_prf_cnts = .;
+      *(*llvm_prf_cnts);
+      __stop___llvm_prf_cnts = .;
+
+      /* Internal Libfuzzer TracePC object which contains the ValueProfileMap */
+      FuzzerTracePC*(.bss*);
+  }
+  .data.fuzz_end : ALIGN(4K)
+  {
+      __FUZZ_COUNTERS_END = .;
+  }
+}
+/* Dont overwrite the SECTIONS in the default linker script. Instead insert the
+ * above into the default script */
+INSERT AFTER .data;