On Tue, May 7, 2024 at 6:32 AM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>
> On Tue, May 7, 2024 at 11:38 AM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Sun, May 5, 2024 at 8:35 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
> > >
> > > Add three new kfuncs for the bits iterator:
> > > - bpf_iter_bits_new
> > > Initialize a new bits iterator for a given memory area. Due to the
> > > limitation of bpf memalloc, the max number of bits that can be iterated
> > > over is limited to (4096 * 8).
> > > - bpf_iter_bits_next
> > > Get the next bit in a bpf_iter_bits
> > > - bpf_iter_bits_destroy
> > > Destroy a bpf_iter_bits
> > >
> > > The bits iterator facilitates the iteration of the bits of a memory area,
> > > such as cpumask. It can be used in any context and on any address.
> > >
> > > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> > > ---
> > > kernel/bpf/helpers.c | 140 +++++++++++++++++++++++++++++++++++++++++++
> > > 1 file changed, 140 insertions(+)
> > >
> > > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > > index 2a69a9a36c0f..83b2a02f795f 100644
> > > --- a/kernel/bpf/helpers.c
> > > +++ b/kernel/bpf/helpers.c
> > > @@ -2744,6 +2744,143 @@ __bpf_kfunc void bpf_preempt_enable(void)
> > > preempt_enable();
> > > }
> > >
> > > +struct bpf_iter_bits {
> > > + __u64 __opaque[2];
> > > +} __aligned(8);
> > > +
> > > +struct bpf_iter_bits_kern {
> > > + union {
> > > + unsigned long *bits;
> > > + unsigned long bits_copy;
> > > + };
> > > + u32 nr_bits;
> > > + int bit;
> > > +} __aligned(8);
> > > +
> > > +/**
> > > + * bpf_iter_bits_new() - Initialize a new bits iterator for a given memory area
> > > + * @it: The new bpf_iter_bits to be created
> > > + * @unsafe_ptr__ign: A ponter pointing to a memory area to be iterated over
> >
> > typo: pointer
>
> Thanks for the fix and the other fixes.
>
> >
> > > + * @nr_bits: The number of bits to be iterated over. Due to the limitation of
> > > + * memalloc, it can't greater than (4096 * 8).
> >
> > typo: can't be greater
> >
> > > + *
> > > + * This function initializes a new bpf_iter_bits structure for iterating over
> > > + * a memory area which is specified by the @unsafe_ptr__ign and @nr_bits. It
> > > + * copy the data of the memory area to the newly created bpf_iter_bits @it for
> >
> > s/copy/copies/
> >
> > > + * subsequent iteration operations.
> > > + *
> > > + * On success, 0 is returned. On failure, ERR is returned.
> > > + */
> > > +__bpf_kfunc int
> > > +bpf_iter_bits_new(struct bpf_iter_bits *it, const void *unsafe_ptr__ign, u32 nr_bits)
> > > +{
> > > + struct bpf_iter_bits_kern *kit = (void *)it;
> > > + u32 words = BITS_TO_LONGS(nr_bits);
> > > + u32 size = BITS_TO_BYTES(nr_bits);
> > > + u32 left, offset;
> > > + int err;
> > > +
> > > + BUILD_BUG_ON(sizeof(struct bpf_iter_bits_kern) != sizeof(struct bpf_iter_bits));
> > > + BUILD_BUG_ON(__alignof__(struct bpf_iter_bits_kern) !=
> > > + __alignof__(struct bpf_iter_bits));
> > > +
> > > + if (!unsafe_ptr__ign || !nr_bits) {
> > > + kit->bits = NULL;
> > > + return -EINVAL;
> > > + }
> > > +
> > > + kit->nr_bits = 0;
> > > + kit->bits_copy = 0;
> > > + /* Optimization for u64/u32 mask */
> > > + if (nr_bits <= 64) {
> > > + /* For big-endian, we must calculate the offset */
> > > + offset = IS_ENABLED(CONFIG_S390) ? sizeof(u64) - size : 0;
> >
> > S390 isn't the only big-endian architecture, it's wrong to hard-code just S390
> >
> > there is __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__ check throughout the
> > kernel to do this detection
>
> I missed that. will check it.
>
> >
> > > +
> > > + err = bpf_probe_read_kernel_common(((char *)&kit->bits_copy) + offset,
> > > + size, unsafe_ptr__ign);
> > > + if (err)
> > > + return -EFAULT;
> >
> > I'd rewrite the above to something like (not tested, but should give
> > the right idea):
> >
> > long bits = 0;
> >
> > err = bpf_probe_read_kernel_common(&bits, size, unsafe_ptr__ign);
> > if (err)
> > return -EFAULT;
> >
> > #if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
> > bits = __swab64(bits);
> > #endif
> >
> > /* deal with bit mask of weird size, ensuring upper bits are zero */
> > bits <<= 64 - nr_bits;
> > bits >>= 64 - nr_bits;
> >
> > kit->bits_copy = bits;
> >
> >
> > This should take care of both big-endianness, and non-multiple-of-8
> > sized bitmasks (I think, we need tests).
