On Mon, Feb 13, 2012 at 3:34 PM, loody <miloody@xxxxxxxxx> wrote:
hi all:
I add below function in free_uid to get which task try to free user_struct
struct task_struct *p;
struct cred *c;
c = container_of(up,struct cred, user);
p = container_of(c,struct task_struct,real_cred);
printk("%s user %p, cred->user = %p p->pid = %d\n",__func__,up, c->user,p->pid);
but p->pid is always 0.
I think it is wrong for me to get cred and task by using container_of.
1. from kernel definition below, is it ok that member is pointer?
#define container_of(ptr, type, member)
it is just a macro, so member can be anything, and compiler will substitute the name during preprocessing time.
2. is there already exist macros or functions I can use for #1 above?
3. what is user_struct used for? When and under what circumstances
kernel will try to release it.
look into kernel/signal.c:__sigqueue_alloc() for example: user_struct is pointer to a user structure for identifying the user running in a particular process context mode, not necessarily itself.
it is free in kernel/user.c:free_user(), which is called by free_uid(). So who called free_uid()?
Look into kernel/sys.c:getpriority() syscall implementation:
} while_each_thread(g, p);
if (who != cred->uid)
free_uid(user); /* for find_user() */
break;
So those who called find_user() will call free_uid() (which then call free_user()....eh...convoluted logic!!!).
See the remark in kernel/user.c:find_user():
107 /*
108 * Locate the user_struct for the passed UID. If found, take a ref on it. The
109 * caller must undo that ref with free_uid().
110 *
111 * If the user_struct could not be found, return NULL.
112 */
113 struct user_struct *find_user(uid_t uid)
114 {
As indicated in remark above, that is the only situation I know when u have to free the user_struct (calling free_user()).
4. since user_struct is allocated by kmem_cache_zalloc, is there api
or tool I can monitor it?
Thanks for your help,
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Regards,
Peter Teoh
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