[llvm-dev] Randomize offset between program segments?

[Topi Miettinen via llvm-dev]

Hi,

I'd like to improve address space randomization (ASLR) by randomizing 
the offset between .text, .data and .bss segments (or more generalized, 
any program segments). With large code generation model (-mcmodel=large) 
on AMD64, the offset could be very large, but even with the default 
model, the segments could be randomized within range of RIP-relative 
accesses (+/-2GB). Currently the dynamic loader can't randomize the 
segments (nothing also tells it if this would be OK) so it maps them 
next to each other, which is predictable and boring.

For this to happen, I think the compiler would have to emit relocations 
for all cross-segment accesses and probably flagging the shared object 
somehow. Then, when detecting the flag, the dynamic loader could load 
the segments at random offsets within 2GB, or if the large model was 
used in compilation (another flag), anywhere in the available virtual 
address space (let OS map the segment anywhere by using mmap(NULL,...)).

Perhaps if GOT would be kept within 2GB range, other data segments could 
still be placed anywhere.

There would be some slowdown because of additional relocations (and the 
OS would not be happy due to increased VM fragmentation) but I think 
otherwise nothing should change (the code should be identical). This 
would be of course an opt-in feature mainly for hardened systems.

So, I wonder how to implement the compiler part. Is this something that 
could be done easily with LLVM/Clang?

-Topi