Merge #249: Upstream PRs 1160, 1193, 1169, 1190, 1192, 1194, 1196, 1195, 1170, 1172, 1200, 1199, 1203, 1201, 1206, 1078, 1209, 979, 1212, 1218, 1217, 1221, 1222

5d8f53e Remove redudent checks. (Russell O'Connor)
d232112 Update Changelog (Tim Ruffing)
b081f7e Add secp256k1_fe_add_int function (Pieter Wuille)
2ef1c9b Update overflow check (Russell O'Connor)
5660c13 prevent optimization in algorithms (Harshil Jani)
ce3cfc7 doc: Describe Jacobi calculation in safegcd_implementation.md (Elliott Jin)
6be0103 Add secp256k1_fe_is_square_var function (Pieter Wuille)
1de2a01 Native jacobi symbol algorithm (Pieter Wuille)
04c6c1b Make secp256k1_modinv64_det_check_pow2 support abs val (Pieter Wuille)
5fffb2c Make secp256k1_i128_check_pow2 support -(2^n) (Pieter Wuille)
e433034 ci: Shutdown wineserver whenever CI script exits (Tim Ruffing)
9a5a611 build: Suppress stupid MSVC linker warning (Tim Ruffing)
739c53b examples: Extend sig examples by call that uses static context (Tim Ruffing)
914276e build: Add SECP256K1_API_VAR to fix importing variables from DLLs (Tim Ruffing)
e089eec group: Further simply gej_add_ge (Tim Ruffing)
ac71020 group: Save a normalize_to_zero in gej_add_ge (Tim Ruffing)
8c7e0fc build: Add -Wreserved-identifier supported by clang (Tim Ruffing)
9b60e31 ci: Do not set git's `user.{email,name}` config options (Hennadii Stepanov)
ef39721 Do not link `bench` and `ctime_tests` to `COMMON_LIB` (Hennadii Stepanov)
c241586 ci: Don't fetch git history (Tim Ruffing)
0ecf318 ci: Use remote pull/merge ref instead of local git merge (Tim Ruffing)
9b7d186 Drop no longer used Autoheader macros (Hennadii Stepanov)
eb6beba scalar: restrict split_lambda args, improve doc and VERIFY_CHECKs (Jonas Nick)
7f49aa7 ci: add test job with -DVERIFY (Jonas Nick)
620ba3d benchmarks: fix bench_scalar_split (Jonas Nick)
e39d954 tests: Add CHECK_ILLEGAL(_VOID) macros and use in static ctx tests (Tim Ruffing)
61841fc contexts: Forbid randomizing secp256k1_context_static (Tim Ruffing)
4b6df5e contexts: Forbid cloning/destroying secp256k1_context_static (Tim Ruffing)
8f51229 ctime_tests: improve output when CHECKMEM_RUNNING is not defined (Jonas Nick)
2cd4e3c Drop no longer used `SECP_{LIBS,INCLUDE}` variables (Hennadii Stepanov)
613626f Drop no longer used `SECP_TEST_{LIBS,INCLUDE}` variables (Hennadii Stepanov)
d6ff738 Ensure safety of ctz_debruijn implementation. (Russell O'Connor)
ce60785 Introduce SECP256K1_B macro for curve b coefficient (Pieter Wuille)
4934aa7 Switch to exhaustive groups with small B coefficient (Pieter Wuille)
e03ef86 Make all non-API functions (except main) static (Pieter Wuille)
0f088ec Rename CTIMETEST -> CTIMETESTS (Pieter Wuille)
74b026f Add runtime checking for DECLASSIFY flag (Pieter Wuille)
5e2e6fc Run ctime test in Linux MSan CI job (Pieter Wuille)
1897406 Make ctime tests building configurable (Pieter Wuille)
5048be1 Rename valgrind_ctime_test -> ctime_tests (Pieter Wuille)
6eed6c1 Update error messages to suggest msan as well (Pieter Wuille)
8e11f89 Add support for msan integration to checkmem.h (Pieter Wuille)
8dc6407 Add compile-time error to valgrind_ctime_test (Pieter Wuille)
0db05a7 Abstract interactions with valgrind behind new checkmem.h (Pieter Wuille)
4f1a54e Move valgrind CPPFLAGS into SECP_CONFIG_DEFINES (Pieter Wuille)
d4a6b58 Add `noverify_tests` to `.gitignore` (Hennadii Stepanov)
e862c4a Makefile: add -I$(top_srcdir)/src to CPPFLAGS for precomputed (Matt Whitlock)

