doubts regarding System Initialization working (SYSINIT)

[John Baldwin]

On Friday 23 January 2009 10:55:32 am Mehul Chadha wrote:
> Hello all,
>                 I have been browsing through the FreeBSD kernel's
> source code trying to understand its working .
> 
> In the mi_startup() in /sys/kern/init_main.c all the SYSINIT objects
> are sorted using bubble sort and then they are executed in order.
> 
> My doubt is that we have declared the pointer to the struct sysinit as
> const pointer to a const in the macro definition of SYSINIT ie  when
> the macro
> 
> SYSINIT(kmem, SI_SUB_KMEM, SI_ORDER_FIRST, kmeminit, NULL)  is
> expanded  completely we get the following
> 
> static struct sysinit kmem_sys_init = { SI_SUB_KMEM, SI_ORDER_FIRST,
> (sysinit_cfunc_t)(sysinit_
> nfunc_t)kmeminit, ((void *)(((void *)0))) }; static void const * const
> __set_sysinit_set_sym_kmem_sys_init __attribute__((__section__("set_"
> "sysinit_set"))) __attribute__((__used__)) = &kmem_sys_init;
> 
> Here we see that the pointer is of type const and to a const but when we 
sort
> and swap using
>                               *sipp=*xipp;
> 
> We are trying to change the address of const pointer to a new address
> in which case it should segfault but it works fine.
> 
> Why does it not segfault it seems I have not understood the concept
> behind using const *const... I will be very thankful if you can help
> me with it.

I'm guessing the startup code doesn't map the SYSINIT pages read only because 
it is not smart enough to honor that request perhaps.  That is, I wouldn't be 
surprised if all of .rodata in the kernel was mapped as R/W instead of R/O.

-- 
John Baldwin