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af0a865d9f
- Adds endian aware functions - Removes references to host endian - Uses binary detected endianness else tries LE and restricts by RAsmPlugin - Fixes gdb debugger endianness when debugging BE qemu gdbserver Signed-off-by: Damien Zammit <damien@zamaudio.com>
69 lines
2.2 KiB
Plaintext
69 lines
2.2 KiB
Plaintext
Endian issues
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=============
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As hackers, we need to be aware of endianness.
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Endianness can become a problem when you try to process buffers or streams
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of bytes and store intermediate values as integers with width larger than
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a single byte.
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It can seem very easy to write the following code:
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ut8 opcode[4] = {0x10, 0x20, 0x30, 0x40};
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ut32 value = *(ut32*)opcode;
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... and then continue to use "value" in the code to represent the opcode.
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This needs to be avoided!
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Why? What is actually happening?
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When you cast the opcode stream to a unsigned int, the compiler uses the endianness
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of the host to interpret the bytes and stores it in host endianness. This leads to
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very unportable code, because if you compile on a different endian machine, the
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value stored in "value" might be 0x40302010 instead of 0x10203040.
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In the past, radare devs were not as strict about this issue, and as a result,
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needed to swap the endian of values regularly in the code.
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Solution
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========
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Use bitshifts and OR instructions to interpret bytes in a known endian.
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Instead of casting streams of bytes to larger width integers, do the following:
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ut8 opcode[4] = {0x10, 0x20, 0x30, 0x40};
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ut32 value = opcode[0] | opcode[1] << 8 | opcode[2] << 16 | opcode[3] << 24;
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or if you prefer the other endian:
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ut32 value = opcode[3] | opcode[2] << 8 | opcode[1] << 16 | opcode[0] << 24;
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This is much better because you actually know which endian your bytes are stored in
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within the integer value, REGARDLESS of the host endian of the machine.
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Endian helper functions
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=======================
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Radare2 now uses helper functions to interpret all byte streams in a known endian.
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Please use these at all times, eg:
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val32 = r_read_be32(buffer) // reads 4 bytes from a stream in BE
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val32 = r_read_le32(buffer) // reads 4 bytes from a stream in LE
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val32 = r_read_ble32(buffer, isbig) // reads 4 bytes from a stream:
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// if isbig is true, reads in BE
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// otherwise reads in LE
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There are a number of helper functions for 64, 32, 16, and 8 bit reads and writes.
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(Note that 8 bit reads are equivalent to casting a single byte of the buffer
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to a ut8 value, ie endian is irrelevant).
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Happy hacking!
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- damo22
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