On Wed, 2023-11-08 at 17:36 +0800, WANG Rui wrote: > > xry111@nanmen2 ~ $ clang t1.c -O2 -fno-pie -no-pie > > xry111@nanmen2 ~ $ ./a.out > > Bus error (core dumped) > > > > I'll consider it a Clang bug then. https://github.com/llvm/llvm-project/issues/71645 > That's it, no copy relocations. As far as I know, copying relocations > has some issues and is not recommended by Fangrui. > > For modules, if distance is not a problem, `no-pic` and > `direct-access-external-data` can be together because the code is > writable. Does it seem reasonable to exist? It may be usable, but the result is generally worse than relying on GOT. For example, consider a module referring two data symbols in vmlinux, foo and bar. The symbol foo is referred 10 times and bar is referred 8 times. With the current GOT-based approach, the total space usage is (2 GOT entry * (8 bytes / GOT entry)) + ((10 + 8) * 2 instruction * 4 (bytes / instruction)) = 160 bytes. With -fdirect-access-external-data, we must add -mcmodel=extreme too because the modules are too far away from vmlinux in the kernel address space, then the total space usage will be (10 + 8) * 5 instruction * 4 (bytes / instruction) = 360 bytes. One possible approach to resolve the issue is relocating vmlinux from XKPRANGE to XKVRANGE and fit vmlinux + all modules into a 2GiB range. Then the total space usage will be (10 + 8) * 2 instruction * 4 (bytes / instruction) = 144 bytes. But I don't know how to implement this, and running vmlinux in XKVRANGE may have a performance penalty. -- Xi Ruoyao <xry111@xxxxxxxxxxx> School of Aerospace Science and Technology, Xidian University
