Re: [PATCH] Monitor: improve check_one_sharer() for checking duplicated process

[Coly Li <colyli@suse.de>]

On 2020/4/13 09:47, Zhong Lidong wrote:
> On 4/11/20 12:24 AM, Coly Li wrote:
>> When running mdadm monitor with scan mode, only one autorebuild process
>> is allowed. check_one_sharer() checks duplicated process by following
>> steps,
>> 1) Read autorebuild.pid file,
>>    - if file does not exist, no duplicated process, go to 3).
>>    - if file exists, continue to next step.
>> 2) Read pid number from autorebuild.pid file, then check procfs pid
>>    directory /proc/<PID>,
>>    - if the directory does not exist, no duplicated process, go to 3)
>>    - if the directory exists, print error message for duplicated process
>>      and exit this mdadm.
>> 3) Write current pid into autorebuild.pid file, continue to monitor in
>>    scan mode.
>>
>> The problem for the above step 2) is, if after system reboots and
>> another different process happens to have exact same pid number which
>> autorebuild.pid file records, check_one_sharer() will treat it as a
>> duplicated mdadm process and returns error with message "Only one
>> autorebuild process allowed in scan mode, aborting".
>>
>> This patch tries to fix the above same-pid-but-different-process issue
>> by one more step to check the process command name,
>> 1) Read autorebuild.pid file
>>    - if file does not exist, no duplicated process, go to 4).
>>    - if file exists, continue to next step.
>> 2) Read pid number from autorebuild.pid file, then check procfs file
>>    comm with the specific pid directory /proc/<PID>/comm
>>    - if the file does not exit, it means the directory /proc/<PID> does
>>      not exist, go to 4)
>>    - if the file exits, continue next step
>> 3) Read process command name from /proc/<PIC>/comm, compare the command
>>    name with "mdadm" process name,
>>    - if not equal, no duplicated process, goto 4)
>>    - if strings are equal, print error message for duplicated process
>>      and exit this mdadm.
>> 4) Write current pid into autorebuild.pid file, continue to monitor in
>>    scan mode.
>>
>> Now check_one_sharer() returns error for duplicated process only when
>> the recorded pid from autorebuild.pid exists, and the process has exact
>> same command name as "mdadm".
>>
> 
> Consider another corner case: what if the recorded pid from
> autorebuild.pid is actually used by other mdadm command, such as "mdadm
> --wait"? It shouldn't report error now.
> 

Hi Lidong,

This is a try-best effort, if there happens to be another mdadm process
has exact same pid number, this mdadm in scan mode has to fail.

There is no perfect method to prevent duplicated process, e.g. rename
the mdadm program to another name, this patch is not able to detect it
as duplicated process neither.

This is an improvement for current check_one_sharer(), not a silver bullet.

Thank.

Coly Li




> 
>> Reported-by: Shinkichi Yamazaki <shinkichi.yamazaki@suse.com>
>> Signed-off-by: Coly Li <colyli@suse.de>
>> ---
>>  Monitor.c | 32 ++++++++++++++++++++------------
>>  1 file changed, 20 insertions(+), 12 deletions(-)
>>
>> diff --git a/Monitor.c b/Monitor.c
>> index b527165..2d6b3b9 100644
>> --- a/Monitor.c
>> +++ b/Monitor.c
>> @@ -301,26 +301,34 @@ static int make_daemon(char *pidfile)
>>  
>>  static int check_one_sharer(int scan)
>>  {
>> -	int pid, rv;
>> +	int pid;
>> +	FILE *comm_fp;
>>  	FILE *fp;
>> -	char dir[20];
>> +	char comm_path[100];
>>  	char path[100];
>> -	struct stat buf;
>> +	char comm[20];
>> +
>>  	sprintf(path, "%s/autorebuild.pid", MDMON_DIR);
>>  	fp = fopen(path, "r");
>>  	if (fp) {
>>  		if (fscanf(fp, "%d", &pid) != 1)
>>  			pid = -1;
>> -		sprintf(dir, "/proc/%d", pid);
>> -		rv = stat(dir, &buf);
>> -		if (rv != -1) {
>> -			if (scan) {
>> -				pr_err("Only one autorebuild process allowed in scan mode, aborting\n");
>> -				fclose(fp);
>> -				return 1;
>> -			} else {
>> -				pr_err("Warning: One autorebuild process already running.\n");
>> +		snprintf(comm_path, sizeof(comm_path),
>> +			 "/proc/%d/comm", pid);
>> +		comm_fp = fopen(comm_path, "r");
>> +		if (comm_fp) {
>> +			if (fscanf(comm_fp, "%s", comm) &&
>> +			    strncmp(basename(comm), Name, strlen(Name)) == 0) {
>> +				if (scan) {
>> +					pr_err("Only one autorebuild process allowed in scan mode, aborting\n");
>> +					fclose(comm_fp);
>> +					fclose(fp);
>> +					return 1;
>> +				} else {
>> +					pr_err("Warning: One autorebuild process already running.\n");
>> +				}
>>  			}
>> +			fclose(comm_fp);
>>  		}
>>  		fclose(fp);
>>  	}
>>