ISO14443aTarget marshalling problem?

[messinm]

See https://gist.github.com/messinm/7086bfce591bf6f1d554
Running under Ubuntu 14.04 64-bit, libnfc 1.7.0

mifare target gives the following from printing Target received from InitiatorListPassiveTargets:
ISO/IEC 14443A (106 kbps) target:
ATQA (SENS_RES): 00 04

• UID size: single
• bit frame anticollision supported
  UID (NFCID1): 3c be 90 75
  SAK (SEL_RES): 08
• Not compliant with ISO/IEC 14443-4
• Not compliant with ISO/IEC 18092

Fingerprinting based on MIFARE type Identification Procedure:

• MIFARE Classic 1K
• MIFARE Plus (4 Byte UID or 4 Byte RID) 2K, Security level 1
• SmartMX with MIFARE 1K emulation
  Other possible matches based on ATQA & SAK values:

Then attempting marshalling to ISO14443aTarget, I get the following:

Atqa =[0 4]
Sak =8
UIDLen =33091718999965696
UID =[112 70 188 32 255 0 0 0 0 0]
Note that 33091718999965696 = 0x7590BE3C000000, so it appears that the UIDLen field actually contains the UID.

Am I doing something wrong in the marshalling code? Or is this a bug in Go NFC lib or libnfc?

[clausecker]

Sounds like a bug in my code to me. Let me check please.

[clausecker]

unsafe_p := (unsafe.Pointer(targets[0].Marshall()))
defer C.free(unsafe_p)
mtarget := (*nfc.ISO14443aTarget)(unsafe_p)

Casting an unsafe.Pointer you get from Marshall() to one of the target types can't work. The pointer you get from Marshall() points to a C type, whereas the target types are entirely different types. As per documentation for Target.Marshall():

Marshall() returns a pointer to an nfc_target allocated with C.malloc() that contains the same data as the Target. Don't forget to C.free() the result of Marshall() afterwards. A runtime panic may occur if any slice referenced by a Target has been made larger than the maximum length mentioned in the respective comments.

Where nfc_target refers to the C type nfc_target, not the Go type. If you want to convert a Target into an object of concrete type, use a type assertion like this (replacing the code above):

mtarget, ok := targets[0].(nfc.ISO14443aTarget)
if !ok {
    // targets[0] is apparently not an ISO 14443a target. Do something here...
}

Please tell me if this solved your problem and then close the issue.

[messinm]

Thanks for the quick response.

I tried your suggestion, but it won't compile. See snippet and error message below. Seems that nfc.ISO14443aTarget should implement nfc.Target, but doesn't.

                            //unsafe_p := (unsafe.Pointer(targets[0].Marshall()))
                            //defer C.free(unsafe_p)
                            //mtarget := (*nfc.ISO14443aTarget)(unsafe_p) 

                            mtarget, ok := targets[0].(nfc.ISO14443aTarget)
                            if !ok {
                               fmt.Printf("Not ISO14443aTarget!")
                            } else {
            fmt.Printf("Atqa =%v\n",mtarget.Atqa)
            fmt.Printf("Sak =%v\n",mtarget.Sak) 
                    fmt.Printf("UIDLen =%v\n",mtarget.UIDLen)  
                    fmt.Printf("UID =%v\n",mtarget.UID) 
                            }

//does not compile:
//../src/messinm2/testnfc/testnfc.go:81: impossible type assertion:
// nfc.ISO14443aTarget does not implement nfc.Target (Marshall method has pointer receiver)

[clausecker]

Sorry. Cast to *nfc.ISO14443aTarget, not nfc.ISO14443aTarget. I don't know why I forgot that star.

[messinm]

That works! Thanks, and thanks very much for the lib.

[clausecker]

It was a pleasure to me.