Subject: Re: SANITIZE=address failure on master (was: [PATCH v8 3/5] config: learn `git_protected_config()`)

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Mon, 25 Jul 2022 22:41:30 +0200

On Mon, Jul 25 2022, Glen Choo wrote:

> Ævar Arnfjörð Bjarmason <[email-masked]> writes:
>
>> On Thu, Jul 14 2022, Glen Choo via GitGitGadget wrote:
>>
>>> +/* Read values into protected_config. */
>>> +static void read_protected_config(void)
>>> +{
>>> +	char *xdg_config = NULL, *user_config = NULL, *system_config = NULL;
>>> +
>>> +	git_configset_init(&protected_config);
>>> +
>>> +	system_config = git_system_config();
>>> +	git_global_config(&user_config, &xdg_config);
>>> +
>>> +	git_configset_add_file(&protected_config, system_config);
>>> +	git_configset_add_file(&protected_config, xdg_config);
>>> +	git_configset_add_file(&protected_config, user_config);
>>> +	git_configset_add_parameters(&protected_config);
>>> +
>>> +	free(system_config);
>>> +	free(xdg_config);
>>> +	free(user_config);
>>> +}
>>
>> Noticed after it landed on master: This change fails with:
>>
>> 	make SANITIZE=address test T=t0410*.sh
>>
>> Running that manually shows that we fail like this:
>> 	
>> 	$ cat trash\ directory.t0410-partial-clone/httpd/error.log | grep -o AH0.*
>> 	AH00163: Apache/2.4.54 (Debian) configured -- resuming normal operations
>> 	AH00094: Command line: '/usr/sbin/apache2 -d /home/avar/g/git/t/trash directory.t0410-partial-clone/httpd -f /home/avar/g/git/t/lib-httpd/apache.conf -c Listen 127.0.0.1:10410'
>> 	AH01215: AddressSanitizer:DEADLYSIGNAL: /home/avar/g/git/git-http-backend
>> 	AH01215: =================================================================: /home/avar/g/git/git-http-backend
>> 	AH01215: ==27820==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7f7af5dc0d66 bp 0x7fff11964450 sp 0x7fff11963be8 T0): /home/avar/g/git/git-http-backend
>> 	AH01215: ==27820==The signal is caused by a READ memory access.: /home/avar/g/git/git-http-backend
>> 	AH01215: ==27820==Hint: address points to the zero page.: /home/avar/g/git/git-http-backend
>> 	AH01215:     #0 0x7f7af5dc0d66 in __sanitizer::internal_strlen(char const*) ../../../../src/libsanitizer/sanitizer_common/sanitizer_libc.cpp:167: /home/avar/g/git/git-http-backend
>> 	AH01215:     #1 0x7f7af5d512f2 in __interceptor_fopen64 ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:6220: /home/avar/g/git/git-http-backend
>> 	AH01215:     #2 0x562a65e37cc8 in git_fopen compat/fopen.c:22: /home/avar/g/git/git-http-backend
>> 	AH01215:     #3 0x562a65df3879 in fopen_or_warn wrapper.c:431: /home/avar/g/git/git-http-backend
>> 	AH01215:     #4 0x562a65a12476 in git_config_from_file_with_options config.c:1982: /home/avar/g/git/git-http-backend
>> 	AH01215:     #5 0x562a65a124f4 in git_config_from_file config.c:1993: /home/avar/g/git/git-http-backend
>> 	AH01215:     #6 0x562a65a15288 in git_configset_add_file config.c:2389: /home/avar/g/git/git-http-backend
>> 	AH01215:     #7 0x562a65a16a37 in read_protected_config config.c:2649: /home/avar/g/git/git-http-backend
>> 	AH01215:     #8 0x562a65a16b5c in git_protected_config config.c:2661: /home/avar/g/git/git-http-backend
>> 	AH01215:     #9 0x562a65dd9f9a in get_upload_pack_config upload-pack.c:1342: /home/avar/g/git/git-http-backend
>> 	AH01215:     #10 0x562a65ddc1cb in upload_pack_v2 upload-pack.c:1706: /home/avar/g/git/git-http-backend
>> 	AH01215:     #11 0x562a65d2eb8a in process_request serve.c:308: /home/avar/g/git/git-http-backend