Pull request description:

ACKs for top commit:
  real-or-random:
    tACK  0d540ec

Tree-SHA512: bc54ccf752163ab6e1a12bb8c4e1f9339f4421d2e4f7716c408549514b3c902f2e9f727655799f1eecb085b0026761b04735b17be3c95c6cf54e07fbf7e86477

Author: jonasnick

.cirrus.yml

@@ -1,4 +1,6 @@
 env:
+  ### cirrus config
+  CIRRUS_CLONE_DEPTH: 1
   ### compiler options
   HOST:
   WRAPPER_CMD:
@@ -30,7 +32,7 @@ env:
   SECP256K1_TEST_ITERS:
   BENCH: yes
   SECP256K1_BENCH_ITERS: 2
-  CTIMETEST: yes
+  CTIMETESTS: yes
   # Compile and run the tests
   EXAMPLES: yes
 
@@ -47,8 +49,8 @@ cat_logs_snippet: &CAT_LOGS
       - cat noverify_tests.log || true
     cat_exhaustive_tests_log_script:
       - cat exhaustive_tests.log || true
-    cat_valgrind_ctime_test_log_script:
-      - cat valgrind_ctime_test.log || true
+    cat_ctime_tests_log_script:
+      - cat ctime_tests.log || true
     cat_bench_log_script:
       - cat bench.log || true
     cat_config_log_script:
@@ -61,10 +63,8 @@ cat_logs_snippet: &CAT_LOGS
 merge_base_script_snippet: &MERGE_BASE
   merge_base_script:
     - if [ "$CIRRUS_PR" = "" ]; then exit 0; fi
-    - git fetch $CIRRUS_REPO_CLONE_URL $CIRRUS_BASE_BRANCH
-    - git config --global user.email "[email protected]"
-    - git config --global user.name "ci"
-    - git merge FETCH_HEAD  # Merge base to detect silent merge conflicts
+    - git fetch --depth=1 $CIRRUS_REPO_CLONE_URL "pull/${CIRRUS_PR}/merge"
+    - git checkout FETCH_HEAD  # Use merged changes to detect silent merge conflicts
 
