Re: [PATCH bpf-next] bpf: Add verifier support for custom callback return range

[sdf@google.com]

On 09/08, Dave Marchevsky wrote:
> Verifier logic to confirm that a callback function returns 0 or 1 was
> added in commit 69c087ba6225b ("bpf: Add bpf_for_each_map_elem() helper").
> At the time, callback return value was only used to continue or stop
> iteration.

> In order to support callbacks with a broader return value range, such as
> those added in rbtree series[0] and others, add a callback_ret_range to
> bpf_func_state. Verifier's helpers which set in_callback_fn will also
> set the new field, which the verifier will later use to check return
> value bounds.

> Default to tnum_range(0, 0) instead of using tnum_unknown as a sentinel
> value as the latter would prevent the valid range (0, U64_MAX) being
> used. Previous global default tnum_range(0, 1) is explicitly set for
> extant callback helpers. The change to global default was made after
> discussion around this patch in rbtree series [1], goal here is to make
> it more obvious that callback_ret_range should be explicitly set.

>    [0]: lore.kernel.org/bpf/20220830172759.4069786-1-davemarchevsky@fb.com/
>    [1]: lore.kernel.org/bpf/20220830172759.4069786-2-davemarchevsky@fb.com/

> Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
> ---
> Sending this separately from rbtree patchset as Joanne also needs this
> change for her usecase.

Not sure you need that given everybody's happy about it in the separate
thread, but for the sake of completion:

Reviewed-by: Stanislav Fomichev <sdf@google.com>