netdev - Re: [PATCH bpf-next 1/2] bpf: introduce helper bpf_find

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Date:   Thu, 4 Nov 2021 05:56:56 +0000
From:   Song Liu <songliubraving@...com>
To:     Daniel Borkmann <daniel@...earbox.net>
CC:     bpf <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "andrii@...nel.org" <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        "kpsingh@...nel.org" <kpsingh@...nel.org>
Subject: Re: [PATCH bpf-next 1/2] bpf: introduce helper bpf_find_vma



> On Nov 3, 2021, at 4:54 PM, Song Liu <songliubraving@...com> wrote:
> 
> 
> 
>> On Nov 1, 2021, at 3:23 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
>> 
>> On 10/28/21 12:00 AM, Song Liu wrote:
>> [...]
>>> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>>> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
>>> index b48750bfba5aa..ad30f2e885356 100644
>>> --- a/kernel/bpf/task_iter.c
>>> +++ b/kernel/bpf/task_iter.c
>>> @@ -8,6 +8,7 @@
>>> #include <linux/fdtable.h>
>>> #include <linux/filter.h>
>>> #include <linux/btf_ids.h>
>>> +#include <linux/irq_work.h>
>>>   struct bpf_iter_seq_task_common {
>>> 	struct pid_namespace *ns;
>>> @@ -21,6 +22,25 @@ struct bpf_iter_seq_task_info {
>>> 	u32 tid;
>>> };
>>> +/* irq_work to run mmap_read_unlock() */
>>> +struct task_iter_irq_work {
>>> +	struct irq_work irq_work;
>>> +	struct mm_struct *mm;
>>> +};
>>> +
>>> +static DEFINE_PER_CPU(struct task_iter_irq_work, mmap_unlock_work);
>>> +
>>> +static void do_mmap_read_unlock(struct irq_work *entry)
>>> +{
>>> +	struct task_iter_irq_work *work;
>>> +
>>> +	if (WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT)))
>>> +		return;
>>> +
>>> +	work = container_of(entry, struct task_iter_irq_work, irq_work);
>>> +	mmap_read_unlock_non_owner(work->mm);
>>> +}
>>> +
>>> static struct task_struct *task_seq_get_next(struct pid_namespace *ns,
>>> 					     u32 *tid,
>>> 					     bool skip_if_dup_files)
>>> @@ -586,9 +606,89 @@ static struct bpf_iter_reg task_vma_reg_info = {
>>> 	.seq_info		= &task_vma_seq_info,
>>> };
>>> +BPF_CALL_5(bpf_find_vma, struct task_struct *, task, u64, start,
>>> +	   bpf_callback_t, callback_fn, void *, callback_ctx, u64, flags)
>>> +{
>>> +	struct task_iter_irq_work *work = NULL;
>>> +	struct mm_struct *mm = task->mm;
>> 
>> Won't this NULL deref if called with task argument as NULL?
> 
> Will fix. 
> 
>> 
>>> +	struct vm_area_struct *vma;
>>> +	bool irq_work_busy = false;
>>> +	int ret = -ENOENT;
>>> +
>>> +	if (flags)
>>> +		return -EINVAL;
>>> +
>>> +	if (!mm)
>>> +		return -ENOENT;
>>> +
>>> +	/*
>>> +	 * Similar to stackmap with build_id support, we cannot simply do
>>> +	 * mmap_read_unlock when the irq is disabled. Instead, we need do
>>> +	 * the unlock in the irq_work.
>>> +	 */
>>> +	if (irqs_disabled()) {
>>> +		if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
>>> +			work = this_cpu_ptr(&mmap_unlock_work);
>>> +			if (irq_work_is_busy(&work->irq_work)) {
>>> +				/* cannot queue more mmap_unlock, abort. */
>>> +				irq_work_busy = true;
>>> +			}
>>> +		} else {
>>> +			/*
>>> +			 * PREEMPT_RT does not allow to trylock mmap sem in
>>> +			 * interrupt disabled context, abort.
>>> +			 */
>>> +			irq_work_busy = true;
>>> +		}
>>> +	}
>>> +
>>> +	if (irq_work_busy || !mmap_read_trylock(mm))
>>> +		return -EBUSY;
>>> +
>>> +	vma = find_vma(mm, start);
>>> +
>>> +	if (vma && vma->vm_start <= start && vma->vm_end > start) {
>>> +		callback_fn((u64)(long)task, (u64)(long)vma,
>>> +			    (u64)(long)callback_ctx, 0, 0);
>>> +		ret = 0;
>>> +	}
>>> +	if (!work) {
>>> +		mmap_read_unlock(current->mm);
>>> +	} else {
>>> +		work->mm = current->mm;
>>> +
>>> +		/* The lock will be released once we're out of interrupt
>>> +		 * context. Tell lockdep that we've released it now so
>>> +		 * it doesn't complain that we forgot to release it.
>>> +		 */
>>> +		rwsem_release(&current->mm->mmap_lock.dep_map, _RET_IP_);
>>> +		irq_work_queue(&work->irq_work);
>>> +	}
>> 
>> Given this is pretty much the same logic around the vma retrieval, could this be
>> refactored/consolidated with stack map build id retrieval into a common function?
> 
> I thought about sharing the irq_work code amount the two. The problem was we need
> to include irq_work.h in bpf.h. But on a second thought, maybe we should just 
> move bpf_find_vma to stackmap.c? This will avoid including irq_work.h. I guess it 
> is not too weird to have bpf_find_vma in stackmap.c.  

Actually, we can just add a local header for it in kernel/bpf. Adding bpf_find_vma
to stackmap.c means bpf_find_vma requires CONFIG_PERF_EVENTS. It is not a real 
issue in most systems, but may break some build tests. 

Song