 linux_container_snippet: &LINUX_CONTAINER
   container:
@@ -88,9 +88,10 @@ task:
     - env: {WIDEMUL: int128,                 ECDH: yes, SCHNORRSIG: yes, EXPERIMENTAL: yes, ECDSA_S2C: yes, RANGEPROOF: yes, WHITELIST: yes, GENERATOR: yes, MUSIG: yes, ECDSAADAPTOR: yes, BPPP: yes}
     - env: {WIDEMUL: int128,  ASM: x86_64}
     - env: {                  RECOVERY: yes,            SCHNORRSIG: yes, EXPERIMENTAL: yes, ECDSA_S2C: yes, RANGEPROOF: yes, WHITELIST: yes, GENERATOR: yes, MUSIG: yes, ECDSAADAPTOR: yes, BPPP: yes}
-    - env: {BUILD: distcheck, WITH_VALGRIND: no, CTIMETEST: no, BENCH: no}
+    - env: {CTIMETESTS: no,    RECOVERY: yes, ECDH: yes, SCHNORRSIG: yes, EXPERIMENTAL: yes, ECDSA_S2C: yes, RANGEPROOF: yes, WHITELIST: yes, GENERATOR: yes, MUSIG: yes, ECDSAADAPTOR: yes, BPPP: yes, CPPFLAGS: -DVERIFY}
+    - env: {BUILD: distcheck, WITH_VALGRIND: no, CTIMETESTS: no, BENCH: no}
     - env: {CPPFLAGS: -DDETERMINISTIC}
-    - env: {CFLAGS: -O0, CTIMETEST: no}
+    - env: {CFLAGS: -O0, CTIMETESTS: no}
     - env: { ECMULTGENPRECISION: 2, ECMULTWINDOW: 2 }
     - env: { ECMULTGENPRECISION: 8, ECMULTWINDOW: 4 }
   matrix:
@@ -145,7 +146,7 @@ task:
   env:
     ASM: no
     WITH_VALGRIND: no
-    CTIMETEST: no
+    CTIMETESTS: no
   matrix:
     - env:
         CC: gcc
@@ -178,7 +179,7 @@ task:
     MUSIG: yes
     ECDSAADAPTOR: yes
     BPPP: yes
-    CTIMETEST: no
+    CTIMETESTS: no
   << : *MERGE_BASE
   test_script:
     # https://sourceware.org/bugzilla/show_bug.cgi?id=27008
@@ -197,7 +198,7 @@ task:
     ECDH: yes
     RECOVERY: yes
     SCHNORRSIG: yes
-    CTIMETEST: no
+    CTIMETESTS: no
   matrix:
     - env: {}
     - env: {EXPERIMENTAL: yes, ASM: arm}
@@ -217,7 +218,7 @@ task:
     ECDH: yes
     RECOVERY: yes
     SCHNORRSIG: yes
-    CTIMETEST: no
+    CTIMETESTS: no
   << : *MERGE_BASE
   test_script:
     - ./ci/cirrus.sh
@@ -234,7 +235,7 @@ task:
     ECDH: yes
     RECOVERY: yes
     SCHNORRSIG: yes
-    CTIMETEST: no
+    CTIMETESTS: no
   << : *MERGE_BASE
   test_script:
     - ./ci/cirrus.sh
@@ -248,7 +249,7 @@ task:
     ECDH: yes
     RECOVERY: yes
     SCHNORRSIG: yes
-    CTIMETEST: no
+    CTIMETESTS: no
   matrix:
     - name: "x86_64 (mingw32-w64): Windows (Debian stable, Wine)"
       env:
@@ -278,7 +279,7 @@ task:
     MUSIG: yes
     ECDSAADAPTOR: yes
     BPPP: yes
-    CTIMETEST: no
+    CTIMETESTS: no
     # Use a MinGW-w64 host to tell ./configure we're building for Windows.
     # This will detect some MinGW-w64 tools but then make will need only
     # the MSVC tools CC, AR and NM as specified below.
@@ -289,7 +290,7 @@ task:
     # Set non-essential options that affect the CLI messages here.
     # (They depend on the user's taste, so we don't want to set them automatically in configure.ac.)
     CFLAGS: -nologo -diagnostics:caret
-    LDFLAGS: -XCClinker -nologo -XCClinker -diagnostics:caret
+    LDFLAGS: -Xlinker -Xlinker -Xlinker -nologo
   matrix:
     - name: "x86_64 (MSVC): Windows (Debian stable, Wine)"
     - name: "x86_64 (MSVC): Windows (Debian stable, Wine, int128_struct)"
@@ -326,7 +327,7 @@ task:
     MUSIG: yes
     ECDSAADAPTOR: yes
     BPPP: yes
-    CTIMETEST: no
+    CTIMETESTS: no
   matrix:
     - name: "Valgrind (memcheck)"
       container:
@@ -379,10 +380,11 @@ task:
     MUSIG: yes
     ECDSAADAPTOR: yes
     BPPP: yes
-    CTIMETEST: no
+    CTIMETESTS: yes
     CC: clang
     SECP256K1_TEST_ITERS: 32
     ASM: no
+    WITH_VALGRIND: no
   container:
     memory: 2G
   matrix:

.gitignore

@@ -5,12 +5,13 @@ bench_generator
 bench_rangeproof
 bench_internal
 bench_whitelist
+noverify_tests
 tests
 example_musig
 exhaustive_tests
 precompute_ecmult_gen
 precompute_ecmult
-valgrind_ctime_test
+ctime_tests
 ecdh_example
 ecdsa_example
 schnorr_example

