malloc的起始地址在LINUX和FREEBSD中的不同

Coolq搞懂了昨天讨论的FREEBSD bss结束地址问题，我来总结一下。
LINUX和FREEBSD在BSS结束地址（也可以说是malloc分配的起始地址）处理方面的不同：

LINUX:
malloc分配的起始地址是通过sys_brk(0)从内核获取，见代码

asmlinkage unsigned long sys_brk(unsigned long brk)

 {

         unsigned long rlim, retval;

         unsigned long newbrk, oldbrk;

         struct mm_struct *mm = current->mm;



         down_write(&mm->mmap_sem);



         if (brk <>end_code)     <------------- 这里       

                 goto out;               <------------- 这里   

         newbrk = PAGE_ALIGN(brk);

         oldbrk = PAGE_ALIGN(mm->brk);

         if (oldbrk == newbrk)

                 goto set_brk;



         /* Always allow shrinking brk. */

         if (brk <= mm->brk) {

                 if (!do_munmap(mm, newbrk, oldbrk-newbrk))

                         goto set_brk;

                 goto out;

         }



         /* Check against rlimit.. */

         rlim = current->signal->rlim[RLIMIT_DATA].rlim_cur;

         if (rlim < RLIM_INFINITY && brk - mm->start_data > rlim)

                 goto out;



         /* Check against existing mmap mappings. */

         if (find_vma_intersection(mm, oldbrk, newbrk+PAGE_SIZE))

                 goto out;



         /* Ok, looks good - let it rip. */

         if (do_brk(oldbrk, newbrk-oldbrk) != oldbrk)

                 goto out;

 set_brk:

         mm->brk = brk;

 out:                       <------------- 这里   

         retval = mm->brk;  <------------- 这里

         up_write(&mm->mmap_sem);

         return retval;

 }

FREEBSD:
malloc分配的起始地址是硬编码在binary中，见代码：

#if defined(SYSLIBC_SCCS) && !defined(lint)
.asciz "@(#)brk.s 5.2 (Berkeley) 12/17/90"
#endif /* SYSLIBC_SCCS and not lint */
#include
__FBSDID("$FreeBSD: src/lib/libc/i386/sys/brk.S,v 1.10 2002/03/23 02:10:28 obrien Exp $");

#include "SYS.h"

.globl HIDENAME(curbrk)
.globl HIDENAME(minbrk)
ENTRY(_brk)
jmp ok

ENTRY(brk)
#ifdef PIC
movl 4(%esp),%eax
PIC_PROLOGUE
movl PIC_GOT(HIDENAME(curbrk)),%edx # set up GOT addressing <------------- 这里
movl PIC_GOT(HIDENAME(minbrk)),%ecx # PIC_EPILOGUE <------------- 这里
cmpl %eax,(%ecx)
jbe ok
movl (%ecx),%eax
movl %eax,4(%esp)
ok:
mov $SYS_break,%eax <------------- 这里
KERNCALL
jb err
movl 4(%esp),%eax
movl %eax,(%edx)
movl $0,%eax
ret
err:
PIC_PROLOGUE
jmp PIC_PLT(HIDENAME(cerror))

#else

movl 4(%esp),%eax
cmpl %eax,HIDENAME(minbrk)
jbe ok
movl HIDENAME(minbrk),%eax
movl %eax,4(%esp)
ok:
mov $SYS_break,%eax
KERNCALL
jb err
movl 4(%esp),%eax
movl %eax,HIDENAME(curbrk)
movl $0,%eax
ret
err:
jmp HIDENAME(cerror)
#endif

因此，要对堆内存分配的起始地址进行随机，在LINUX只需在内核修改brk的初始值，而在FREEBSD中则必需在用户层做处理（可以改libc或ld）。