Re: [PATCH 1/1] mmap: fix uninitialized entities warnings.

[Matthew Wilcox <matthew@wil.cx>]

On Thu, May 01, 2008 at 10:43:25PM +0300, Adrian Bunk wrote:
> > > -	struct vm_area_struct *vma, *prev;
> > > +	struct vm_area_struct *vma, *prev = NULL;
> > 
> > gcc bug -- it's failing to notice that find_vma_prepare() will always
> > initialise pprev.
> >...
> 
> You miss the "return" inside the while() loop in find_vma_prepare().
> 
> I do not know whether the code is correct, but the answer is not that 
> easy.

You're right, it isn't that easy.

However, the code is not buggy.  Just compare:

                        vma = vma_tmp;
                        if (vma_tmp->vm_start <= addr)
                                return vma;

with

munmap_back:
        vma = find_vma_prepare(mm, addr, &prev, &rb_link, &rb_parent);
        if (vma && vma->vm_start < addr + len) {
                if (do_munmap(mm, addr, len))
                        return -ENOMEM;
                goto munmap_back;
        }

Now, we know that addr + len does not wrap, and that len > 0, so we know
that this warning is incorrect.  But it's not reasonable to expect gcc to
be able to deduce this.

See also the CERT warning threads recently on the GCC lists ;-)

-- 
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