Re: [PATCH v3 bpf-next 2/3] bpf: Introduce task_vma open-coded iterator kfuncs

[David Marchevsky <david.marchevsky@linux.dev>]

On 8/22/23 1:42 PM, Yonghong Song wrote:
> 
> 
> On 8/21/23 10:05 PM, Dave Marchevsky wrote:
>> This patch adds kfuncs bpf_iter_task_vma_{new,next,destroy} which allow
>> creation and manipulation of struct bpf_iter_task_vma in open-coded
>> iterator style. BPF programs can use these kfuncs directly or through
>> bpf_for_each macro for natural-looking iteration of all task vmas.
>>
>> The implementation borrows heavily from bpf_find_vma helper's locking -
>> differing only in that it holds the mmap_read lock for all iterations
>> while the helper only executes its provided callback on a maximum of 1
>> vma. Aside from locking, struct vma_iterator and vma_next do all the
>> heavy lifting.
>>
>> The newly-added struct bpf_iter_task_vma has a name collision with a
>> selftest for the seq_file task_vma iter's bpf skel, so the selftests/bpf/progs
>> file is renamed in order to avoid the collision.
>>
>> A pointer to an inner data struct, struct bpf_iter_task_vma_kern_data, is the
>> only field in struct bpf_iter_task_vma. This is because the inner data
>> struct contains a struct vma_iterator (not ptr), whose size is likely to
>> change under us. If bpf_iter_task_vma_kern contained vma_iterator directly
>> such a change would require change in opaque bpf_iter_task_vma struct's
>> size. So better to allocate vma_iterator using BPF allocator, and since
>> that alloc must already succeed, might as well allocate all iter fields,
>> thereby freezing struct bpf_iter_task_vma size.
>>
>> Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
>> Cc: Nathan Slingerland <slinger@meta.com>
>> ---
>>   include/uapi/linux/bpf.h                      |  4 +
>>   kernel/bpf/helpers.c                          |  3 +
>>   kernel/bpf/task_iter.c                        | 84 +++++++++++++++++++
>>   tools/include/uapi/linux/bpf.h                |  4 +
>>   tools/lib/bpf/bpf_helpers.h                   |  8 ++
>>   .../selftests/bpf/prog_tests/bpf_iter.c       | 26 +++---
>>   ...f_iter_task_vma.c => bpf_iter_task_vmas.c} |  0
>>   7 files changed, 116 insertions(+), 13 deletions(-)
>>   rename tools/testing/selftests/bpf/progs/{bpf_iter_task_vma.c => bpf_iter_task_vmas.c} (100%)
>>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index 8790b3962e4b..49fc1989a548 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -7311,4 +7311,8 @@ struct bpf_iter_num {
>>       __u64 __opaque[1];
>>   } __attribute__((aligned(8)));
>>   +struct bpf_iter_task_vma {
>> +    __u64 __opaque[1]; /* See bpf_iter_num comment above */
>> +} __attribute__((aligned(8)));
> 
> In the future, we might have bpf_iter_cgroup, bpf_iter_task, bpf_iter_cgroup_task, etc. They may all use the same struct
> like
>   struct bpf_iter_<...> {
>     __u64 __opaque[1];
>   } __attribute__((aligned(8)));
> 
> Maybe we want a generic one instead of having lots of
> structs with the same underline definition? For example,
>   struct bpf_iter_generic
> ?
> 

The bpf_for_each macro assumes a consistent naming scheme for opaque iter struct
and associated kfuncs. Having a 'bpf_iter_generic' shared amongst multiple types
of iters would break the scheme. We could:

  * Add bpf_for_each_generic that only uses bpf_iter_generic
    * This exposes implementation details in an ugly way, though.
  * Do some macro magic to pick bpf_iter_generic for some types of iters, and
    use consistent naming pattern for others.
    * I'm not sure how to do this with preprocessor
  * Migrate all opaque iter structs to only contain pointer to bpf_mem_alloc'd
    data struct, and use bpf_iter_generic for all of them
    * Probably need to see more iter implementation / usage before making such
      a change
  * Do 'typedef __u64 __aligned(8) bpf_iter_<...>
    * BTF_KIND_TYPEDEF intead of BTF_KIND_STRUCT might throw off some verifier
      logic. Could do similar typedef w/ struct to try to work around
      it.

Let me know what you think. Personally I considered doing typedef while
implementing this, so that's the alternative I'd choose. 

