[PATCH] fuzz: do not use POSIX shm for coverage bitmap

[Alexander Bulekov]

We used shm_open with mmap to share libfuzzer's coverage bitmap with
child (runner) processes. The same functionality can be achieved with
MAP_SHARED | MAP_ANONYMOUS, since we do not care about naming or
permissioning the shared memory object.

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
---
This might fix:
qemu-fuzz-i386-target-virtio-net-socket: Unexpected-exit in
counter_shm_init 
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=23636 (private link)

oss-fuzz does not provide access to /dev/, so it is likely that shm_open
breaks, when it tries to access /dev/shm. This seems likely, based on
the oss-fuzz minijail setup:
https://github.com/google/oss-fuzz/blob/3740c751fd9edea138c17783995d370d6b1b89bc/infra/base-images/base-runner/run_minijail

 tests/qtest/fuzz/fork_fuzz.c | 40 ++++++++++++------------------------
 1 file changed, 13 insertions(+), 27 deletions(-)

diff --git a/tests/qtest/fuzz/fork_fuzz.c b/tests/qtest/fuzz/fork_fuzz.c
index 2bd0851903..6ffb2a7937 100644
--- a/tests/qtest/fuzz/fork_fuzz.c
+++ b/tests/qtest/fuzz/fork_fuzz.c
@@ -17,39 +17,25 @@
 
 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,
+    /* Copy what's in the counter region to a temporary buffer.. */
+    void *copy = malloc(&__FUZZ_COUNTERS_END - &__FUZZ_COUNTERS_START);
+    memcpy(copy,
            &__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) {
+    /* Map a shared region over the counter region */
+    if (mmap(&__FUZZ_COUNTERS_START,
+             &__FUZZ_COUNTERS_END - &__FUZZ_COUNTERS_START,
+             PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED | MAP_ANONYMOUS,
+             0, 0) == MAP_FAILED) {
         perror("Error: ");
         exit(1);
     }
+
+    /* Copy the original data back to the counter-region */
+    memcpy(&__FUZZ_COUNTERS_START, copy,
+           &__FUZZ_COUNTERS_END - &__FUZZ_COUNTERS_START);
+    free(copy);
 }
 
 
-- 
2.26.2