>> 	AH01215:     #12 0x562a65d2ec18 in protocol_v2_serve_loop serve.c:323: /home/avar/g/git/git-http-backend
>> 	AH01215:     #13 0x562a6593c5ae in cmd_upload_pack builtin/upload-pack.c:55: /home/avar/g/git/git-http-backend
>> 	AH01215:     #14 0x562a656cf8ff in run_builtin git.c:466: /home/avar/g/git/git-http-backend
>> 	AH01215:     #15 0x562a656d02ab in handle_builtin git.c:720: /home/avar/g/git/git-http-backend
>> 	AH01215:     #16 0x562a656d09d5 in run_argv git.c:787: /home/avar/g/git/git-http-backend
>> 	AH01215:     #17 0x562a656d174f in cmd_main git.c:920: /home/avar/g/git/git-http-backend
>> 	AH01215:     #18 0x562a6594b0b9 in main common-main.c:56: /home/avar/g/git/git-http-backend
>> 	AH01215:     #19 0x7f7af5a5681c in __libc_start_main ../csu/libc-start.c:332: /home/avar/g/git/git-http-backend
>> 	AH01215:     #20 0x562a656cb209 in _start (git+0x1d1209): /home/avar/g/git/git-http-backend
>> 	AH01215: : /home/avar/g/git/git-http-backend
>> 	AH01215: AddressSanitizer can not provide additional info.: /home/avar/g/git/git-http-backend
>> 	AH01215: SUMMARY: AddressSanitizer: SEGV
>> ../../../../src/libsanitizer/sanitizer_common/sanitizer_libc.cpp:167
>> in __sanitizer::internal_strlen(char const*):
>> /home/avar/g/git/git-http-backend
>> 	AH01215: ==27820==ABORTING: /home/avar/g/git/git-http-backend
>> 	AH01215: error: upload-pack died of signal 6: /home/avar/g/git/git-http-backend
>>
>> (We really should have a SANITIZE=address in CI, but it takes a while...)
>
> Thanks. I narrowed the failure down to the hunk above, specifically this
> line:
>
>   git_configset_add_file(&protected_config, xdg_config);
>
> Since xdg_config can be NULL, this results in the failing call
> fopen_or_warn(NULL, "r").
>
> This logic was lifted  from do_git_config_sequence(), which checks that
> each of the paths are not NULL. So a fix might be something like:
>
> ----- >8 --------- >8 --------- >8 --------- >8 --------- >8 ----
>
>   diff --git a/config.c b/config.c
>   index 015bec360f..208a3dd7a7 100644
>   --- a/config.c
>   +++ b/config.c
>   @@ -2645,9 +2645,13 @@ static void read_protected_config(void)
>     system_config = git_system_config();
>     git_global_config(&user_config, &xdg_config);
>
>   -	git_configset_add_file(&protected_config, system_config);
>   -	git_configset_add_file(&protected_config, xdg_config);
>   -	git_configset_add_file(&protected_config, user_config);
>   +
>   +	if (system_config)
>   +		git_configset_add_file(&protected_config, system_config);
>   +	if (xdg_config)
>   +		git_configset_add_file(&protected_config, xdg_config);
>   +	if (user_config)
>   +		git_configset_add_file(&protected_config, user_config);
>     git_configset_add_parameters(&protected_config);
>
>     free(system_config);
>
> ----- >8 --------- >8 --------- >8 --------- >8 --------- >8 ----
>
> I'm not sure if system_config can ever be NULL, but (xdg|user)_config is
> NULL when $HOME is unset, and xdg_config is also unset if
> $GIT_CONFIG_GLOBAL is set.

Not having looked into it much at all: Doesn't this then introduce
another logic error where git_protected_config() is now buggy, i.e. it's
a "lazy load" method where we'll expect to read_protected_config()
first.

The assumption with that seems to have been that it's invariant within a
single process, is that still the case, or can e.g. HOME be set during
our runtime when we rely on these functions?

(I don't know)