On Sun, 2017-04-16 at 11:51 +0200, Nicolai Stange wrote: > > +++ b/fs/debugfs/file.c > @@ -53,6 +53,7 @@ const struct file_operations > *debugfs_real_fops(const struct file *filp) > { > struct debugfs_fsdata *fsd = F_DENTRY(filp)->d_fsdata; > > + WARN_ON((unsigned long)fsd & > DEBUGFS_FSDATA_IS_REAL_FOPS_BIT); > return fsd->real_fops;
I'm not a fan of BUG_ON(), but in this case, if you have a completely bogus pointer here, and then you return fsd->real_fops which will be even more bogus, and *then* you call a function from within it... that seems like a recipe for disaster. So either you could return some valid ops (perhaps debugfs_noop_file_operations although those don't have .name or .poll, so it doesn't cover everything), or you can just BUG_ON() here directly, saving the incomprehensible crash later. johannes > EXPORT_SYMBOL_GPL(debugfs_real_fops); > @@ -74,9 +75,35 @@ EXPORT_SYMBOL_GPL(debugfs_real_fops); > */ > int debugfs_file_get(struct dentry *dentry) > { > - struct debugfs_fsdata *fsd = dentry->d_fsdata; > + struct debugfs_fsdata *fsd; > + void *d_fsd; > + > + d_fsd = READ_ONCE(dentry->d_fsdata); > + if (!((unsigned long)d_fsd & > DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)) { > + fsd = d_fsd; > + } else { > + fsd = kmalloc(sizeof(*fsd), GFP_KERNEL); > + if (!fsd) > + return -ENOMEM; > + > + fsd->real_fops = (void *)((unsigned long)d_fsd & > + ~DEBUGFS_FSDATA_IS_REAL_FOPS > _BIT); > + refcount_set(&fsd->active_users, 1); > + init_completion(&fsd->active_users_drained); > + if (cmpxchg(&dentry->d_fsdata, d_fsd, fsd) != d_fsd) > { > + kfree(fsd); > + fsd = READ_ONCE(dentry->d_fsdata); > + } > + } > > - /* Avoid starvation of removers. */ > + /* > + * In case of a successful cmpxchg() above, this check is > + * strictly necessary and must follow it, see the comment in > + * __debugfs_remove_file(). > + * OTOH, if the cmpxchg() hasn't been executed or wasn't > + * successful, this serves the purpose of not starving > + * removers. > + */ > if (d_unlinked(dentry)) > return -EIO; > > @@ -98,7 +125,7 @@ EXPORT_SYMBOL_GPL(debugfs_file_get); > */ > void debugfs_file_put(struct dentry *dentry) > { > - struct debugfs_fsdata *fsd = dentry->d_fsdata; > + struct debugfs_fsdata *fsd = READ_ONCE(dentry->d_fsdata); > > if (refcount_dec_and_test(&fsd->active_users)) > complete(&fsd->active_users_drained); > @@ -109,10 +136,11 @@ static int open_proxy_open(struct inode *inode, > struct file *filp) > { > struct dentry *dentry = F_DENTRY(filp); > const struct file_operations *real_fops = NULL; > - int r = 0; > + int r; > > - if (debugfs_file_get(dentry)) > - return -ENOENT; > + r = debugfs_file_get(dentry); > + if (r) > + return r == -EIO ? -ENOENT : r; > > real_fops = debugfs_real_fops(filp); > real_fops = fops_get(real_fops); > @@ -233,10 +261,11 @@ static int full_proxy_open(struct inode *inode, > struct file *filp) > struct dentry *dentry = F_DENTRY(filp); > const struct file_operations *real_fops = NULL; > struct file_operations *proxy_fops = NULL; > - int r = 0; > + int r; > > - if (debugfs_file_get(dentry)) > - return -ENOENT; > + r = debugfs_file_get(dentry); > + if (r) > + return r == -EIO ? -ENOENT : r; > > real_fops = debugfs_real_fops(filp); > real_fops = fops_get(real_fops); > diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c > index 5550f11d60bd..2360c17ec00a 100644 > --- a/fs/debugfs/inode.c > +++ b/fs/debugfs/inode.c > @@ -184,7 +184,10 @@ static const struct super_operations > debugfs_super_operations = { > > static void debugfs_release_dentry(struct dentry *dentry) > { > - kfree(dentry->d_fsdata); > + void *fsd = dentry->d_fsdata; > + > + if (!((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)) > + kfree(dentry->d_fsdata); > } > > static struct vfsmount *debugfs_automount(struct path *path) > @@ -346,35 +349,25 @@ static struct dentry > *__debugfs_create_file(const char *name, umode_t mode, > { > struct dentry *dentry; > struct inode *inode; > - struct debugfs_fsdata *fsd; > - > - fsd = kmalloc(sizeof(*fsd), GFP_KERNEL); > - if (!fsd) > - return NULL; > > if (!(mode & S_IFMT)) > mode |= S_IFREG; > BUG_ON(!S_ISREG(mode)); > dentry = start_creating(name, parent); > > - if (IS_ERR(dentry)) { > - kfree(fsd); > + if (IS_ERR(dentry)) > return NULL; > - } > > inode = debugfs_get_inode(dentry->d_sb); > - if (unlikely(!inode)) { > - kfree(fsd); > + if (unlikely(!inode)) > return failed_creating(dentry); > - } > > inode->i_mode = mode; > inode->i_private = data; > > inode->i_fop = proxy_fops; > - fsd->real_fops = real_fops; > - refcount_set(&fsd->active_users, 1); > - dentry->d_fsdata = fsd; > + dentry->d_fsdata = (void *)((unsigned long)real_fops | > + DEBUGFS_FSDATA_IS_REAL_FOPS_BIT); > > d_instantiate(dentry, inode); > fsnotify_create(d_inode(dentry->d_parent), dentry); > @@ -637,8 +630,17 @@ static void __debugfs_remove_file(struct dentry > *dentry, struct dentry *parent) > > simple_unlink(d_inode(parent), dentry); > d_delete(dentry); > - fsd = dentry->d_fsdata; > - init_completion(&fsd->active_users_drained); > + > + /* > + * Paired with the closing smp_mb() implied by a successful > + * cmpxchg() in debugfs_file_get(): either > + * debugfs_file_get() must see a dead dentry or we must see > a > + * debugfs_fsdata instance at ->d_fsdata here (or both). > + */ > + smp_mb(); > + fsd = READ_ONCE(dentry->d_fsdata); > + if ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT) > + return; > if (!refcount_dec_and_test(&fsd->active_users)) > wait_for_completion(&fsd->active_users_drained); > } > diff --git a/fs/debugfs/internal.h b/fs/debugfs/internal.h > index 0eea99432840..cb1e8139c398 100644 > --- a/fs/debugfs/internal.h > +++ b/fs/debugfs/internal.h > @@ -25,4 +25,12 @@ struct debugfs_fsdata { > struct completion active_users_drained; > }; > > +/* > + * A dentry's ->d_fsdata either points to the real fops or to a > + * dynamically allocated debugfs_fsdata instance. > + * In order to distinguish between these two cases, a real fops > + * pointer gets its lowest bit set. > + */ > +#define DEBUGFS_FSDATA_IS_REAL_FOPS_BIT BIT(0) > + > #endif /* _DEBUGFS_INTERNAL_H_ */