[PATCH] state: check return value of chown

[yeholmes at outlook.com]

From: Ye Holmes <yeholmes at outlook.com>

wrap around chown with `assert, this avoids unused result
warnings being treated as errors by prebuilt GCC (with
glibc-2.31, version 10.2):

procd-2020-12-12-7f12c89d/state.c: In function 'state_enter':
procd-2020-12-12-7f12c89d/state.c:147:4: error: ignoring
  return value of 'chown' declared with attribute
  'warn_unused_result' [-Werror=unused-result]
  147 |    chown(p->pw_dir, p->pw_uid, p->pw_gid);
      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Ye Holmes <yeholmes at outlook.com>
---
Thanks for your note, I will keep in mind that not to
increase code size by introducing unnecessary strings.
I've verified adding `assert resolves the error.

>On Sat, Jan 02, 2021 at 10:01:36PM +0800, yeholmes at outlook.com wrote:
>> From: Ye Holmes <yeholmes at outlook.com>
>>
>> Output warning in case changing the ownership of ubus' home
>> directory has failed. Besides, this avoids treating unused
>> result warning as error by GCC (with glibc-2.31, version 10.2):
>>
>> procd-2020-12-12-7f12c89d/state.c: In function 'state_enter':
>> procd-2020-12-12-7f12c89d/state.c:147:4: error: ignoring
>>   return value of 'chown' declared with attribute
>>   'warn_unused_result' [-Werror=unused-result]
>>   147 |    chown(p->pw_dir, p->pw_uid, p->pw_gid);
>>       |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> Signed-off-by: Ye Holmes <yeholmes at outlook.com>
>> ---
>>  state.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/state.c b/state.c
>> index 44f56c6..d68776f 100644
>> --- a/state.c
>> +++ b/state.c
>> @@ -144,7 +144,8 @@ static void state_enter(void)
>>  		if (p) {
>>  			LOG("- ubus -\n");
>>  			mkdir(p->pw_dir, 0755);
>> -			chown(p->pw_dir, p->pw_uid, p->pw_gid);
>> +			if (chown(p->pw_dir, p->pw_uid, p->pw_gid))
>> +				fprintf(stderr, "Failed to change ownership for %s\n", p->pw_dir);
>
>Please let's not have a custom error message for cases which
>practically never occur. If we would really cover all that, around
>80% of the size of executables like procd would be error messages.
>Imho an assertion is the right thing to do here.
>
>>  		} else {
>>  			LOG("- ubus (running as root!) -\n");
>>  		}
>> --
>> 2.25.1
---
 state.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/state.c b/state.c
index 44f56c6..e9f101f 100644
--- a/state.c
+++ b/state.c
@@ -12,6 +12,7 @@
  * GNU General Public License for more details.
  */
 
+#include <assert.h>
 #include <fcntl.h>
 #include <pwd.h>
 #include <sys/reboot.h>
@@ -144,7 +145,7 @@ static void state_enter(void)
 		if (p) {
 			LOG("- ubus -\n");
 			mkdir(p->pw_dir, 0755);
-			chown(p->pw_dir, p->pw_uid, p->pw_gid);
+			assert(chown(p->pw_dir, p->pw_uid, p->pw_gid) == 0);
 		} else {
 			LOG("- ubus (running as root!) -\n");
 		}
-- 
2.25.1