Hi, Kosaki-san
> Which version are you looking at? Current code seems to don't have
> sys_mmap().
This sys_mmap() means the entrance of mmap system call for x86_64.
----------------------------------------------------------------------
arch/x86/kernel/sys_x86_64.c:
84 SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
85 unsigned long, prot, unsigned long, flags,
86 unsigned long, fd, unsigned long, off)
87 {
88 long error;
89 error = -EINVAL;
90 if (off & ~PAGE_MASK)
91 goto out;
92
93 error = sys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
94 out:
95 return error;
96 }
----------------------------------------------------------------------
This function calls sys_mmap_pgoff, which has the argument
"off >> PAGE_SHIFT". It means that sys_mmap_pgoff does not use off,
which is the argument of sys_mmap, with no change, but uses the value
obtained after off is shifted right by PAGE_SHIFT bits.
In mmap system call for x86, the following sys_mmap_pgoff is the
entrance in kernel.
----------------------------------------------------------------------
arch/x86/kernel/syscall_table_32.S:
...
194 .long sys_mmap_pgoff
...
mm/mmap.c:
1080 SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
1081 unsigned long, prot, unsigned long, flags,
1082 unsigned long, fd, unsigned long, pgoff)
...
1111 down_write(¤t->mm->mmap_sem);
1112 retval = do_mmap_pgoff(file, addr, len, prot, flags, pgoff);
1113 up_write(¤t->mm->mmap_sem);
----------------------------------------------------------------------
> value. We have
> no reason to make artificial limit. Why don't you meke a overflow
> check in sys_mmap()?
I consider it is better to make an overflow check in do_mmap_pgoff.
There are two reasons:
1. If we make an overflow check in the entrance of system call, we
have to check in sys_mmap for x86_64 and in sys_mmap_pgoff for
x86. It means that we have to check for each architecture
individually. Therefore, it is more effective to make an
overflow check in do_mmap_pgoff because both sys_mmap and
sys_mmap_pgoff call do_mmap_pgoff.
2. Because the argument "offset" of sys_mmap is a multiple
of the page size(otherwise, EINVAL is returned.), no information
is lost after shifting right by PAGE_SHIFT bits. Therefore
to make an overflow check in do_mmap_pgoff is equivalent
to check in sys_mmap.
Best Regards,
Naotaka Hamaguchi
(2011/12/23 2:41), KOSAKI Motohiro wrote:
>> The argument "offset" is shifted right by PAGE_SHIFT bits
>> in sys_mmap(mmap systemcall).
>>
>> ------------------------------------------------------------------------
>> sys_mmap(unsigned long addr, unsigned long len,
>> unsigned long prot, unsigned long flags,
>> unsigned long fd, unsigned long off)
>> {
>> error = sys_mmap_pgoff(addr, len, prot, flags, fd, off>> PAGE_SHIFT);
>> }
>> ------------------------------------------------------------------------
>
> Hm.
> Which version are you looking at? Current code seems to don't have
> sys_mmap().
>
>
>
>
>
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