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Full Discussion: getgroups usage on linux
Top Forums Programming getgroups usage on linux Post 302517998 by jim mcnamara on Thursday 28th of April 2011 12:14:19 PM
Old 04-28-2011
You have to seteuid to the user, get the group, then return to uid 0. Because setuid() will change all of the uid settings (real, effective, etc.) you may think you can call seteuid() instead, so that you can later call setuid to root. But seteuid() does not change groups. Which does not work for what you want....

So, you have to exec(), setuid() to nobody, write the result of getgroups to shared memory, exit the child process.

IMO, It is probably less complicated to try this:
Code:
char *
get_gid(char *dest, const char *uid)
{
   FILE *sh=NULL;
   char cmd[40]={0x0};
   char result[80]={0x0};
   
   sprintf(cmd, "/usr/bin/id -G %s";, uid);
   sh=popen(cmd, "r");
   if(fgets(result, sizeof(result), sh)!=NULL)
       strcpy(dest, result);
   pclose(sh);
   return dest;
}

Then parse out the uids, usually nobody is a single group so one atoi call returns the group.

Last edited by jim mcnamara; 04-28-2011 at 02:15 PM.. Reason: change -g to -R see red text
 

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getgroups(2)							   System Calls 						      getgroups(2)

NAME
getgroups, setgroups - get or set supplementary group access list IDs SYNOPSIS
#include <unistd.h> int getgroups(int gidsetsize, gid_t *grouplist); int setgroups(int ngroups, const gid_t *grouplist); DESCRIPTION
The getgroups() function gets the current supplemental group access list of the calling process and stores the result in the array of group IDs specified by grouplist. This array has gidsetsize entries and must be large enough to contain the entire list. This list cannot be larger than NGROUPS_MAX. If gidsetsize equals 0, getgroups() will return the number of groups to which the calling process belongs without modifying the array pointed to by grouplist. The setgroups() function sets the supplementary group access list of the calling process from the array of group IDs specified by grou- plist. The number of entries is specified by ngroups and can not be greater than NGROUPS_MAX. RETURN VALUES
Upon successful completion, getgroups() returns the number of supplementary group IDs set for the calling process and setgroups() returns 0. Otherwise, -1 is returned and errno is set to indicate the error. ERRORS
The getgroups() and setgroups() functions will fail if: EFAULT A referenced part of the array pointed to by grouplist is an illegal address. The getgroups() function will fail if: EINVAL The value of gidsetsize is non-zero and less than the number of supplementary group IDs set for the calling process. The setgroups() function will fail if: EINVAL The value of ngroups is greater than {NGROUPS_MAX}. EPERM The {PRIV_PROC_SETID} privilege is not asserted in the effective set of the calling process. USAGE
Use of the setgroups() function requires the {PRIV_PROC_SETID} privilege. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |getgroups() is Standard. | +-----------------------------+-----------------------------+ |MT-Level |Async-Signal-Safe | +-----------------------------+-----------------------------+ SEE ALSO
groups(1), chown(2), getuid(2), setuid(2), getgrnam(3C), initgroups(3C), attributes(5), privileges(5), standards(5) SunOS 5.10 1 Feb 2003 getgroups(2)
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