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55 lines
2.3 KiB
55 lines
2.3 KiB
7 months ago
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HiddenApi
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=========
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This tool iterates over all class members inside given DEX files and modifies
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their access flags if their signatures appear on one of two lists - unsupported and
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blocklist - provided as text file inputs. These access flags denote to the
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runtime that the marked methods/fields should be treated as internal APIs with
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access restricted only to platform code. Methods/fields not mentioned on the two
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lists are assumed to be part of the SDK and left accessible by all code.
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API signatures
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==============
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The methods/fields to be marked are specified in two text files (unsupported,
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blocklist) provided an input. Only one signature per line is allowed.
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Types are expected in their DEX format - class descriptors are to be provided in
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"slash" form, e.g. "Ljava/lang/Object;", primitive types in their shorty form,
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e.g. "I" for "int", and a "[" prefix denotes an array type. Lists of types do
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not use any separators, e.g. "ILxyz;F" for "int, xyz, float".
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Methods are encoded as:
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`class_descriptor->method_name(parameter_types)return_type`
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Fields are encoded as:
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`class_descriptor->field_name:field_type`
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Bit encoding
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============
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Two bits of information are encoded in the DEX access flags. These are encoded
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as unsigned LEB128 values in DEX and so as to not increase the size of the DEX,
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different modifiers were chosen for different kinds of methods/fields.
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First bit is encoded as the inversion of visibility access flags (bits 2:0).
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At most one of these flags can be set at any given time. Inverting these bits
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therefore produces a value where at least two bits are set and there is never
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any loss of information.
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Second bit is encoded differently for each given type of class member as there
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is no single unused bit such that setting it would not increase the size of the
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LEB128 encoding. The following bits are used:
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* bit 5 for fields as it carries no other meaning
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* bit 5 for non-native methods, as `synchronized` can only be set on native
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methods (the Java `synchronized` modifier is bit 17)
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* bit 9 for native methods, as it carries no meaning and bit 8 (`native`) will
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make the LEB128 encoding at least two bytes long
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Two following bit encoding is used to denote the membership of a method/field:
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* sdk: `false`, `false`
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* unsupported: `true`, `false`
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* blocklist: `true`, `true`
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