The Teensy Executable Revisited

6 hours ago

The author's original 45-byte file was not a proper ELF executable but was mistakenly accepted by the Linux kernel.
To conform to ELF standards, the author revisits a 91-byte version and reduces it by overlapping headers and exploiting unused fields.
By overlapping the ELF header and program header table by 8 bytes, the size is reduced to 83 bytes.
Exploiting the 'p_paddr' field (with unspecified contents) allows embedding a 2-byte instruction and a jump, reducing to 81 bytes.
Changing the load address to 0x2AB30000 and rearranging instructions reduces the size to 79 bytes.
Using a jump into the 'p_memsz' field and adding a dummy byte achieves a 76-byte version, but with an 8MB memory allocation.
Replacing the jump with a 5-byte 'cmp' instruction allows crossing the 'p_filesz' field without extra memory overhead, finalizing at 76 bytes.
Purists note that direct system calls (e.g., 'int 0x80') are not part of the documented ABI, suggesting dynamic linking with libc is needed for full compliance.
Future work will explore dynamic linking to adhere strictly to ELF standards, adding complexity but ensuring compatibility.

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