Re: [tracepoint] cargo-culting considered harmful...

Mathieu Desnoyers
2013-01-25T14:49:57Z

* Al Viro (viro@ZenIV.linux.org.uk) wrote:
> On Wed, Jan 23, 2013 at 03:51:47PM -0800, Andrew Morton wrote:
>
> > > note that
> > > * file->f_path is already pinned down by open(), path_get() does not
> > > provide anything extra.
> > > * file->f_path.dentry is already pinned by open() *and* path_get()
> > > just above that dget().
> > > * ->d_name.name *IS* *NOT* *PROTECTED* by pinning dentry down,
> > > whether it's done once or thrice.
> >
> > I guess the first two are obvious (or at least, expected). But the
> > third isn't.

Hi Al,

I agree that the tracepoint example should be removed. There is one
extra piece of module code I think would require fixing (see below).

>
> ->d_name.name is changed by rename() (as one could expect). Grabbing
> a reference to dentry will not prevent rename() from happening. ->i_mutex
> on parent will, but you either need to play with retries (grab reference
> to parent, grab ->i_mutex, check that it's still our parent, if we'd lost
> the race and someone had renamed the sucker - unlock ->i_mutex, dput,
> repeat) *or* to have our dentry looked up with parent locked, with ->i_mutex
> on said parent still held (which happens to cover the majority of valid
> uses in fs code - ->lookup(), ->create(), ->unlink(), rename(), etc. are
> all called that way, so the name of dentry passed to such methods is stable
> for the duration of the method).
>
> ->d_lock on dentry is also sufficient, but that obviously means that you
> can't block while holding it.
>
> > Where should a kernel developer go to learn these things?
> > include/linux/dcache.h doesn't mention d_name locking rules, nor does
> > Documentation/filesystems/vfs.txt.
>
> See directory locking rules in there; the crucial point is that dentry
> name is changed by rename() *and* that results of a race can be worse than
> just running into a partially rewritten name - long names are allocated
> separately and walking through a stale pointer you might end up in freed
> memory.
>
> It's a mess, unfortunately, and $BIGNUM other uses of ->i_mutex make it only
> nastier. Once in a while I go hunting for races in that area, usally with
> a bunch of fixes coming out of such run ;-/

In the light of what you are saying here, am I right to think that the
following code is broken wrt locking wrt use of
filp->f_dentry->d_name.name ?

static
void lttng_enumerate_task_fd(struct lttng_session *session,
struct task_struct *p, char *tmp)
{
struct fdtable *fdt;
struct file *filp;
unsigned int i;
const unsigned char *path;

task_lock(p);
if (!p->files)
goto unlock_task;
spin_lock(&p->files->file_lock);
fdt = files_fdtable(p->files);
for (i = 0; i < fdt->max_fds; i++) {
filp = fcheck_files(p->files, i);
if (!filp)
continue;
path = d_path(&filp->f_path, tmp, PAGE_SIZE);
/* Make sure we give at least some info */
trace_lttng_statedump_file_descriptor(session, p, i,
IS_ERR(path) ?
filp->f_dentry->d_name.name :
path);
}
spin_unlock(&p->files->file_lock);
unlock_task:
task_unlock(p);
}

Since tracepoints never block, holding the ->d_lock around the
trace_lttng_statedump_file_descriptor() tracepoint should probably be
enough to make it correct. Am I missing anything ?

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.