On Sat, Feb 08, 2014 at 12:27:37PM +0200, Pekka Enberg wrote: > Hi Paul, > > On 01/02/2014 10:33 PM, Paul E. McKenney wrote: > > From what I can see, the Linux-kernel's SLAB, SLOB, and SLUB memory > >allocators would deal with the following sort of race: > > > >A. CPU 0: r1 = kmalloc(...); ACCESS_ONCE(gp) = r1; > > > > CPU 1: r2 = ACCESS_ONCE(gp); if (r2) kfree(r2); > > > >However, my guess is that this should be considered an accident of the > >current implementation rather than a feature. The reason for this is > >that I cannot see how you would usefully do (A) above without also allowing > >(B) and (C) below, both of which look to me to be quite destructive: > > > >B. CPU 0: r1 = kmalloc(...); ACCESS_ONCE(shared_x) = r1; > > > > CPU 1: r2 = ACCESS_ONCE(shared_x); if (r2) kfree(r2); > > > > CPU 2: r3 = ACCESS_ONCE(shared_x); if (r3) kfree(r3); > > > > This results in the memory being on two different freelists. > > > >C. CPU 0: r1 = kmalloc(...); ACCESS_ONCE(shared_x) = r1; > > > > CPU 1: r2 = ACCESS_ONCE(shared_x); r2->a = 1; r2->b = 2; > > > > CPU 2: r3 = ACCESS_ONCE(shared_x); if (r3) kfree(r3); > > > > CPU 3: r4 = kmalloc(...); r4->s = 3; r4->t = 4; > > > > This results in the memory being used by two different CPUs, > > each of which believe that they have sole access. > > > >But I thought I should ask the experts. > > > >So, am I correct that kernel hackers are required to avoid "drive-by" > >kfree()s of kmalloc()ed memory? > > So to be completely honest, I don't understand what is the race in > (A) that concerns the *memory allocator*. I also don't what the > memory allocator can do in (B) and (C) which look like double-free > and use-after-free, respectively, to me. :-) >From what I can see, (A) works by accident, but is kind of useless because you allocate and free the memory without touching it. (B) and (C) are the lightest touches I could imagine, and as you say, both are bad. So I believe that it is reasonable to prohibit (A). Or is there some use for (A) that I am missing? Thanx, Paul -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>