========================= Implementation ========================= Checking if an object should resist fingerprinting is ideally done by referencing the `Document's ShouldResistFingerprinting `_ method. This is both fast and correct. In certain other situations, you may need to call some of the ``nsContentUtils::ShouldResistFingerprinting`` functions. When doing so, you should avoid calling either of the functions marked *dangerous*. As you can see in the callgraph below, directly calling a *dangerous* function will skip some of the checks that occur further up-stack. .. mermaid:: graph TD SRFP["ShouldResistFingerprinting()"] click SRFP href "https://searchfox.org/mozilla-central/search?q=symbol:_ZN14nsContentUtils26ShouldResistFingerprintingEN7mozilla9RFPTargetE&redirect=false" SRGP_GO["ShouldResistFingerprinting(nsIGlobalObject* aGlobalObject"] click SRGP_GO href "https://searchfox.org/mozilla-central/search?q=symbol:_ZN14nsContentUtils26ShouldResistFingerprintingEP15nsIGlobalObjectN7mozilla9RFPTargetE&redirect=false" GO_SRFP["nsIGlobalObject*::ShouldResistFingerprinting()"] click GO_SRFP href "https://searchfox.org/mozilla-central/search?q=symbol:_ZNK15nsIGlobalObject26ShouldResistFingerprintingEN7mozilla9RFPTargetE&redirect=false" Doc_SRFP["Document::ShouldResistFingerprinting()
System Principal Check"] click Doc_SRFP href "https://searchfox.org/mozilla-central/search?q=symbol:_ZNK7mozilla3dom8Document26ShouldResistFingerprintingENS_9RFPTargetE&redirect=false" SRFP_char["ShouldResistFingerprinting(const char*)"] click SRFP_char href "https://searchfox.org/mozilla-central/search?q=symbol:_ZN14nsContentUtils26ShouldResistFingerprintingEPKcN7mozilla9RFPTargetE&redirect=false" SRFP_callertype_go["ShouldResistFingerprinting(CallerType, nsIGlobalObject*)
System Principal Check"] click SRFP_callertype_go href "https://searchfox.org/mozilla-central/search?q=symbol:_ZN14nsContentUtils26ShouldResistFingerprintingEN7mozilla3dom10CallerTypeEP15nsIGlobalObjectNS0_9RFPTargetE&redirect=false" SRFP_docshell["ShouldResistFingerprinting(nsIDocShell*)"] click SRFP_docshell href "https://searchfox.org/mozilla-central/search?q=symbol:_ZN14nsContentUtils26ShouldResistFingerprintingEP11nsIDocShellN7mozilla9RFPTargetE&redirect=false" SRFP_channel["ShouldResistFingerprinting(nsIChannel*)
ETPSaysShouldNotResistFingerprinting Check
CookieJarSettingsSaysShouldResistFingerprinting Check"] click SRFP_channel href "https://searchfox.org/mozilla-central/search?q=symbol:_ZN14nsContentUtils26ShouldResistFingerprintingEP10nsIChannelN7mozilla9RFPTargetE&redirect=false" SRFP_uri["ShouldResistFingerprinting_dangerous(nsIURI*, OriginAttributes)
PBM Check
Scheme (inc WebExtension) Check
About Page Check
URI & Partition Key Exempt Check"] click SRFP_uri href "https://searchfox.org/mozilla-central/search?q=symbol:_ZN14nsContentUtils36ShouldResistFingerprinting_dangerousEP6nsIURIRKN7mozilla16OriginAttributesEPKcNS2_9RFPTargetE&redirect=false" SRFP_principal["ShouldResistFingerprinting_dangerous(nsIPrincipal*)
System Principal Check
PBM Check
Scheme Check
About Page Check
Web Extension Principal Check
URI & Partition Key Exempt Check"] click SRFP_principal href "https://searchfox.org/mozilla-central/search?q=symbol:_ZN14nsContentUtils36ShouldResistFingerprinting_dangerousEP12nsIPrincipalPKcN7mozilla9RFPTargetE&redirect=false" SRFP_principal --> |null| SRFP_char SRFP_uri --> |null| SRFP_char SRFP_channel -->|null| SRFP_char SRFP_channel --> |Document Load| SRFP_uri SRFP_channel --> |Subresource Load| SRFP_principal SRFP_docshell -->|null| SRFP_char SRFP_docshell --> Doc_SRFP SRFP_callertype_go --> SRGP_GO SRFP_char --> SRFP SRGP_GO -->|null| SRFP_char SRGP_GO --> GO_SRFP GO_SRFP --> |innerWindow, outerWindow| Doc_SRFP Doc_SRFP --> SRFP_channel Exemptions and Targets ~~~~~~~~~~~~~~~~~~~~~~ Fingerprinting Resistance takes into account many things to determine if we should alter behavior: * Whether we are the System Principal * Whether we are a Web Extension * Whether Fingerprinting Resistance is applied to all browsing modes or only Private Browsing Mode * Whether the specific site you are visiting has been granted an exemption (taking into account the framing page) * Whether the specific **activity** is granted an exemption All callsites for ``ShouldResistFingerprinting`` take a (currently) optional ``RFPTarget`` value, which defaults to ``Unknown``. While arguments such as ``Document`` or ``nsIChannel`` provide context for the first four exemptions above, the Target provides context for the final one. A Target is a Web API or an activity - such as a Pointer Event, the Screen Orientation, or plugging a gamepad into your computer (and therefore producing a `gamepadconnected event `_). Most Targets correlate strongly to a specific Web API, but not all do: for example whether or not to automatically reject Canvas extraction requests from third parties is a separate Target from prompting to reject canvas extraction. In some situations we may *not* alter our behavior for a certain activity - this could be based on the fingerprinting resistance mode you are using, or per-site overrides to correct breakage. Targets are defined `RFPTargets.inc `_.