Re: don't let mmap allocate down to zero

[William Lee Irwin III <wli@holomorphy.com>]

On Thu, 27 Jan 2005, William Lee Irwin III wrote:
>> The intention was to disallow vmas starting at 0 categorically. i.e. it
>> is very intentional to deny the MREMAP_FIXED to 0 case of mremap().
>> It was also the intention to deny the MAP_FIXED to 0 case of mmap(),
>> though I didn't actually sweep that much (if at all).

On Thu, Jan 27, 2005 at 04:28:19PM -0500, Rik van Riel wrote:
> We can't do that, look at line 944 of fs/binfmt_elf.c:
>         if (current->personality & MMAP_PAGE_ZERO) {
>                 /* Why this, you ask???  Well SVr4 maps page 0 as read-only,
>                    and some applications "depend" upon this behavior.
>                    Since we do not have the power to recompile these, we
>                    emulate the SVr4 behavior.  Sigh.  */
>                 down_write(&current->mm->mmap_sem);
>                 error = do_mmap(NULL, 0, PAGE_SIZE, PROT_READ | PROT_EXEC,
>                                 MAP_FIXED | MAP_PRIVATE, 0);
>                 up_write(&current->mm->mmap_sem);
>         }

You seem to be on about something else, e.g. only forbidding the vma
allocator to return a vma starting at 0 when not specifically requested.
In that case vma->vm_start < mm->brk and similar are all fine.


-- wli