Messages in this thread | | | Subject | Re: [RFC 1/4] net/ipv4/fib: Remove run-time check in tnode_alloc() | From | Alexander Duyck <> | Date | Mon, 01 Apr 2019 10:50:48 -0700 |
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On Mon, 2019-04-01 at 16:55 +0100, Dmitry Safonov wrote: > Hi Alexander, > > On 4/1/19 4:40 PM, Alexander Duyck wrote: > > > @@ -333,8 +328,7 @@ static struct tnode *tnode_alloc(int bits) > > > { > > > size_t size; > > > > > > - /* verify bits is within bounds */ > > > - if (bits > TNODE_VMALLOC_MAX) > > > + if ((BITS_PER_LONG <= KEYLENGTH) && unlikely(bits >= BITS_PER_LONG)) > > > return NULL; > > > > > > /* determine size and verify it is non-zero and didn't overflow */ > > > > I think it would be better if we left TNODE_VMALLOC_MAX instead of > > replacing it with BITS_PER_LONG. This way we know that we are limited > > by the size of the node on 32b systems, and by the KEYLENGTH on 64b > > systems. The basic idea is to maintain the logic as to why we are doing > > it this way instead of just burying things by using built in constants > > that are close enough to work. > > > > So for example I believe TNODE_VMALLOC_MAX is 31 on a 32b system. > > This is also true after the change: bits == 31 will *not* return.
Actually now that I think about it TNODE_VMALLOC_MAX is likely much less than 31. The logic that we have to be concerned with is: size = TNODE_SIZE(1ul << bits);
If size is a 32b value, and the size of a pointer is 4 bytes, then our upper limit is roughly ilog2((4G - 28) / 4), which comes out to 29. What we are trying to avoid is overflowing the size variable, not actually limiting the vmalloc itself.
> > The > > main reason for that is because we have to subtract the TNODE_SIZE from > > the upper limit for size. By replacing TNODE_VMALLOC_MAX with > > BITS_PER_LONG that becomes abstracted away and it becomes more likely > > that somebody will mishandle it later. > > So, I wanted to remove run-time check here on x86_64.. > I could do it by adding !CONFIG_64BIT around the check.
I have no problem with that. All I am suggesting is that if at all possible we should use TNODE_VMALLOC_MAX instead of BITS_PER_LONG.
> But, I thought about the value of the check: > I believe it's here not to limit maximum allocated size: > kzalloc()/vzalloc() will fail and we should be fine with that.
No, the problem is we don't want to overflow size. The allocation will succeed, but give us a much smaller allocation then we expected.
> In my opinion it's rather to check that (1UL << bits) wouldn't result in UB.
Sort of, however we have to keep mind that 1ul << bits is an index so it is also increased by the size of a pointer. As such the logic might be better expressed as sizeof(void*) << bits.
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