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SubjectRe: [RFC] [PATCH] Pre-emption control for userspace
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On Tue, 2014-03-04 at 18:38 +0000, Al Viro wrote:
> On Tue, Mar 04, 2014 at 10:44:54AM -0700, Khalid Aziz wrote:
>
> > do_exit() unmaps mmap_state->uaddr, and frees up mmap_state->kaddr
> > and mmap_state. mmap_state should not be NULL after unmap. vfree()
> > and kfree() are tolerant of pointers that have already been freed.
>
> Huh? Double free() is a bug, plain and simple. Never do that - not
> in userland and especially not in the kernel. Think what happens if
> some code gets executed between those two and asks to allocate something.
> If it gets the area you'd just freed, your second free will leave it
> with all kinds of nasty surprises. Starting with "who the hell has
> started to modify the object I'd allocated and hadn't freed?"
>
> A: p = alloc();
> A: free(p);
> B: q = alloc(); /* q == p now */
> B: *q = 0; /* *q is zero */
> A: free(p); /* same as free(q) */
> C: r = alloc(); /* r == q now */
> C: *r = 1; /* *q is one */
> B: if (*q != 0) panic("somebody's buggering my memory");
>
> It's always a bug, whether the implementation catches it or not.

Agreed, you are right. I will fix it.

--
Khalid





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