Add fuzzer for dbm module

Fuzzes CPython's dbm backend — whichever implementation dbm.open()
selects on the build, typically the gdbm C module
(Modules/_gdbmmodule.c) or _dbm (Modules/_dbmmodule.c). Opens a fresh
GDBM-style database in a temp directory with mode "c", then drives a
sequence of up to 20 fuzzed operations — store (with fuzzed-length
keys and values), get, key listing via db.keys(), membership-guarded
delete, and iteration with value lookup — to exercise the hash-table
insert/lookup/delete paths and the on-disk file handling in the C
backend.

Author: AdamKorcz

Makefile

@@ -1,4 +1,4 @@
-all : fuzzer-html fuzzer-email fuzzer-httpclient fuzzer-json fuzzer-difflib fuzzer-csv fuzzer-decode fuzzer-ast fuzzer-tarfile fuzzer-tarfile-hypothesis fuzzer-zipfile fuzzer-zipfile-hypothesis fuzzer-re fuzzer-configparser fuzzer-tomllib fuzzer-plistlib fuzzer-xml fuzzer-zoneinfo fuzzer-binascii
+all : fuzzer-html fuzzer-email fuzzer-httpclient fuzzer-json fuzzer-difflib fuzzer-csv fuzzer-decode fuzzer-ast fuzzer-tarfile fuzzer-tarfile-hypothesis fuzzer-zipfile fuzzer-zipfile-hypothesis fuzzer-re fuzzer-configparser fuzzer-tomllib fuzzer-plistlib fuzzer-xml fuzzer-zoneinfo fuzzer-binascii fuzzer-dbm
 
 PYTHON_CONFIG_PATH=$(CPYTHON_INSTALL_PATH)/bin/python3-config
 CXXFLAGS += $(shell $(PYTHON_CONFIG_PATH) --cflags)
@@ -43,3 +43,6 @@ fuzzer-zoneinfo:
 
 fuzzer-binascii:
 	clang++ $(CXXFLAGS) $(LIB_FUZZING_ENGINE) -std=c++17 fuzzer.cpp -DPYTHON_HARNESS_PATH="\"binascii.py\"" -ldl $(LDFLAGS) -o fuzzer-binascii
+
+fuzzer-dbm:
+	clang++ $(CXXFLAGS) $(LIB_FUZZING_ENGINE) -std=c++17 fuzzer.cpp -DPYTHON_HARNESS_PATH="\"dbm.py\"" -ldl $(LDFLAGS) -o fuzzer-dbm

dbm.py

@@ -0,0 +1,56 @@
+from fuzzeddataprovider import FuzzedDataProvider
+import os
+import dbm
+import tempfile
+
+OP_STORE = 0
+OP_GET = 1
+OP_LIST_KEYS = 2
+OP_DELETE = 3
+OP_ITERATE = 4
+
+
+# Fuzzes the _gdbm C module (Modules/_gdbmmodule.c).
+# Exercises key-value store operations on a temporary GDBM database:
+# store, get, key listing, deletion, and iteration with fuzzed
+# keys and values.
+def FuzzerRunOne(FuzzerInput):
+    if len(FuzzerInput) < 1 or len(FuzzerInput) > 0x10000:
+        return
+    fdp = FuzzedDataProvider(FuzzerInput)
+    try:
+        with tempfile.TemporaryDirectory() as tmpdir:
+            dbpath = os.path.join(tmpdir, "fuzzdb")
+            with dbm.open(dbpath, "c") as db:
+                num_ops = fdp.ConsumeIntInRange(1, 20)
+                for _ in range(num_ops):
+                    if fdp.remaining_bytes() == 0:
+                        break
+                    op = fdp.ConsumeIntInRange(OP_STORE, OP_ITERATE)
+                    if op == OP_STORE:
+                        n = fdp.ConsumeIntInRange(1, min(fdp.remaining_bytes(), 100))
+                        key = fdp.ConsumeBytes(n)
+                        n2 = (
+                            fdp.ConsumeIntInRange(1, min(fdp.remaining_bytes(), 1000))
+                            if fdp.remaining_bytes() > 0
+                            else 0
+                        )
+                        val = fdp.ConsumeBytes(n2) if n2 > 0 else b""
+                        db[key] = val
+                    elif op == OP_GET:
+                        n = fdp.ConsumeIntInRange(1, min(fdp.remaining_bytes(), 100))
+                        key = fdp.ConsumeBytes(n)
+                        _ = db.get(key)
+                    elif op == OP_LIST_KEYS:
+                        _ = list(db.keys())
+                    elif op == OP_DELETE:
+                        n = fdp.ConsumeIntInRange(1, min(fdp.remaining_bytes(), 100))
+                        key = fdp.ConsumeBytes(n)
+                        if key in db:
+                            del db[key]
+                    elif op == OP_ITERATE:
+                        for k in db:
+                            _ = db[k]
+                            break
+    except Exception:
+        pass

fuzz_targets.txt

@@ -2,6 +2,7 @@ ast ast.py
 binascii binascii.py
 configparser configparser.py
 csv csv.py
+dbm dbm.py
 decode decode.py
 difflib difflib.py
 email email.py