> -----Original Message----- > From: Al Viro [mailto:viro@xxxxxxxxxxxxxxxx] On Behalf Of Al Viro > Sent: Saturday, August 31, 2013 3:36 PM > To: Liu, Chuansheng > Cc: Eric Dumazet; linux-fsdevel@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx > Subject: Re: [PATCH] Fix the race between the fget() and close() > > On Sat, Aug 31, 2013 at 07:01:33AM +0000, Liu, Chuansheng wrote: > > > My scenario is: > > P1 files_struct refcount is 1, P2's is 1 also. > > P1 get_files_struct(P2) > > P1 install one file into P2's files_struct > > P1 put_files_struct(P2) > > > > Then P1 and P2's files_struct refcount are 1, then when P1 is doing ioctl() and > P2 is exiting > > with put_files_struct(P2), the race will occur, my understanding is wrong? > > First of all, this wouldn't have been a problem (so you get a new reference > to file inserted in P2's files_struct; file refcount had been bumped, so > destruction of P2's files_struct will undo that increment of file refcount > and we are still fine). _Removal_ in a similar scenario would have been > a problem, with P2 doing fdget() while its table isn't shared, then P1 > removing a reference from it and dropping a file - the last one, at that, > since fdget() assumed that the reference would've stayed in P2's descriptor > table. HOWEVER, P1 does not do get_files_struct(P2) at all - it's only > done by P2 in binder_mmap(). Got it, thanks. In other process, the fget() + _fd_install() should be the same as the process call open() directly, and the file reference count will be at least 2. > > Again, the invariant to look for is this: > * if current->files had not been shared at fdget() time, it won't > be shared at matching fdput() and no entries will have been removed in > between. > > task_fd_install()/task_close_fd() are done on proc->files, which contributes > to descriptor table refcount. All other modifications are done to > current->files, which also contributes to refcount. If at fdget() time > current->files had refcount 1, we had no other processes with task->files > pointing to this descriptor table *and* no binder_proc had their ->files > pointint to it. No new ones may appear, since new process could get > such a reference only from do_fork() called by us and new binder_proc could > get such a reference only from binder_mmap() called by us. Neither is > called between fdget() and fdput(). So in that case the only reference > to this descriptor table will remain current->files and all removals > would have to be done by ourselves (and not via task_close_fd(), at that). > > And AFAICS, binder_lock() prevents proc->files being dropped under > task_close_fd() and task_fd_install(). Hell knows... > > How reproducible it is? Do you have any more instances, or had that > been a one-off panic? Just meet once yet. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
