内核模块中的符号

Question

I built linux kernel module with SSP support for mips architecture. I added -fstack-protector-all to compilation flags. But after loading this module I've got undefined references to __stack_chk_guard and __stack_chk_fail . But I added libssp.so to linker. It looks like I should export those symbols in kernel something like this:

EXPORT_SYMBOL(__stack_chk_guard);

Because my kernel is old and didn't contain them yet. But unfortunately I should use this version.

My question is: why user space can use this symbols from toolchain library, but kernel space don't ?

I think, I missed some linux kernel essentials.

Answer 1

You can't link the kernel to shared libraries. If you have a static library of libssp, it MAY work - but it would require that the library isn't calling something else that would cause problems in the kernel.

In general, stack-checking isn't something that you should be doing on the kernel - I'm pretty sure it serves no particularly good purpose [I'm also pretty sure that the kernel uses a "guard page" for each kernel stack].

Answer 2

You cannot use shared libraries anywhere in kernel space (including as part of kernel modules).

You could think of kernel modules themselves as an equivalent of shared libraries in kernel space but with lot of differences.

Kernel modules can depend on exported symbols from other kernel modules.

My question is: why user space can use this symbols from toolchain library, but kernel space don't ?

Nothing in kernel space has access to the libc C library. Kernel has its own set of builtin standard string manipulation functions, etc. that you could use instead. The toolchain libraries are built on top of libc .

+1 on Mats's answer. You could use a static library as long as it does not depend on standard C libraries like libc