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Date: Mon, 1 Nov 2021 23:23:56 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Song Liu <songliubraving@...com>, bpf@...r.kernel.org,
netdev@...r.kernel.org
Cc: ast@...nel.org, andrii@...nel.org, kernel-team@...com,
kpsingh@...nel.org
Subject: Re: [PATCH bpf-next 1/2] bpf: introduce helper bpf_find_vma
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?
> + 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(¤t->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?
> + return ret;
> +}
> +
> +BTF_ID_LIST_SINGLE(btf_find_vma_ids, struct, task_struct)
> +
> +const struct bpf_func_proto bpf_find_vma_proto = {
> + .func = bpf_find_vma,
> + .ret_type = RET_INTEGER,
> + .arg1_type = ARG_PTR_TO_BTF_ID,
> + .arg1_btf_id = &btf_find_vma_ids[0],
> + .arg2_type = ARG_ANYTHING,
> + .arg3_type = ARG_PTR_TO_FUNC,
> + .arg4_type = ARG_PTR_TO_STACK_OR_NULL,
> + .arg5_type = ARG_ANYTHING,
> +};
[...]
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index c108200378834..056c00da1b5d6 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -4915,6 +4915,24 @@ union bpf_attr {
> * Dynamically cast a *sk* pointer to a *unix_sock* pointer.
> * Return
> * *sk* if casting is valid, or **NULL** otherwise.
> + * long bpf_find_vma(struct task_struct *task, u64 addr, void *callback_fn, void *callback_ctx, u64 flags)
nit: Wrongly copied uapi header over to tooling?
> + * Description
> + * Find vma of *task* that contains *addr*, call *callback_fn*
> + * function with *task*, *vma*, and *callback_ctx*.
> + * The *callback_fn* should be a static function and
> + * the *callback_ctx* should be a pointer to the stack.
> + * The *flags* is used to control certain aspects of the helper.
> + * Currently, the *flags* must be 0.
> + *
> + * The expected callback signature is
> + *
> + * long (\*callback_fn)(struct task_struct \*task, struct vm_area_struct \*vma, void \*ctx);
> + *
> + * Return
> + * 0 on success.
> + * **-ENOENT** if *task->mm* is NULL, or no vma contains *addr*.
> + * **-EBUSY** if failed to try lock mmap_lock.
> + * **-EINVAL** for invalid **flags**.
> */
> #define __BPF_FUNC_MAPPER(FN) \
> FN(unspec), \
> @@ -5096,6 +5114,7 @@ union bpf_attr {
> FN(get_branch_snapshot), \
> FN(trace_vprintk), \
> FN(skc_to_unix_sock), \
> + FN(find_vma), \
> /* */
>
> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>
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