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[146.198.22.11]) by smtp.googlemail.com with ESMTPSA id t16sm15227197wmi.2.2021.06.20.11.14.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 20 Jun 2021 11:14:37 -0700 (PDT) Subject: Re: [PATCH 11/12] builtin/rebase: fix options.strategy memory lifecycle To: andrzej@ahunt.org, git@vger.kernel.org References: <20210620151204.19260-1-andrzej@ahunt.org> <20210620151204.19260-12-andrzej@ahunt.org> From: Phillip Wood Message-ID: <6e02fc85-42a4-8b19-1fe7-3527c2308a24@gmail.com> Date: Sun, 20 Jun 2021 19:14:36 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210620151204.19260-12-andrzej@ahunt.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org Hi Andrzej Thanks for working on removing memory leaks from git. On 20/06/2021 16:12, andrzej@ahunt.org wrote: > From: Andrzej Hunt > > This change: > - xstrdup()'s all string being used for replace_opts.strategy, to I think you mean replay_opts rather than replace_opts. > guarantee that replace_opts owns these strings. This is needed because > sequencer_remove_state() will free replace_opts.strategy, and it's > usually called as part of the usage of replace_opts. > - Removes xstrdup()'s being used to populate options.strategy in > cmd_rebase(), which avoids leaking options.strategy, even in the > case where strategy is never moved/copied into replace_opts. > These changes are needed because: > - We would always create a new string for options.strategy if we either > get a strategy via options (OPT_STRING(...strategy...), or via > GIT_TEST_MERGE_ALGORITHM. > - But only sometimes is this string copied into replace_opts - in which > case it did get free()'d in sequencer_remove_state(). > - The rest of the time, the newly allocated string would remain unused, > causing a leak. But we can't just add a free because that can result > in a double-free in those cases where replace_opts was populated. > > An alternative approach would be to set options.strategy to NULL when > moving the pointer to replace_opts.strategy, combined with always > free()'ing options.strategy, but that seems like a more > complicated and wasteful approach. read_basic_state() contains if (file_exists(state_dir_path("strategy", opts))) { strbuf_reset(&buf); if (!read_oneliner(&buf, state_dir_path("strategy", opts), READ_ONELINER_WARN_MISSING)) return -1; free(opts->strategy); opts->strategy = xstrdup(buf.buf); } So we do try to free opts->strategy when reading the state from disc and we allocate a new string. I suspect that opts->strategy is actually NULL in when this function is called but I haven't checked. Given that we are allocating a copy above I think maybe your alternative approach of always freeing opts->strategy would be better. Best Wishes Phillip > This was first seen when running t0021 with LSAN, but t2012 helped catch > the fact that we can't just free(options.strategy) at the end of > cmd_rebase (as that can cause a double-free). LSAN output from t0021: > > LSAN output from t0021: > > Direct leak of 4 byte(s) in 1 object(s) allocated from: > #0 0x486804 in strdup ../projects/compiler-rt/lib/asan/asan_interceptors.cpp:452:3 > #1 0xa71eb8 in xstrdup wrapper.c:29:14 > #2 0x61b1cc in cmd_rebase builtin/rebase.c:1779:22 > #3 0x4ce83e in run_builtin git.c:475:11 > #4 0x4ccafe in handle_builtin git.c:729:3 > #5 0x4cb01c in run_argv git.c:818:4 > #6 0x4cb01c in cmd_main git.c:949:19 > #7 0x6b3fad in main common-main.c:52:11 > #8 0x7f267b512349 in __libc_start_main (/lib64/libc.so.6+0x24349) > > SUMMARY: AddressSanitizer: 4 byte(s) leaked in 1 allocation(s). > > Signed-off-by: Andrzej Hunt > --- > builtin/rebase.c | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff --git a/builtin/rebase.c b/builtin/rebase.c > index 12f093121d..9d81db0f3a 100644 > --- a/builtin/rebase.c > +++ b/builtin/rebase.c > @@ -139,7 +139,7 @@ static struct replay_opts get_replay_opts(const struct rebase_options *opts) > replay.ignore_date = opts->ignore_date; > replay.gpg_sign = xstrdup_or_null(opts->gpg_sign_opt); > if (opts->strategy) > - replay.strategy = opts->strategy; > + replay.strategy = xstrdup_or_null(opts->strategy); > else if (!replay.strategy && replay.default_strategy) { > replay.strategy = replay.default_strategy; > replay.default_strategy = NULL; > @@ -1723,7 +1723,6 @@ int cmd_rebase(int argc, const char **argv, const char *prefix) > } > > if (options.strategy) { > - options.strategy = xstrdup(options.strategy); > switch (options.type) { > case REBASE_APPLY: > die(_("--strategy requires --merge or --interactive")); > @@ -1776,7 +1775,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix) > if (options.type == REBASE_MERGE && > !options.strategy && > getenv("GIT_TEST_MERGE_ALGORITHM")) > - options.strategy = xstrdup(getenv("GIT_TEST_MERGE_ALGORITHM")); > + options.strategy = getenv("GIT_TEST_MERGE_ALGORITHM"); > > switch (options.type) { > case REBASE_MERGE: >