CHANGELOG.md

@@ -10,6 +10,20 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+#### Added
+ - Usage examples: Added a recommended method for securely clearing sensitive data, e.g., secret keys, from memory.
+ - Tests: Added a new test binary `noverify_tests`. This binary runs the tests without some additional checks present in the ordinary `tests` binary and is thereby closer to production binaries. The `noverify_tests` binary is automatically run as part of the `make check` target. 
+
+#### Fixed
+ - Fixed declarations of API variables for MSVC (`__declspec(dllimport)`). This fixes MSVC builds of programs which link against a libsecp256k1 DLL dynamically and use API variables (and not only API functions). Unfortunately, the MSVC linker now will emit warning `LNK4217` when trying to link against libsecp256k1 statically. Pass `/ignore:4217` to the linker to suppress this warning.
+
+#### Changed
+ - Forbade cloning or destroying `secp256k1_context_static`. Create a new context instead of cloning the static context. (If this change breaks your code, your code is probably wrong.)
+ - Forbade randomizing (copies of) `secp256k1_context_static`. Randomizing a copy of `secp256k1_context_static` did not have any effect and did not provide defense-in-depth protection against side-channel attacks. Create a new context if you want to benefit from randomization.
+
+#### Removed
+ - Removed the configuration header `src/libsecp256k1-config.h`. We recommend passing flags to `./configure` to set configuration options (see `./configure --help`). If you cannot or do not want to use `./configure`, pass configuration flags such as `-DSECP256K1_ENABLE_MODULE_SCHNORRSIG` manually to the compiler (see the file `configure.ac` for supported flags). 
+
 ## [0.2.0] - 2022-12-12
 
 #### Added

Makefile.am

@@ -49,6 +49,7 @@ noinst_HEADERS += src/modinv64_impl.h
 noinst_HEADERS += src/precomputed_ecmult.h
 noinst_HEADERS += src/precomputed_ecmult_gen.h
 noinst_HEADERS += src/assumptions.h
+noinst_HEADERS += src/checkmem.h
 noinst_HEADERS += src/util.h
 noinst_HEADERS += src/int128.h
 noinst_HEADERS += src/int128_impl.h
@@ -70,12 +71,14 @@ noinst_HEADERS += contrib/lax_der_parsing.h
 noinst_HEADERS += contrib/lax_der_parsing.c
 noinst_HEADERS += contrib/lax_der_privatekey_parsing.h
 noinst_HEADERS += contrib/lax_der_privatekey_parsing.c
-noinst_HEADERS += examples/random.h
+noinst_HEADERS += examples/examples_util.h
 
 PRECOMPUTED_LIB = libsecp256k1_precomputed.la
 noinst_LTLIBRARIES = $(PRECOMPUTED_LIB)
 libsecp256k1_precomputed_la_SOURCES =  src/precomputed_ecmult.c src/precomputed_ecmult_gen.c
-libsecp256k1_precomputed_la_CPPFLAGS = $(SECP_INCLUDES) $(SECP_CONFIG_DEFINES)
+# We need `-I$(top_srcdir)/src` in VPATH builds if libsecp256k1_precomputed_la_SOURCES have been recreated in the build tree.
+# This helps users and packagers who insist on recreating the precomputed files (e.g., Gentoo).
+libsecp256k1_precomputed_la_CPPFLAGS = -I$(top_srcdir)/src $(SECP_CONFIG_DEFINES)
 
 if USE_EXTERNAL_ASM
 COMMON_LIB = libsecp256k1_common.la
@@ -94,42 +97,32 @@ endif
 endif
 
 libsecp256k1_la_SOURCES = src/secp256k1.c
-libsecp256k1_la_CPPFLAGS = $(SECP_INCLUDES) $(SECP_CONFIG_DEFINES)
-libsecp256k1_la_LIBADD = $(SECP_LIBS) $(COMMON_LIB) $(PRECOMPUTED_LIB)
+libsecp256k1_la_CPPFLAGS = $(SECP_CONFIG_DEFINES)
+libsecp256k1_la_LIBADD = $(COMMON_LIB) $(PRECOMPUTED_LIB)
 libsecp256k1_la_LDFLAGS = -no-undefined -version-info $(LIB_VERSION_CURRENT):$(LIB_VERSION_REVISION):$(LIB_VERSION_AGE)
 
