I want to call printk from arch/x86/lib/copy_user_64.S. The only
example in assembly that I can find for calling something like
printk is from: arch/x86/kernel/head_64.S (code follows)
here is what I tried:
ENTRY(jj_print)
CFI_STARTPROC
ASM_STAC
leaq jj_buf(%rip),%rdi
mov $0, %rsi
call printk
xorl %eax,%eax
ASM_CLAC
ret
CFI_ENDPROC
ENDPROC(jj_print)
jj_buf:
.ascii "hello asm\n\0"
Then I added this to where I wanted to run the code:
ALTERNATIVE_JUMP X86_FEATURE_REP_GOOD,X86_FEATURE_ERMS, \
jj_print, jj_print, jj_print
I wasn't sure how to make the call to jj_print so I just used the
ALTERNATIVE_JUMP without trying to understand it, but from what I can
tell, it seems like it should work (I know it is not ideal and lame)
sample workig code which I based my jj_print func on:
#ifdef CONFIG_EARLY_PRINTK
GET_CR2_INTO(%r9) # can clobber any volatile register if pv
movl 80(%rsp),%r8d # error code
movl 72(%rsp),%esi # vector number
movl 96(%rsp),%edx # %cs
movq 88(%rsp),%rcx # %rip
xorl %eax,%eax
leaq early_idt_msg(%rip),%rdi
call early_printk
cmpl $2,early_recursion_flag(%rip)
jz 1f
call dump_stack
#ifdef CONFIG_KALLSYMS
leaq early_idt_ripmsg(%rip),%rdi
movq 40(%rsp),%rsi # %rip again
call __print_symbol
#endif
#endif /* EARLY_PRINTK */
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