It also uses the int 0x80 interface instead of the faster sysenter opcode. (AMD calls this opcode syscall.)

http://wiki.osdev.org/SYSENTER

I say "faster" above, and that's true for all chips since the Pentium 4, if not earlier, but it isn't true on all 32-bit x86 chips. It isn't even true on all 32-bit x86 chips which have the opcode. So the Linux kernel has a special trick such that all binaries always use the fastest syscall method, regardless of which kind of system they were compiled for: linux-gate.so.1 also known as linux-vdso.so.1

This isn't actually a file on disk, it's just some machine code the kernel maps into the process address space which syscalls indirect through, because the kernel knows more about the hardware than applications.

http://www.trilithium.com/johan/2005/08/linux-gate/

vDSO even stands for virtual Dynamic Shared Object:

http://man7.org/linux/man-pages/man7/vdso.7.html

Aside from being virtual, it's a normal ELF shared object, with symbol names and versoning and the functions use the normal C calling convention.