Re: [RFCv2 PATCH bpf-next 01/18] bpf: Add verifier support for custom callback return range

[Dave Marchevsky <davemarchevsky@fb.com>]

On 9/1/22 5:01 PM, Joanne Koong wrote:
> On Tue, Aug 30, 2022 at 11:03 AM Dave Marchevsky <davemarchevsky@fb.com> 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 further in this series, 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, 1) instead of using tnum_unknown as a sentinel
>> value as the latter would prevent the valid range (0, U64_MAX) being
>> used.
>>
>> Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
>> ---
>>  include/linux/bpf_verifier.h | 1 +
>>  kernel/bpf/verifier.c        | 4 +++-
>>  2 files changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
>> index 2e3bad8640dc..9c017575c034 100644
>> --- a/include/linux/bpf_verifier.h
>> +++ b/include/linux/bpf_verifier.h
>> @@ -237,6 +237,7 @@ struct bpf_func_state {
>>          */
>>         u32 async_entry_cnt;
>>         bool in_callback_fn;
>> +       struct tnum callback_ret_range;
>>         bool in_async_callback_fn;
>>
>>         /* The following fields should be last. See copy_func_state() */
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 9bef8b41e737..68bfa7c28048 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -1745,6 +1745,7 @@ static void init_func_state(struct bpf_verifier_env *env,
>>         state->callsite = callsite;
>>         state->frameno = frameno;
>>         state->subprogno = subprogno;
>> +       state->callback_ret_range = tnum_range(0, 1);
>>         init_reg_state(env, state);
>>         mark_verifier_state_scratched(env);
>>  }
>> @@ -6879,6 +6880,7 @@ static int set_find_vma_callback_state(struct bpf_verifier_env *env,
>>         __mark_reg_not_init(env, &callee->regs[BPF_REG_4]);
>>         __mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
>>         callee->in_callback_fn = true;
>> +       callee->callback_ret_range = tnum_range(0, 1);
> 
> Thanks for removing this restriction for callback functions!
> 
> One quick question: is this line above needed? I think in
> __check_func_call, we always call init_func_state() first before
> calling set_find_vma_callback_state(), so after the init_func_state()
> call, the callee->callback_ret_range will already be set to
> tnum_range(0,1).
> 

You're right, it's not strictly necessary. I think that the default range being
tnum_range(0, 1) - although necessary for backwards compat - is unintuitive. So
decided to be explicit with existing callbacks so folks didn't have to go
searching for the default to understand what the ret_range is, and it's more
obvious that callback_ret_range should be changed if existing helper code is
reused.

>>         return 0;
>>  }
>>
>> @@ -6906,7 +6908,7 @@ static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
>>         caller = state->frame[state->curframe];
>>         if (callee->in_callback_fn) {
>>                 /* enforce R0 return value range [0, 1]. */
>> -               struct tnum range = tnum_range(0, 1);
>> +               struct tnum range = callee->callback_ret_range;
>>
>>                 if (r0->type != SCALAR_VALUE) {
>>                         verbose(env, "R0 not a scalar value\n");
>> --
>> 2.30.2
>>