On Mon, Nov 6, 2023 at 2:00 PM Song Liu <songliubraving@xxxxxxxx> wrote:
>
>
>
> > On Nov 6, 2023, at 1:07 PM, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Fri, Nov 3, 2023 at 5:13 PM Song Liu <song@xxxxxxxxxx> wrote:
> >>
> >> Different types of bpf dynptr have different internal data storage.
> >> Specifically, SKB and XDP type of dynptr may have non-continuous data.
> >> Therefore, it is not always safe to directly access dynptr->data.
> >>
> >> Add __bpf_dynptr_data and __bpf_dynptr_data_rw to replace direct access to
> >> dynptr->data.
> >>
> >> Update bpf_verify_pkcs7_signature to use __bpf_dynptr_data instead of
> >> dynptr->data.
> >>
> >> Signed-off-by: Song Liu <song@xxxxxxxxxx>
> >> ---
> >> include/linux/bpf.h | 2 ++
> >> kernel/bpf/helpers.c | 47 ++++++++++++++++++++++++++++++++++++++++
> >> kernel/trace/bpf_trace.c | 12 ++++++----
> >> 3 files changed, 57 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> >> index b4825d3cdb29..eb84caf133df 100644
> >> --- a/include/linux/bpf.h
> >> +++ b/include/linux/bpf.h
> >> @@ -1222,6 +1222,8 @@ enum bpf_dynptr_type {
> >>
> >> int bpf_dynptr_check_size(u32 size);
> >> u32 __bpf_dynptr_size(const struct bpf_dynptr_kern *ptr);
> >> +const void *__bpf_dynptr_data(const struct bpf_dynptr_kern *ptr, u32 len);
> >> +void *__bpf_dynptr_data_rw(const struct bpf_dynptr_kern *ptr, u32 len);
> >>
> >> #ifdef CONFIG_BPF_JIT
> >> int bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_trampoline *tr);
> >> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> >> index e46ac288a108..c569c4c43bde 100644
> >> --- a/kernel/bpf/helpers.c
> >> +++ b/kernel/bpf/helpers.c
> >> @@ -2611,3 +2611,50 @@ static int __init kfunc_init(void)
> >> }
> >>
> >> late_initcall(kfunc_init);
> >> +
> >> +/* Get a pointer to dynptr data up to len bytes for read only access. If
> >> + * the dynptr doesn't have continuous data up to len bytes, return NULL.
> >> + */
> >> +const void *__bpf_dynptr_data(const struct bpf_dynptr_kern *ptr, u32 len)
> >> +{
> >> + enum bpf_dynptr_type type;
> >> + int err;
> >> +
> >> + if (!ptr->data)
> >> + return NULL;
> >> +
> >> + err = bpf_dynptr_check_off_len(ptr, 0, len);
> >> + if (err)
> >> + return NULL;
> >> + type = bpf_dynptr_get_type(ptr);
> >> +
> >> + switch (type) {
> >> + case BPF_DYNPTR_TYPE_LOCAL:
> >> + case BPF_DYNPTR_TYPE_RINGBUF:
> >> + return ptr->data + ptr->offset;
> >> + case BPF_DYNPTR_TYPE_SKB:
> >> + return skb_pointer_if_linear(ptr->data, ptr->offset, len);
> >> + case BPF_DYNPTR_TYPE_XDP:
> >> + {
> >> + void *xdp_ptr = bpf_xdp_pointer(ptr->data, ptr->offset, len);
> >> +
> >> + if (IS_ERR_OR_NULL(xdp_ptr))
> >> + return NULL;
> >> + return xdp_ptr;
> >> + }
> >> + default:
> >> + WARN_ONCE(true, "unknown dynptr type %d\n", type);
> >> + return NULL;
> >> + }
> >> +}
> >> +
> >
> > Song, you basically reimplemented bpf_dynptr_slice() but didn't unify
> > the code. Now we have two almost identical non-trivial functions we'd
> > need to update every time someone adds a new type of dynptr. Why not
> > have common helper that does everything both bpf_dynptr_slice() kfunc
> > needs and __bpf_dynptr_data() needs. And then call into it from both,
> > keeping all the LOCAL vs RINGBUF vs SKB vs XDP logic in one place?
> >
> > Is there some problem unifying them?
>
> Initially, I was thinking "buffer__opt == NULL && buffer__szk != 0" was
> a problem for bpf_dynptr_slice(). And the buffer__opt == NULL case may
> make a common helper more complicated. So I decided to not unify the two.
>
> After a second look at it, I agree it shouldn't be a problem. And actually
> we can do: (though you may argue against)
>
> const void *__bpf_dynptr_data(const struct bpf_dynptr_kern *ptr, u32 len)
> {
> return bpf_dynptr_slice(ptr, 0, NULL, len);
> }
yeah, let's do this, at least for now. If we have a problem with this,
we can extract a common helper function later. It's more about
interfaces (__bpf_dynptr_data() vs bpf_dynptr_slice()) staying
separate
>
>
> As we are on this, shall we update bpf_dynptr_slice() to return
> "const void *"? This is a little weird for buffer_opt != NULL, case as
> buffer_opt is not const. But the compiler (clang) doesn't seem to
> complain about it.
Good question, but I don't know the answer, so I'd just leave it as is.
>
> If we cannot have bpf_dynptr_slice() return const pointer, we will need
> a little more casting for __bpf_dynptr_data().
>
> Thanks,
> Song
>
>
> >
> >> +/* Get a pointer to dynptr data up to len bytes for read write access. If
> >> + * the dynptr doesn't have continuous data up to len bytes, or the dynptr
> >> + * is read only, return NULL.
> >> + */
>
>
>