>> +
>>   #endif /* _UAPI__LINUX_BPF_H__ */
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index eb91cae0612a..7a06dea749f1 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -2482,6 +2482,9 @@ BTF_ID_FLAGS(func, bpf_dynptr_slice_rdwr, KF_RET_NULL)
>>   BTF_ID_FLAGS(func, bpf_iter_num_new, KF_ITER_NEW)
>>   BTF_ID_FLAGS(func, bpf_iter_num_next, KF_ITER_NEXT | KF_RET_NULL)
>>   BTF_ID_FLAGS(func, bpf_iter_num_destroy, KF_ITER_DESTROY)
>> +BTF_ID_FLAGS(func, bpf_iter_task_vma_new, KF_ITER_NEW)
>> +BTF_ID_FLAGS(func, bpf_iter_task_vma_next, KF_ITER_NEXT | KF_RET_NULL)
>> +BTF_ID_FLAGS(func, bpf_iter_task_vma_destroy, KF_ITER_DESTROY)
>>   BTF_ID_FLAGS(func, bpf_dynptr_adjust)
>>   BTF_ID_FLAGS(func, bpf_dynptr_is_null)
>>   BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
>> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
>> index c4ab9d6cdbe9..51c2dce435c1 100644
>> --- a/kernel/bpf/task_iter.c
>> +++ b/kernel/bpf/task_iter.c
>> @@ -7,7 +7,9 @@
>>   #include <linux/fs.h>
>>   #include <linux/fdtable.h>
>>   #include <linux/filter.h>
>> +#include <linux/bpf_mem_alloc.h>
>>   #include <linux/btf_ids.h>
>> +#include <linux/mm_types.h>
>>   #include "mmap_unlock_work.h"
>>     static const char * const iter_task_type_names[] = {
>> @@ -823,6 +825,88 @@ const struct bpf_func_proto bpf_find_vma_proto = {
>>       .arg5_type    = ARG_ANYTHING,
>>   };
>>   +struct bpf_iter_task_vma_kern_data {
>> +    struct task_struct *task;
>> +    struct mm_struct *mm;
>> +    struct mmap_unlock_irq_work *work;
>> +    struct vma_iterator vmi;
>> +};
>> +
>> +/* Non-opaque version of uapi bpf_iter_task_vma */
>> +struct bpf_iter_task_vma_kern {
>> +    struct bpf_iter_task_vma_kern_data *data;
>> +} __attribute__((aligned(8)));
>> +
>> +__bpf_kfunc int bpf_iter_task_vma_new(struct bpf_iter_task_vma *it,
>> +                      struct task_struct *task, u64 addr)
>> +{
>> +    struct bpf_iter_task_vma_kern *kit = (void *)it;
>> +    bool irq_work_busy = false;
>> +    int err;
>> +
>> +    BUILD_BUG_ON(sizeof(struct bpf_iter_task_vma_kern) != sizeof(struct bpf_iter_task_vma));
>> +    BUILD_BUG_ON(__alignof__(struct bpf_iter_task_vma_kern) != __alignof__(struct bpf_iter_task_vma));
>> +
>> +    /* is_iter_reg_valid_uninit guarantees that kit hasn't been initialized
>> +     * before, so non-NULL kit->data doesn't point to previously
>> +     * bpf_mem_alloc'd bpf_iter_task_vma_kern_data
>> +     */
>> +    kit->data = bpf_mem_alloc(&bpf_global_ma, sizeof(struct bpf_iter_task_vma_kern_data));
>> +    if (!kit->data)
>> +        return -ENOMEM;
>> +    kit->data->task = NULL;
>> +
>> +    if (!task) {
>> +        err = -ENOENT;
>> +        goto err_cleanup_iter;
>> +    }
>> +
>> +    kit->data->task = get_task_struct(task);
> 
> The above is not safe. Since there is no restriction on 'task',
> the 'task' could be in a state for destruction with 'usage' count 0
> and then get_task_struct(task) won't work since it unconditionally
> tries to increase 'usage' count from 0 to 1.
> 
> Or, 'task' may be valid at the entry of the funciton, but when
> 'task' is in get_task_struct(), 'task' may have been destroyed
> and 'task' memory is reused by somebody else.
> 
> I suggest that we check input parameter 'task' must be
> PTR_TRUSTED or MEM_RCU. This way, the above !task checking
> is not necessary and get_task_struct() can correctly
> hold a reference to 'task'.
> 

Adding a PTR_TRUSTED or MEM_RCU check seems reasonable. I'm curious
whether there's any way to feed a 'plain' struct task_struct PTR_TO_BTF_ID
to this kfunc currently.

* bpf_get_current_task_btf helper returns PTR_TRUSTED | PTR_TO_BTF_ID
* ptr hop from trusted task_struct to 'real_parent' or similar should
  yield MEM_RCU (due to BTF_TYPE_SAFE_RCU(struct task_struct) def
* if task kptr is in map_val, direct reference to it should result
  in PTR_UNTRUSTED PTR_TO_BTF_ID, must kptr_xchg it or acquire again
  using bpf_task_from_pid (?)

But regardless, better to be explicit. Will change.

>> +    kit->data->mm = task->mm;
>> +    if (!kit->data->mm) {
>> +        err = -ENOENT;
>> +        goto err_cleanup_iter;
>> +    }
>> +
>> +    /* kit->data->work == NULL is valid after bpf_mmap_unlock_get_irq_work */
>> +    irq_work_busy = bpf_mmap_unlock_get_irq_work(&kit->data->work);
>> +    if (irq_work_busy || !mmap_read_trylock(kit->data->mm)) {
>> +        err = -EBUSY;
>> +        goto err_cleanup_iter;
>> +    }
>> +
>> +    vma_iter_init(&kit->data->vmi, kit->data->mm, addr);
>> +    return 0;
>> +
>> +err_cleanup_iter:
>> +    if (kit->data->task)
>> +        put_task_struct(kit->data->task);
>> +    bpf_mem_free(&bpf_global_ma, kit->data);
>> +    /* NULL kit->data signals failed bpf_iter_task_vma initialization */
>> +    kit->data = NULL;
>> +    return err;
>> +}
>> +
> [...]