Re: [PATCH] generic/484: Need another process to check record locks

[Xiao Yang <yangx.jy@cn.fujitsu.com>]

On 2018/05/22 10:36, Xiong Murphy Zhou wrote:
> On Mon, May 21, 2018 at 01:42:00PM +0800, Xiao Yang wrote:
>> 1) According to fcntl(2) manpage, A single process always gets F_UNLCK in
>>     the l_type field when using fcntl(F_GETLK) to acquire the existing lock
>>     set by itself because it could convert the existing lock to a new lock
>>     unconditionally.  So we need another process to check if the lock exists.
> I used to do that, eventaully I deleted it because we don't need
> to check in another process in this case.
Hi Xiong,

Even if the fix patch[1] has been applied, you always gets F_UNLCK without another process.

According to fcntl(2) manpage:
F_GETLK
	On  input  to this call, lock describes a lock we would like to place on the file.
	If the lock could be placed, fcntl() does not actually place it, but returns F_UNLCK
	in the l_type field of lock and leaves the other fields of the structure unchanged.

A single process can always place a new lock on the file and return F_UNLCK because the existing
lock is set by itself.

[1] https://www.spinics.net/lists/linux-fsdevel/msg123144.html

Thanks,
Xiao Yang

> Thanks,
> Xiong
>
>> 2) Remove redundant exit(0).
>>
>> Signed-off-by: Xiao Yang<yangx.jy@cn.fujitsu.com>
>> ---
>>   src/t_locks_execve.c | 24 ++++++++++++++++++------
>>   1 file changed, 18 insertions(+), 6 deletions(-)
>>
>> diff --git a/src/t_locks_execve.c b/src/t_locks_execve.c
>> index 9ad2dc3..d99d7de 100644
>> --- a/src/t_locks_execve.c
>> +++ b/src/t_locks_execve.c
>> @@ -8,6 +8,7 @@
>>   #include<errno.h>
>>   #include<pthread.h>
>>   #include<unistd.h>
>> +#include<sys/types.h>
>>   #include<sys/wait.h>
>>
>>   static void err_exit(char *op, int errn)
>> @@ -32,12 +33,24 @@ struct flock fl = {
>>
>>   static void checklock(int fd)
>>   {
>> -	if (fcntl(fd, F_GETLK,&fl)<  0)
>> -		err_exit("getlk", errno);
>> -	if (fl.l_type == F_UNLCK) {
>> -		printf("record lock is not preserved across execve(2)\n");
>> -		exit(1);
>> +	pid_t pid;
>> +
>> +	pid = fork();
>> +	if (pid<  0)
>> +		err_exit("fork", errno);
>> +
>> +	if (!pid) {
>> +		if (fcntl(fd, F_GETLK,&fl)<  0)
>> +			err_exit("getlk", errno);
>> +		if (fl.l_type == F_UNLCK) {
>> +			printf("record lock is not preserved across execve(2)\n");
>> +			exit(1);
>> +		}
>> +		exit(0);
>>   	}
>> +
>> +	waitpid(pid, NULL, 0);
>> +
>>   	exit(0);
>>   }
>>
>> @@ -52,7 +65,6 @@ int main(int argc, char **argv)
>>   	if (argc == 3) {
>>   		fd = atoi(argv[2]);
>>   		checklock(fd);
>> -		exit(0);
>>   	}
>>
>>   	fd = open(argv[1], O_WRONLY|O_CREAT, 0755);
>> -- 
>> 1.8.3.1
>>
>>
>>
>
> .
>