x86_64 soft float kernel module POC

.foo.c

#include <stdint.h>

double ker(void) { return 42.0; }

// Can't link with compiler-rt because their built-ins also use SSE registers.
// A super naive double to int conversion function that only works on small positive integers.
int64_t naive_dtoi(double x) {
        uint64_t u = (union { double d; uint64_t u; }) { x }.u;
        return ((u & 0xfffffffffffff) | (1ULL << 52)) >> (1075 - ((u >> 52) & 0x7ff));
}

int foo(void) { return naive_dtoi(ker()); }

Makefile

obj-m += mymodule.o

mymodule-y := module.o foo.o

KDIR := /lib/modules/$(shell uname -r)/build
GENS := foo.o_shipped

all: gen
	$(MAKE) -C $(KDIR) M=$(PWD) modules

clean:
	$(MAKE) -C $(KDIR) M=$(PWD) clean
	$(RM) $(GENS)

gen: $(GENS)

foo.o_shipped: .foo.c
	clang -Wall -Wextra -pedantic -msoft-float -mno-sse -mno-red-zone $< -emit-llvm -S -o - | sed -e 's/"use-soft-float"="false"/"use-soft-float"="true"/' | llc -filetype=obj -o $@ # | clang -xir - -rtlib=compiler-rt -no-pie -nostartfiles -Wl,-r -o $@

.PHONY: all clean gen

module.c

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>

int foo(void);

int init_module(void) {
	printk(KERN_INFO "init_module(): %d\n", foo());
	return 0;
}
void cleanup_module(void) {
	printk(KERN_INFO "cleanup_module()\n");
}

MODULE_LICENSE("GPL");
MODULE_AUTHOR("PkmX");
MODULE_DESCRIPTION("Test module");
MODULE_VERSION("0.0");

zoutput.txt

$ make && sudo insmod mymodule.ko && sudo rmmod mymodule && dmesg -T | tail -n2
[Tue Dec 11 02:28:11 2018] init_module(): 42
[Tue Dec 11 02:28:11 2018] cleanup_module()

$ objdump -xd mymodule.ko
...
Disassembly of section .text:

0000000000000000 <init_module>:
   0:	e8 00 00 00 00       	callq  5 <init_module+0x5>
			1: R_X86_64_PLT32	__fentry__-0x4
   5:	e8 00 00 00 00       	callq  a <init_module+0xa>
			6: R_X86_64_PLT32	foo-0x4
   a:	48 c7 c7 00 00 00 00 	mov    $0x0,%rdi
			d: R_X86_64_32S	.rodata.str1.1
  11:	89 c6                	mov    %eax,%esi
  13:	e8 00 00 00 00       	callq  18 <init_module+0x18>
			14: R_X86_64_PLT32	printk-0x4
  18:	31 c0                	xor    %eax,%eax
  1a:	c3                   	retq   
  1b:	0f 1f 44 00 00       	nopl   0x0(%rax,%rax,1)

0000000000000020 <ker>:
  20:	55                   	push   %rbp
  21:	48 89 e5             	mov    %rsp,%rbp
  24:	48 b8 00 00 00 00 00 	movabs $0x4045000000000000,%rax
  2b:	00 45 40 
  2e:	5d                   	pop    %rbp
  2f:	c3                   	retq   

0000000000000030 <naive_dtoi>:
  30:	55                   	push   %rbp
  31:	48 89 e5             	mov    %rsp,%rbp
  34:	48 83 ec 18          	sub    $0x18,%rsp
  38:	b9 33 04 00 00       	mov    $0x433,%ecx
  3d:	48 89 7d e8          	mov    %rdi,-0x18(%rbp)
  41:	48 8b 45 e8          	mov    -0x18(%rbp),%rax
  45:	48 89 45 f0          	mov    %rax,-0x10(%rbp)
  49:	48 8b 45 f0          	mov    -0x10(%rbp),%rax
  4d:	48 89 45 f8          	mov    %rax,-0x8(%rbp)
  51:	48 b8 ff ff ff ff ff 	movabs $0xfffffffffffff,%rax
  58:	ff 0f 00 
  5b:	48 23 45 f8          	and    -0x8(%rbp),%rax
  5f:	48 ba 00 00 00 00 00 	movabs $0x10000000000000,%rdx
  66:	00 10 00 
  69:	48 09 d0             	or     %rdx,%rax
  6c:	48 8b 55 f8          	mov    -0x8(%rbp),%rdx
  70:	48 c1 ea 34          	shr    $0x34,%rdx
  74:	48 81 e2 ff 07 00 00 	and    $0x7ff,%rdx
  7b:	48 29 d1             	sub    %rdx,%rcx
  7e:	48 d3 e8             	shr    %cl,%rax
  81:	48 83 c4 18          	add    $0x18,%rsp
  85:	5d                   	pop    %rbp
  86:	c3                   	retq   
  87:	90                   	nop
  88:	90                   	nop
  89:	90                   	nop
  8a:	90                   	nop
  8b:	90                   	nop
  8c:	90                   	nop
  8d:	90                   	nop
  8e:	90                   	nop
  8f:	90                   	nop

0000000000000090 <foo>:
  90:	55                   	push   %rbp
  91:	48 89 e5             	mov    %rsp,%rbp
  94:	e8 87 ff ff ff       	callq  20 <ker>
  99:	48 89 c7             	mov    %rax,%rdi
  9c:	e8 8f ff ff ff       	callq  30 <naive_dtoi>
  a1:	5d                   	pop    %rbp
  a2:	c3                   	retq