On Tue, Aug 22, 2023 at 10:42 PM David Marchevsky
<david.marchevsky@xxxxxxxxx> wrote:
>
> On 8/22/23 8:04 PM, Andrii Nakryiko wrote:
> > On Tue, Aug 22, 2023 at 12:20 PM David Marchevsky
> > <david.marchevsky@xxxxxxxxx> wrote:
> >>
> >> 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@xxxxxx>
> >>>> Cc: Nathan Slingerland <slinger@xxxxxxxx>
> >>>> ---
> >>>> 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.
> >
> > How horrible would it be to base an interface on TID/PID (i.e., int)
> > as input argument to specify a task? I'm just thinking it might be
> > more generic and easy to use in more situations:
> > - for all the cases where we have struct task_struct, getting its
> > pid is trivial: `task->pid`;
> > - but in some situations PID might be coming from outside: either
> > as an argument to CLI tool, or from old-style tracepoint (e.g.,
> > context_switch where we have prev/next task pid), etc.
> >
> > The downside is that we'd need to look up a task, right? But on the
> > other hand we get more generality and won't have to rely on having
> > PTR_TRUSTED task_struct.
> >
> > Thoughts?
> >
>
> Meh, taking tid here feels like the 'old-school' approach, before recent
> efforts to teach the verifier more about resource acquisition, locking,
> iteration, trustedness, etc. All allowing us to push more important logic
> out of 'opaque' helper impl and into BPF programs.
>
> In this tid -> struct task_struct case I think the provided resource acquisition
> is sufficient. Using your examples:
>
> * We have a TRUSTED or RCU struct task_struct
> * No need to do anything, can pass to bpf_iter_task_vma_new
>
> * We have a struct task_struct, but it's UNTRUSTED or has no trustedness
> type flags
> * Use bpf_task_acquire or bpf_task_from_pid
>
> * We just have a pid ('coming from outside')
> * Use bpf_task_from_pid
>
> If there is some scenario where we can't get from pid to properly acquired task
> in the BPF program, let's improve the resource acquisition instead of pushing
> it into the kfunc impl.
>
> Also, should we look up (and refcount incr) the task using task->rcu_users
> refcount w/ bpf_task_{acquire,release}, or using task->usage refcount w/
> {get,put}_task_struct ? More generally, if there are >1 valid ways to acquire
> task_struct or some other resource, pushing acquisition to the BPF program
> gives us the benefit of not having to pick one (or do possibly ugly / complex
> flags). As long as type flags express that the resource will not go away,
> this kfunc impl can ignore the details of how that property came about.
Agree with above reasoning. pid->task should be separate set of kfunc.
It's not a trivial task either. there are pid namespaces. there is u32 pid
and there is 'struct pid'. u32_pid->struct_pid->task has to be designed first.
Probably in some case the trusted task pointer is ready to be accessed,
but context could be such that pid->task cannot be done.