-if VALGRIND_ENABLED
-libsecp256k1_la_CPPFLAGS += -DVALGRIND
-endif
-
 noinst_PROGRAMS =
 if USE_BENCHMARK
 noinst_PROGRAMS += bench bench_internal bench_ecmult
 bench_SOURCES = src/bench.c
-bench_LDADD = libsecp256k1.la $(SECP_LIBS) $(SECP_TEST_LIBS) $(COMMON_LIB)
+bench_LDADD = libsecp256k1.la
 bench_CPPFLAGS = $(SECP_CONFIG_DEFINES)
 bench_internal_SOURCES = src/bench_internal.c
-bench_internal_LDADD = $(SECP_LIBS) $(COMMON_LIB) $(PRECOMPUTED_LIB)
-bench_internal_CPPFLAGS = $(SECP_INCLUDES) $(SECP_CONFIG_DEFINES)
+bench_internal_LDADD = $(COMMON_LIB) $(PRECOMPUTED_LIB)
+bench_internal_CPPFLAGS = $(SECP_CONFIG_DEFINES)
 bench_ecmult_SOURCES = src/bench_ecmult.c
-bench_ecmult_LDADD = $(SECP_LIBS) $(COMMON_LIB) $(PRECOMPUTED_LIB)
-bench_ecmult_CPPFLAGS = $(SECP_INCLUDES) $(SECP_CONFIG_DEFINES)
+bench_ecmult_LDADD = $(COMMON_LIB) $(PRECOMPUTED_LIB)
+bench_ecmult_CPPFLAGS = $(SECP_CONFIG_DEFINES)
 endif
 
 TESTS =
 if USE_TESTS
 TESTS += noverify_tests
 noinst_PROGRAMS += noverify_tests
 noverify_tests_SOURCES = src/tests.c
-noverify_tests_CPPFLAGS = $(SECP_INCLUDES) $(SECP_TEST_INCLUDES) $(SECP_CONFIG_DEFINES)
-noverify_tests_LDADD = $(SECP_LIBS) $(SECP_TEST_LIBS) $(COMMON_LIB) $(PRECOMPUTED_LIB)
+noverify_tests_CPPFLAGS = $(SECP_CONFIG_DEFINES)
+noverify_tests_LDADD = $(COMMON_LIB) $(PRECOMPUTED_LIB)
 noverify_tests_LDFLAGS = -static
-if VALGRIND_ENABLED
-noverify_tests_CPPFLAGS += -DVALGRIND
-noinst_PROGRAMS += valgrind_ctime_test
-valgrind_ctime_test_SOURCES = src/valgrind_ctime_test.c
-valgrind_ctime_test_LDADD = libsecp256k1.la $(SECP_LIBS) $(COMMON_LIB)
-endif
 if !ENABLE_COVERAGE
 TESTS += tests
 noinst_PROGRAMS += tests
@@ -140,15 +133,22 @@ tests_LDFLAGS  = $(noverify_tests_LDFLAGS)
 endif
 endif
 
+if USE_CTIME_TESTS
+noinst_PROGRAMS += ctime_tests
+ctime_tests_SOURCES = src/ctime_tests.c
+ctime_tests_LDADD = libsecp256k1.la
+ctime_tests_CPPFLAGS = $(SECP_CONFIG_DEFINES)
+endif
+
 if USE_EXHAUSTIVE_TESTS
 noinst_PROGRAMS += exhaustive_tests
 exhaustive_tests_SOURCES = src/tests_exhaustive.c
-exhaustive_tests_CPPFLAGS = $(SECP_INCLUDES) $(SECP_CONFIG_DEFINES)
+exhaustive_tests_CPPFLAGS = $(SECP_CONFIG_DEFINES)
 if !ENABLE_COVERAGE
 exhaustive_tests_CPPFLAGS += -DVERIFY
 endif
 # Note: do not include $(PRECOMPUTED_LIB) in exhaustive_tests (it uses runtime-generated tables).
-exhaustive_tests_LDADD = $(SECP_LIBS) $(COMMON_LIB)
+exhaustive_tests_LDADD = $(COMMON_LIB)
 exhaustive_tests_LDFLAGS = -static
 TESTS += exhaustive_tests
 endif
@@ -203,12 +203,12 @@ EXTRA_PROGRAMS = precompute_ecmult precompute_ecmult_gen
 CLEANFILES = $(EXTRA_PROGRAMS)
 
 precompute_ecmult_SOURCES = src/precompute_ecmult.c
-precompute_ecmult_CPPFLAGS = $(SECP_INCLUDES) $(SECP_CONFIG_DEFINES)
-precompute_ecmult_LDADD = $(SECP_LIBS) $(COMMON_LIB)
+precompute_ecmult_CPPFLAGS = $(SECP_CONFIG_DEFINES)
+precompute_ecmult_LDADD = $(COMMON_LIB)
 
 precompute_ecmult_gen_SOURCES = src/precompute_ecmult_gen.c
-precompute_ecmult_gen_CPPFLAGS = $(SECP_INCLUDES) $(SECP_CONFIG_DEFINES)
-precompute_ecmult_gen_LDADD = $(SECP_LIBS) $(COMMON_LIB)
+precompute_ecmult_gen_CPPFLAGS = $(SECP_CONFIG_DEFINES)
+precompute_ecmult_gen_LDADD = $(COMMON_LIB)
 
