[edk2-devel] [PATCH 2/3] MdePkg/BaseLib: rewrite Base64Decode()

[Laszlo Ersek]

Hi Marvin,

On 07/15/19 20:44, Marvin Häuser wrote:
> I feel like my rushed message mentioning 'MAX_ADDRESS' was misleading
> a little - the point with that was a potential index overflow (I may
> actually have meant 'MAX_UINTN', I am not sure about the details
> anymore) in the original code, I personally see a lot of sense in
> *not* checking whether the buffer wraps around (similarly the overlap
> condition). For one consistency with similar code, where no such
> checks exist, and then a sanity-trust in the caller (which, as this is
> a library function, is interior as opposed to an external protocol
> caller which should naturally be trusted less).
>
> Generally, I am rather confused about the edk2 trust model for static
> calls. A bunch of libraries verify input parameter sanity via ASSERTs,
> another bunch by runtime checks and appropriate return statuses. Is
> there any kind of policy I am unaware of?

Good points, and you are right, the landscape is not consistent.

I think the edk2 practice can be characterized as follows (again, the
picture is not uniform / consistent across the codebase):

(1) Functions are supposed to have detailed interface contracts, but
    many don't. In some cases, cutting corners appears at least
    moderately defensible, because a function can be really simple, and
    writing documentation could be the larger part of the effort. I
    still prefer if we document all new functions painstakingly.

(2) Caller responsibilities are frequently checked by callees. The
    checks vary between ASSERT()s and explicit conditions / return
    values. The 2nd kind is better, of course, and that is actually
    exemplified by the UEFI spec.

    Namely, a large proportion of EFI_INVALID_PARAMETER return codes
    correspond to cases when the callee catches the caller breaking the
    interface contract. See for example EFI_BOOT_SERVICES.CreateEvent().

    In some other cases, similar efforts / spec requirements look
    dubious (see EFI_BOOT_SERVICES.FreePool() -- "Buffer was invalid").
    If the callee gets a pointer to freed storage, it can't even
    *evaluate* that pointer without depending on undefined behavior. So,
    in theory, there's no way to enforce the contract, beyond trusting
    the caller, and so there can be no *substitute* for a detailed
    contract; see (1).

    I think the approach ("watch your caller") is generally acceptable
    still, if there is not a large runtime cost, because the environment
    is extremely unforgiving, and because the firmware can, in practice,
    recover from *some* undefined behavior this way, even.

(3) Genuine failure conditions are occasionally checked with
    ASSERT(). This should never be done.


Now, considering "wrapped arrays" and such -- obviously such a thing is
not even an object in C (no valid memory allocation would ever produce
it), so normally I would consider checks against it pointless.

However, in the present case, there are three arguments for including
the MAX_ADDRESS checks:

- The original code included similar MAX_ADDRESS checks, and I didn't
  want to "weaken" the implementation in any way. (I simply didn't want
  to defend such choices -- so I didn't make them.)

- The conditions are not costly, and as long as the buffer pointers are
  into valid storage, the conditions do catch -- without invoking
  undefined behavior -- *sizes* that would cause wrap-around. This is in
  line with (2).

- This is MdePkg/BaseLib, so general distrust (with negligible runtime
  cost) cannot hurt. Widely used library -- yes, not a protocol, but
  still linked (statically) into a bunch of 3rd party code --, and
  (again) an unforgiving environment.

I introduced the overlap check myself (IIRC). That was because:

- There is no language-level reason why decoding back into the same
  buffer couldn't work -- it's just that this implementation doesn't aim
  to support that. Hence the corresponding natural language restriction
  in the interface contract. (In edk2, we don't have the "restrict"
  keyword, from C99.) Otherwise a programmer might cleverly say, "aha,
  base64 decoding never *inflates* data, so I'll just decode "in-place".

- And then, although it's a caller responsibility, catch the overlap
  explicitly with RETURN_INVALID_PARAMETER, in line with the above: the
  checks are cheap, and BaseLib is quite central.

Now I'm not trying to present any of this as "policy" -- just my 2
cents. I hope it makes sense.

Thanks
Laszlo

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