Re: [PATCH bpf-next v3 6/7] bpf: task work scheduling kfuncs

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2025-09-08 at 14:13 +0100, Mykyta Yatsenko wrote:

[...]

> > > +static void bpf_task_work_irq(struct irq_work *irq_work)
> > > +{
> > > +	struct bpf_task_work_ctx *ctx = container_of(irq_work, struct bpf_task_work_ctx, irq_work);
> > > +	enum bpf_task_work_state state;
> > > +	int err;
> > > +
> > > +	guard(rcu_tasks_trace)();
> > > +
> > > +	if (cmpxchg(&ctx->state, BPF_TW_PENDING, BPF_TW_SCHEDULING) != BPF_TW_PENDING) {
> > > +		bpf_task_work_ctx_put(ctx);
> > > +		return;
> > > +	}
> > Why are separate PENDING and SCHEDULING states needed?
> > Both indicate that the task had not been yet but is ready to be
> > submitted to task_work_add(). So, on a first glance it seems that
> > merging the two won't change the behaviour, what do I miss?

> Yes, this is right, we may drop SCHEDULING state, it does not change any 
> behavior compared to PENDING.
> The state check before task_work_add is needed anyway, so we won't 
> remove much code here.
> I kept it just to be more consistent: with every state check we also 
> transition state machine forward.

Why is state check before task_work_add() mandatory?
You check for FREED in both branches of task_work_add(),
so there seem to be no issues with leaking ctx.

> > > +	err = task_work_add(ctx->task, &ctx->work, ctx->mode);
> > > +	if (err) {
> > > +		bpf_task_work_ctx_reset(ctx);
> > > +		/*
> > > +		 * try to switch back to STANDBY for another task_work reuse, but we might have
> > > +		 * gone to FREED already, which is fine as we already cleaned up after ourselves
> > > +		 */
> > > +		(void)cmpxchg(&ctx->state, BPF_TW_SCHEDULING, BPF_TW_STANDBY);
> > > +
> > > +		/* we don't have RCU protection, so put after switching state */
> > > +		bpf_task_work_ctx_put(ctx);
> > > +	}
> > > +
> > > +	/*
> > > +	 * It's technically possible for just scheduled task_work callback to
> > > +	 * complete running by now, going SCHEDULING -> RUNNING and then
> > > +	 * dropping its ctx refcount. Instead of capturing extra ref just to
> > > +	 * protected below ctx->state access, we rely on RCU protection to
> > > +	 * perform below SCHEDULING -> SCHEDULED attempt.
> > > +	 */
> > > +	state = cmpxchg(&ctx->state, BPF_TW_SCHEDULING, BPF_TW_SCHEDULED);
> > > +	if (state == BPF_TW_FREED)
> > > +		bpf_task_work_cancel(ctx); /* clean up if we switched into FREED state */
> > > +}
> > [...]
> > 
> > > +static struct bpf_task_work_ctx *bpf_task_work_acquire_ctx(struct bpf_task_work *tw,
> > > +							   struct bpf_map *map)
> > > +{
> > > +	struct bpf_task_work_ctx *ctx;
> > > +
> > > +	/* early check to avoid any work, we'll double check at the end again */
> > > +	if (!atomic64_read(&map->usercnt))
> > > +		return ERR_PTR(-EBUSY);
> > > +
> > > +	ctx = bpf_task_work_fetch_ctx(tw, map);
> > > +	if (IS_ERR(ctx))
> > > +		return ctx;
> > > +
> > > +	/* try to get ref for task_work callback to hold */
> > > +	if (!bpf_task_work_ctx_tryget(ctx))
> > > +		return ERR_PTR(-EBUSY);
> > > +
> > > +	if (cmpxchg(&ctx->state, BPF_TW_STANDBY, BPF_TW_PENDING) != BPF_TW_STANDBY) {
> > > +		/* lost acquiring race or map_release_uref() stole it from us, put ref and bail */
> > > +		bpf_task_work_ctx_put(ctx);
> > > +		return ERR_PTR(-EBUSY);
> > > +	}
> > > +
> > > +	/*
> > > +	 * Double check that map->usercnt wasn't dropped while we were
> > > +	 * preparing context, and if it was, we need to clean up as if
> > > +	 * map_release_uref() was called; bpf_task_work_cancel_and_free()
> > > +	 * is safe to be called twice on the same task work
> > > +	 */
> > > +	if (!atomic64_read(&map->usercnt)) {
> > > +		/* drop ref we just got for task_work callback itself */
> > > +		bpf_task_work_ctx_put(ctx);
> > > +		/* transfer map's ref into cancel_and_free() */
> > > +		bpf_task_work_cancel_and_free(tw);
> > > +		return ERR_PTR(-EBUSY);
> > > +	}
> > I don't understand how the above check is useful.
> > Is map->usercnt protected from being changed during execution of
> > bpf_task_work_schedule()?
> > There are two such checks in this function, so apparently it is not.
> > Then what's the point of checking usercnt value if it can be
> > immediately changed after the check?

> BPF map implementation calls bpf_task_work_cancel_and_free() for each 
> value when map->usercnt goes to 0.
> We need to make sure that after mutating map value (attaching a ctx, 
> setting state and refcnt), we do not
> leak memory to a newly allocated ctx.
> If bpf_task_work_cancel_and_free() runs concurrently with 
> bpf_task_work_acquire_ctx(), there is a chance that map cleans up the 
> value first and then we attach a ctx with refcnt=2, memory will leak. 
> Alternatively, if map->usercnt is set to 0 right after this check, we 
> are guaranteed to have the initialized context attached already, so the 
> refcnts will be properly decremented (once by 
> bpf_task_work_cancel_and_free()
> and once by bpf_task_work_irq() and clean up is safe).
> 
> In other words, initialization of the ctx in struct bpf_task_work is 
> multi-step operation, those steps could be
> interleaved with cancel_and_free(), in such case the value may leak the 
> ctx. Check map->usercnt==0 after initialization,
> to force correct cleanup preventing the leak. Calling cancel_and_free() 
> for the same value twice is safe.

Ack, thank you for explaining.

> > 
> > > +
> > > +	return ctx;
> > > +}

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