On 8/9/2024 8:51 AM, Amery Hung wrote:
> From: Dave Marchevsky <davemarchevsky@xxxxxx>
>
> Currently btf_parse_fields is used in two places to create struct
> btf_record's for structs: when looking at mapval type, and when looking
> at any struct in program BTF. The former looks for kptr fields while the
> latter does not. This patch modifies the btf_parse_fields call made when
> looking at prog BTF struct types to search for kptrs as well.
>
> Before this series there was no reason to search for kptrs in non-mapval
> types: a referenced kptr needs some owner to guarantee resource cleanup,
> and map values were the only owner that supported this. If a struct with
> a kptr field were to have some non-kptr-aware owner, the kptr field
> might not be properly cleaned up and result in resources leaking. Only
> searching for kptr fields in mapval was a simple way to avoid this
> problem.
>
> In practice, though, searching for BPF_KPTR when populating
> struct_meta_tab does not expose us to this risk, as struct_meta_tab is
> only accessed through btf_find_struct_meta helper, and that helper is
> only called in contexts where recognizing the kptr field is safe:
>
> * PTR_TO_BTF_ID reg w/ MEM_ALLOC flag
> * Such a reg is a local kptr and must be free'd via bpf_obj_drop,
> which will correctly handle kptr field
>
> * When handling specific kfuncs which either expect MEM_ALLOC input or
> return MEM_ALLOC output (obj_{new,drop}, percpu_obj_{new,drop},
> list+rbtree funcs, refcount_acquire)
> * Will correctly handle kptr field for same reasons as above
>
> * When looking at kptr pointee type
> * Called by functions which implement "correct kptr resource
> handling"
>
> * In btf_check_and_fixup_fields
> * Helper that ensures no ownership loops for lists and rbtrees,
> doesn't care about kptr field existence
>
> So we should be able to find BPF_KPTR fields in all prog BTF structs
> without leaking resources.
>
> Further patches in the series will build on this change to support
> kptr_xchg into non-mapval local kptr. Without this change there would be
> no kptr field found in such a type.
>
> Acked-by: Martin KaFai Lau <martin.lau@xxxxxxxxxx>
> Signed-off-by: Dave Marchevsky <davemarchevsky@xxxxxx>
> Signed-off-by: Amery Hung <amery.hung@xxxxxxxxxxxxx>
Acked-by: Hou Tao <houtao1@xxxxxxxxxx>