>
> looks good, will change it.
>
> >
> > pw-bot: cr
> >
> >
> > > +
> > > + kit->nr_bits = nr_bits;
> > > + kit->bit = -1;
> > > + return 0;
> > > + }
> > > +
> > > + /* Fallback to memalloc */
> > > + kit->bits = bpf_mem_alloc(&bpf_global_ma, size);
> > > + if (!kit->bits)
> > > + return -ENOMEM;
> > > +
> > > + err = bpf_probe_read_kernel_common(kit->bits, words * sizeof(u64), unsafe_ptr__ign);
> > > + if (err) {
> > > + bpf_mem_free(&bpf_global_ma, kit->bits);
> > > + return err;
> > > + }
> > > +
> > > + /* long-aligned */
> > > + left = size & (sizeof(u64) - 1);
> > > + if (!left)
> > > + goto out;
> > > +
> > > + offset = IS_ENABLED(CONFIG_S390) ? sizeof(u64) - left : 0;
> > > + err = bpf_probe_read_kernel_common((char *)(kit->bits + words - 1) + offset, left,
> > > + unsafe_ptr__ign + (words - 1) * sizeof(u64));
> > > + if (err) {
> > > + bpf_mem_free(&bpf_global_ma, kit->bits);
> > > + return err;
> > > + }
> >
> > tbh, I'm not sure what's the desired behavior here is. David (cc'ed),
> > you were dealing with cpumasks, how is the bit mask specified there?
> > Is it considered to be an long[] array or byte[] array? And how is
> > that working on big-endian, because I think it makes a difference?
> > Please take a look, thanks.
>
> The function find_next_bit() requires the pointer to be of type
> "unsigned long *", hence, we must ensure consistency by converting it
> here as well. As cpumask represents a bitmap and is always of type
> "unsigned long *", it remains unaffected by endianness considerations.
>
Right, but the question is whether this iterator should make the same
simplifying assumption or not? I think the motivation for this
iterator was the ability to iterate over CPU masks, so I'm asking (and
that's why I cc'ed David) what we should do to make it work well for
CPU masks.
> >
> > > +
> > > +out:
> > > + kit->nr_bits = nr_bits;
> > > + kit->bit = -1;
> > > + return 0;
> > > +}
> > > +
> > > +/**
> > > + * bpf_iter_bits_next() - Get the next bit in a bpf_iter_bits
> > > + * @it: The bpf_iter_bits to be checked
> > > + *
> > > + * This function returns a pointer to a number representing the value of the
> > > + * next bit in the bits.
> > > + *
> > > + * If there are no further bit available, it returns NULL.
> > > + */
> > > +__bpf_kfunc int *bpf_iter_bits_next(struct bpf_iter_bits *it)
> > > +{
> > > + struct bpf_iter_bits_kern *kit = (void *)it;
> > > + u32 nr_bits = kit->nr_bits;
> > > + const unsigned long *bits;
> > > + int bit;
> > > +
> > > + if (nr_bits == 0)
> > > + return NULL;
> > > +
> > > + bits = nr_bits <= 64 ? &kit->bits_copy : kit->bits;
> > > + bit = find_next_bit(bits, nr_bits, kit->bit + 1);
> > > + if (bit >= nr_bits) {
> > > + kit->nr_bits = 0;
> > > + return NULL;
> > > + }
> > > +
> > > + kit->bit = bit;
> > > + return &kit->bit;
> > > +}
> > > +
> > > +/**
> > > + * bpf_iter_bits_destroy() - Destroy a bpf_iter_bits
> > > + * @it: The bpf_iter_bits to be destroyed
> > > + *
> > > + * Destroy the resource associated with the bpf_iter_bits.
> > > + */
> > > +__bpf_kfunc void bpf_iter_bits_destroy(struct bpf_iter_bits *it)
> > > +{
> > > + struct bpf_iter_bits_kern *kit = (void *)it;
> > > +
> > > + if (kit->nr_bits <= 64)
> > > + return;
> > > + bpf_mem_free(&bpf_global_ma, kit->bits);
> > > +}
> > > +
> > > __bpf_kfunc_end_defs();
> > >
> > > BTF_KFUNCS_START(generic_btf_ids)
> > > @@ -2826,6 +2963,9 @@ BTF_ID_FLAGS(func, bpf_wq_set_callback_impl)
> > > BTF_ID_FLAGS(func, bpf_wq_start)
> > > BTF_ID_FLAGS(func, bpf_preempt_disable)
> > > BTF_ID_FLAGS(func, bpf_preempt_enable)
> > > +BTF_ID_FLAGS(func, bpf_iter_bits_new, KF_ITER_NEW)
> > > +BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT | KF_RET_NULL)
> > > +BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY)
> > > BTF_KFUNCS_END(common_btf_ids)
> > >
> > > static const struct btf_kfunc_id_set common_kfunc_set = {
> > > --
> > > 2.30.1 (Apple Git-130)
> > >
>
>
>
> --
> Regards
> Yafang
