Re: [PATCH bpf-next v3 4/7] bpf: Allow reads through trusted-or-null BTF pointers

[bot+bpf-ci]

> bpf: Allow reads through trusted-or-null BTF pointers
>
> Currently, a trusted-or-null pointer
> (i.e. PTR_TO_BTF_ID|PTR_TRUSTED|PTR_MAYBE_NULL) has to be checked
> for NULL before it can be dereferenced. Marking a field from
> PTR_TO_BTF_ID typing to trusted-or-null can reject programs that
> previously dereferenced the pointer directly. This is useful as we
> need to mark new fields as trusted in order to pass those as arguments
> to kfuncs.
>
> This patch adds support to dereference pointers marked as
> PTR_TO_BTF_ID|PTR_TRUSTED|PTR_MAYBE_NULL without the need for a NULL
> check. We define that those pointers can fault on dereference, so
> reads need to happen through BPF_PROBE_MEM. If the dereferenced pointer
> is another BTF pointer, we clear trusted flags and mark the result as
> PTR_UNTRUSTED.
>
> This applies only to reads. Other cases still require an explicit NULL
> check. After such a check, the pointer retains PTR_TRUSTED and can be
> used normally.
>
> To summarise, before that patch, reads to PTR_TO_BTF_ID|PTR_UNTRUSTED
> are allowed through BPF_PROBE_MEM and reads to
> PTR_TO_BTF_ID|PTR_TRUSTED are normal loads. This patch allows
> PTR_TO_BTF_ID|PTR_TRUSTED|PTR_MAYBE_NULL to happen through
> BPF_PROBE_MEM.
>
> The unchecked read path has two consequences:
>
> 1. It uses BPF_PROBE_MEM, which is slower than a normal load. An
>    explicit NULL check refines the pointer to PTR_TRUSTED and allows a
>    normal load.
>
> 2. A faulting read returns zero, which is indistinguishable from a
>    legitimately zero-valued field. Programs that need to distinguish
>    those cases must check the pointer before reading the field.
>
> The next patch updates current tests and also introduces more checks to
> ensure this change does not break anything.
>
> Signed-off-by: Anastasios Papagiannis <tasos.papagiannnis@xxxxxxxxx>

This isn't a bug, but could the changelog use the imperative mood here
("Allow reads through PTR_TO_BTF_ID|PTR_TRUSTED|PTR_MAYBE_NULL without a
NULL check") and drop the "To summarise" paragraph, since the preceding
paragraph already states the before/after behaviour?


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