Force rust compiler to use RIP relative addressing for global variables

help
1

Hello,

I am building a very simple kernel, and I am using globals here and there like so:

// some mod.rs

static FOO: () = ()

I am building my code using the following config.toml:

[build]
target = "x86_64-unknown-none"

[target.x86_64-unknown-none]
rustflags = [
    "-C", '''link-args=
 --image-base=0xffffffff80000000
 --no-nmagic
 --entry=_start
 -z separate-loadable-segments''',
    "-C", "relocation-model=static",
    "-C", "code-model=kernel"
]

[profile.dev]
panic = "abort"
debug = true

[profile.release]
panic = "abort"
opt-level = "z"
lto = "fat"
debug = true

This was working fine a couple of weeks ago, the compiler would produce some rip relative code and everything was good.
Now, however, the compiler is issuing gs relative code for some reason, f.e.:

<my_func_doing_smth>:
ffffffff800023c0:       48 83 ec 48             sub    $0x48,%rsp
ffffffff800023c4:       65 48 8b 04 25 00 00    mov    %gs:0x0,%rax
...

This obviously is pretty terrible because gs is used for my own structures.
Two questions:

  1. Why did this behavior change?
  2. Can I somehow get the compiler to do the right thing?
2

Compiler doesn't issue gs accesses “for some reason”, it uses it for TLS.

If your OS doesn't provide TLS then you need to find and remove code that uses it.

3

That is some reason. No reason to be antagonistic like that.

5 Likes
4

Are you showing the output from objdump applied to an object file? If you use objdump -Cdr on an object file it will show relocations in its disassembly (and demangle) which makes the output more readable. In this case would you have gotten a relocation for thread-local storage?

1 Like
5

If you want rip-relative addressing, don't use -Crelocation-model=static. That is for when the address of the executable is fixed at link time. The default -Crelocation-model=pie of the x86_64-unknown-none target should work for using rip-relative addressing.

x86_64 uses %fs for TLS, not %gs. Also unless you use the unstable #[thread_local], on no_std targets no TLS usage is possible at all.

5 Likes
6

Alright. Thanks everyone. I was indeed just being stupid and messed up %gs references which I miss-interpreted.

If you use objdump -Cdr on an object file it will show relocations in its disassembly

Thanks. That's a nice trick I did not know.

1 Like
7

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