Messages in this thread | | | Subject | Re: [RFC] [PATCH] Pre-emption control for userspace | From | Khalid Aziz <> | Date | Tue, 04 Mar 2014 12:01:19 -0700 |
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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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