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?
>
> >> + 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;
> >> +}
> >> +
> > [...]
