Date: Fri, 23 Jan 2009 17:55:15 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-hackers@freebsd.org Cc: Mehul Chadha <mehulc87@gmail.com>, freebsd-ia32@freebsd.org Subject: Re: doubts regarding System Initialization working (SYSINIT) Message-ID: <200901231755.15548.jhb@freebsd.org> In-Reply-To: <251d650c0901230755n769de861u1fe2e76dd28bbde8@mail.gmail.com> References: <251d650c0901230755n769de861u1fe2e76dd28bbde8@mail.gmail.com>
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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
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