Re: [edk2-devel] [PATCH] OvmfPkg/PvScsiDxe: Fix VS2019 build error because of implicit cast

["Liran Alon" <liran.alon@oracle.com>]

On 01/04/2020 0:56, Laszlo Ersek wrote:
> On 03/31/20 17:53, Sean via Groups.Io wrote:
>> A couple of thoughts.
>> 1. I would suggest that ASSERT should not be the only protection for an invalid operation as ASSERT is usually disabled on release builds.
>> 2. We do have a library to make this more explicit and common. https://urldefense.com/v3/__https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Library/SafeIntLib.h*L548__;Iw!!GqivPVa7Brio!LNp76poqa4dly7M8C8F3aX66wzZA68yStF_9jS-pD3izw3uJ24i_mDygFJEBsLc$
> In this case, when "Response->ScsiStatus" does not fit in
> "Packet->TargetStatus", the device model is obviously (and blatantly)
> misbehaving, so I would agree with Liran that trying to recover from
> that (or to cover it up with a nice error code passed out) is futile.
Exactly.
>
> I do agree with the observation however that ASSERT()s disappear from
> RELEASE builds.
>
> Mike Kinney taught me a pattern to deal with this. There are various
> ways to write it; one example (for this case) is:
>
>    ASSERT (Response->ScsiStatus <= MAX_UINT8);
>    if (Response->ScsiStatus > MAX_UINT8) {
>      CpuDeadLoop ();
>    }
>    Packet->TargetStatus = (UINT8)Response->ScsiStatus;
>
> An alternative way to write it is (by moving the ASSERT into the block):
>
>    if (Response->ScsiStatus > MAX_UINT8) {
>      ASSERT (Response->ScsiStatus <= MAX_UINT8);
>      CpuDeadLoop ();
>    }
>    Packet->TargetStatus = (UINT8)Response->ScsiStatus;
>
> Yet another (simply assert FALSE in the block):
>
>    if (Response->ScsiStatus > MAX_UINT8) {
>      ASSERT (FALSE);
>      CpuDeadLoop ();
>    }
>    Packet->TargetStatus = (UINT8)Response->ScsiStatus;
>
>
> Why:
>
> - in DEBUG builds, the assertion failure will be logged, and the proper
> assertion failure action will be triggered (CpuDeadLoop / exception /
> ..., as configured by the platform)
>
> - in RELEASE builds, we'll still hang, and might have a chance to
> investigate (get a stack dump perhaps).
>
> Regarding SafeIntLib, I'm a fan in general. In this case, I did not
> think of it (possible integer truncations seem so rare in this driver).
> For this patch, I'm OK either way (with or without using SafeIntLib), as
> long as we add both the ASSERT and the explicit CpuDeadLoop (either
> variant of the three).
>
> Thanks
> Laszlo
Honestly, I don't quite understand why using SafeIntLib is useful in 
this case.
It just internally does even more branches and checks for exactly same 
overflow and return RETURN_BUFFER_TOO_SMALL in case value is bigger than 
MAX_UINT8.
Externally, I would still need to do a check on SafeUint16ToUint8() 
return-value. So what's the benefit?... Seems to me to just be an 
useless overhead.
I believe checking against MAX_UINT8 and casting immediately one line 
afterwards, is explicit enough.

Regarding above comment that ASSERT() doesn't do anything for RELEASE 
builds:
The point in ASSERT() is to be able to check a condition early to assist 
debugging but not worth putting this condition in RELEASE as it should 
never happen and just waste CPU cycles.
I thought this is the case we have here. If a weird ScsiStatus would 
return, it is highly unlikely that boot would just succeed as usual, and 
if it does, does the user really care?
In contrast, when boot fails because of this, it makes sense to build in 
DEBUG in attempt to verify what happened.
Note that if this condition will ever evaluate to FALSE (i.e. ScsiStatus 
is bigger than MAX_UINT8), then it is probably very deterministic. As it 
means PVSCSI device emulation on host is broken
and broke because of a very specific commit on host userspace VMM (E.g. 
QEMU).
Therefore, I still think an ASSERT() is what fits here best. But if you 
still think otherwise, then I have no objection to change it (Or Laszlo 
change it when applying).
I would say though, that if the pattern above is common, why isn't there 
a macro in EDK2 for that instead of manually writing it? Something like:

#define RELEASE_ASSERT(Cond) \
                if (!(Cond)) {                \
                  ASSERT (FALSE);        \
                  CpuDeadLoop ();       \
                }

That would be more elegant.

-Liran