Re: don't let mmap allocate down to zero

[Rik van Riel]

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).

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);
        }


--
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan
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