Re: [PATCH/RFC 1/2] anon-inodes: Remove fd_install() from anon_inode_getfd()

Roland Dreier
2008-02-28T20:25:15Z

> If we let the caller call fd_install(), then it may be messed up WRT
> cleanup (fd, file, inode).

Yes, that is a tiny bit tricky (need to call put_unused_fd() if you
don't install the fd).

> How about removing the inode pointer handout altogether, and *doing*
> fd_install() inside anon_inode_getfd() like:
>
> if (pfile != NULL) {
> get_file(file);
> *pfile = file;
> }
> fd_install(fd, file);
>
> In this way, if the caller want the file* back, he gets the reference
> bumped before fd_install().

I think that may be a bit cleaner than Al's approach, but it still
leaves the same trap that create_vcpu_fd() falls into. The current
code is:

static int create_vcpu_fd(struct kvm_vcpu *vcpu)
{
int fd, r;
struct inode *inode;
struct file *file;

r = anon_inode_getfd(&fd, &inode, &file,
"kvm-vcpu", &kvm_vcpu_fops, vcpu);
if (r)
return r;
atomic_inc(&vcpu->kvm->filp->f_count);
return fd;
}

and with your proposal, the natural way to write that becomes:

static int create_vcpu_fd(struct kvm_vcpu *vcpu)
{
int fd, r;

r = anon_inode_getfd(&fd, NULL,
"kvm-vcpu", &kvm_vcpu_fops, vcpu);
if (r)
return r;
atomic_inc(&vcpu->kvm->filp->f_count);
return fd;
}

which still has the same bug.

Maybe a good way to handle this is just to make the get_file() not
optional. I dunno... I feel like we've spent more discussion on this
point than it deserves, so someone should just make a decision and
I'll adapt the ib_uverbs code to work with whatever it is.

- R.