Re: [PATCH v2 bpf-next 1/2] bpf: Fix release_on_unlock release logic for multiple refs

[Yonghong Song <yhs@meta.com>]

On 12/1/22 10:34 AM, Dave Marchevsky wrote:
> Consider a verifier state with three acquired references, all with
> release_on_unlock = true:
> 
>              idx  0 1 2
>    state->refs = [2 4 6]
> 
> (with 2, 4, and 6 being the ref ids).
> 
> When bpf_spin_unlock is called, process_spin_lock will loop through all
> acquired_refs and, for each ref, if it's release_on_unlock, calls
> release_reference on it. That function in turn calls
> release_reference_state, which removes the reference from state->refs by
> swapping the reference state with the last reference state in
> refs array and decrements acquired_refs count.
> 
> process_spin_lock's loop logic, which is essentially:
> 
>    for (i = 0; i < state->acquired_refs; i++) {
>      if (!state->refs[i].release_on_unlock)
>        continue;
>      release_reference(state->refs[i].id);
>    }
> 
> will fail to release release_on_unlock references which are swapped from
> the end. Running this logic on our example demonstrates:
> 
>    state->refs = [2 4 6] (start of idx=0 iter)
>      release state->refs[0] by swapping w/ state->refs[2]
> 
>    state->refs = [6 4]   (start of idx=1)
>      release state->refs[1], no need to swap as it's the last idx
> 
>    state->refs = [6]     (start of idx=2, loop terminates)
> 
> ref_id 6 should have been removed but was skipped.
> 
> Fix this by looping from back-to-front, which results in refs that are
> candidates for removal being swapped with refs which have already been
> examined and kept.
> 
> If we modify our initial example such that ref 6 is replaced with ref 7,
> which is _not_ release_on_unlock, and loop from the back, we'd see:
> 
>    state->refs = [2 4 7] (start of idx=2)
> 
>    state->refs = [2 4 7] (start of idx=1)
> 
>    state->refs = [2 7]   (start of idx=0, refs 7 and 4 swapped)
> 
>    state->refs = [7]     (after idx=0, 7 and 2 swapped, loop terminates)

Thanks, new description is much better.

> 
> Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
> Acked-by: Yonghong Song <yhs@fb.com>
> cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> Fixes: 534e86bc6c66 ("bpf: Add 'release on unlock' logic for bpf_list_push_{front,back}")
> ---
> v1 -> v2 lore.kernel.org/bpf/b5d46fd5-2693-cd46-9515-700fef1a110b@meta.com:
> 
>    * Update second example in patch summary to use a new ref_id for the
>      non-release_on_unlock ref. (Yonghong)
>    * Add Yonghong's ack
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