Doubt about ucred structure in Kernel

GroupInode · Feb 17, 2012

I would like to know more about the ucred structure used in FreeBSD kernel for managing user credentials in kernel. I want to know how can use to get the user ID and also from the structure gets its data.

Thanking in anticipation.

GroupInode · Feb 20, 2012

As far as I know, ucred family of functions is used to manage user credential structures (struct ucred) within kernel.
I saw /usr/src/sys/sys/ucred.h file

Code:

struct ucred {
	u_int	cr_ref;			/* reference count */
#define	cr_startcopy cr_uid
	uid_t	cr_uid;			/* effective user id */
	uid_t	cr_ruid;		/* real user id */
	uid_t	cr_svuid;		/* saved user id */
	int	cr_ngroups;		/* number of groups */
	gid_t	cr_rgid;		/* real group id */
	gid_t	cr_svgid;		/* saved group id */
	struct uidinfo	*cr_uidinfo;	/* per euid resource consumption */
	struct uidinfo	*cr_ruidinfo;	/* per ruid resource consumption */
	struct prison	*cr_prison;	/* jail(2) */
	void		*cr_pspare;	/* general use */
	u_int		cr_flags;	/* credential flags */
	void 		*cr_pspare2[2];	/* general use 2 */
#define	cr_endcopy	cr_label
	struct label	*cr_label;	/* MAC label */
	struct auditinfo_addr	cr_audit;	/* Audit properties. */
	gid_t	*cr_groups;		/* groups */
	int	cr_agroups;		/* Available groups */
};

From the above structure I want to access real user id information.

Basically I'm changing the ufs.ko module trying to get information of the user who last modified the file. The change would be made in ufs_itimes_locked() which updates the modified time in the inode structure and when the modified time is changed I wanted to store information of who/which user (uid) modifies the file.

I'm stuck here as to which function to use get cr_uid/cr_ruid information in kernel space. I also tried getuid() but it is working only in user space not kernel space.

Any help would be appreciated.

jake_t · Mar 26, 2012

Hey GroupInode,

I personally haven't worked with ufs_itimes_locked() but could you not just access cr_uid/cr_ruid via the current thread?

IE:

Code:

uprintf("UID: %d\n", td->td_proc->p_ucred->cr_uid);

Hope this helps.

GroupInode · Mar 27, 2012

Solved

Thank you jake_t for replying to my thread. I had already figured out how to do this: the same way you are telling. That is by the cr_ruid field of curthread defined in </usr/src/sys/sys/pcpu.h>. The only thing was, I knew there was a current pointer in Linux, but we didn't know what was the name of the structure.

Thanks anyway