It's not that much work; after all, every libc needs to have its own implementation. The kernel maps the vDSO into memory for you, and gives you the base address as an entry in the auxiliary vector.
But using it does require some basic knowledge of the ELF format on the current platform, in order to parse the symbol table. (Alongside knowledge of which functions are available in the first place.)
It's hard work to NOT have the damn vDSO invade your address space. Only kludge part of Linux, well, apart from Nagle's, dlopen, and that weird zero copy kernel patch that mmap'd -each- socket recv(!) for a while.
It's possible, but tedious: if you disable ASLR to put the stack at the top of virtual memory, then use ELF segments to fill up everything from the mmap base downward (or upward, if you've set that), then the kernel will have nowhere left to put the vDSO, and give up.
(I investigated vDSO placement quite a lot for my x86-64 tiny ELF project: I had to rule out the possibility of a tiny ELF placing its entry point in the vDSO, to bounce back out somewhere else in the address space. It can be done, but not in any ELF file shorter than one that enters its own mapping directly.)
Curiously, there are undocumented arch_prctl() commands to map any of the three vDSOs (32, 64, x32) into your address space, if they are not already mapped, or have been unmapped.
It's not that much work; after all, every libc needs to have its own implementation. The kernel maps the vDSO into memory for you, and gives you the base address as an entry in the auxiliary vector.
But using it does require some basic knowledge of the ELF format on the current platform, in order to parse the symbol table. (Alongside knowledge of which functions are available in the first place.)
It's hard work to NOT have the damn vDSO invade your address space. Only kludge part of Linux, well, apart from Nagle's, dlopen, and that weird zero copy kernel patch that mmap'd -each- socket recv(!) for a while.
It's possible, but tedious: if you disable ASLR to put the stack at the top of virtual memory, then use ELF segments to fill up everything from the mmap base downward (or upward, if you've set that), then the kernel will have nowhere left to put the vDSO, and give up.
(I investigated vDSO placement quite a lot for my x86-64 tiny ELF project: I had to rule out the possibility of a tiny ELF placing its entry point in the vDSO, to bounce back out somewhere else in the address space. It can be done, but not in any ELF file shorter than one that enters its own mapping directly.)
Curiously, there are undocumented arch_prctl() commands to map any of the three vDSOs (32, 64, x32) into your address space, if they are not already mapped, or have been unmapped.