ELF binaries sometimes require an extra step of ``branding''. If you attempt to run an unbranded ELF binary, you will get an error message like the following:
% ./my-linux-elf-binary ELF binary type not known Abort %
To help the FreeBSD kernel distinguish between a FreeBSD ELF binary from a Linux binary, use the brandelf(1) utility:
% brandelf -t Linux my-linux-elf-binary
The GNU toolchain now places the appropriate branding information into ELF binaries automatically, so you should be needing to do this step increasingly rarely in the future.