 # See Automake manual, Section "Errors with distclean".
 # We don't list any dependencies for the prebuilt files here because

ci/cirrus.sh

@@ -14,7 +14,7 @@ print_environment() {
             ECMULTWINDOW ECMULTGENPRECISION ASM WIDEMUL WITH_VALGRIND EXTRAFLAGS \
             EXPERIMENTAL ECDH RECOVERY SCHNORRSIG \
             ECDSA_S2C GENERATOR RANGEPROOF WHITELIST MUSIG ECDSAADAPTOR BPPP \
-            SECP256K1_TEST_ITERS BENCH SECP256K1_BENCH_ITERS CTIMETEST\
+            SECP256K1_TEST_ITERS BENCH SECP256K1_BENCH_ITERS CTIMETESTS\
             EXAMPLES \
             HOST WRAPPER_CMD \
             CC CFLAGS CPPFLAGS AR NM
@@ -35,6 +35,8 @@ print_environment
 # This speeds up jobs with many invocations of wine (e.g., ./configure with MSVC) tremendously.
 case "$WRAPPER_CMD" in
     *wine*)
+        # Make sure to shutdown wineserver whenever we exit.
+        trap "wineserver -k || true" EXIT INT HUP
         # This is apparently only reliable when we run a dummy command such as "hh.exe" afterwards.
         wineserver -p && wine hh.exe
         ;;
@@ -67,6 +69,7 @@ fi
     --enable-module-schnorrsig="$SCHNORRSIG"  --enable-module-musig="$MUSIG" --enable-module-ecdsa-adaptor="$ECDSAADAPTOR" \
     --enable-module-schnorrsig="$SCHNORRSIG" \
     --enable-examples="$EXAMPLES" \
+    --enable-ctime-tests="$CTIMETESTS" \
     --with-valgrind="$WITH_VALGRIND" \
     --host="$HOST" $EXTRAFLAGS
 
@@ -83,14 +86,15 @@ export LOG_COMPILER="$WRAPPER_CMD"
 
 make "$BUILD"
 
+# Using the local `libtool` because on macOS the system's libtool has nothing to do with GNU libtool
+EXEC='./libtool --mode=execute'
+if [ -n "$WRAPPER_CMD" ]
+then
+    EXEC="$EXEC $WRAPPER_CMD"
+fi
+
 if [ "$BENCH" = "yes" ]
 then
-    # Using the local `libtool` because on macOS the system's libtool has nothing to do with GNU libtool
-    EXEC='./libtool --mode=execute'
-    if [ -n "$WRAPPER_CMD" ]
-    then
-        EXEC="$EXEC $WRAPPER_CMD"
-    fi
     {
         $EXEC ./bench_ecmult
         $EXEC ./bench_internal
@@ -102,9 +106,13 @@ then
     } >> bench.log 2>&1
 fi
 
-if [ "$CTIMETEST" = "yes" ]
+if [ "$CTIMETESTS" = "yes" ]
 then
-    ./libtool --mode=execute valgrind --error-exitcode=42 ./valgrind_ctime_test > valgrind_ctime_test.log 2>&1
+    if [ "$WITH_VALGRIND" = "yes" ]; then
+        ./libtool --mode=execute valgrind --error-exitcode=42 ./ctime_tests > ctime_tests.log 2>&1
+    else
+        $EXEC ./ctime_tests > ctime_tests.log 2>&1
+    fi
 fi
 
 # Rebuild precomputed files (if not cross-compiling).
@@ -114,9 +122,6 @@ then
     make precomp
 fi
 
-# Shutdown wineserver again
-wineserver -k || true
-
 # Check that no repo files have been modified by the build.
 # (This fails for example if the precomp files need to be updated in the repo.)
 git diff --exit-code