Re: [PATCH bpf 3/7] bpf: Free dynamically allocated bits in bpf_iter_bits_destroy()

[Yafang Shao <laoar.shao@xxxxxxxxx>]

On Wed, Oct 9, 2024 at 2:26 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Tue, Oct 8, 2024 at 2:05 AM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote:
> >
> > From: Hou Tao <houtao1@xxxxxxxxxx>
> >
> > bpf_iter_bits_destroy() uses "kit->nr_bits <= 64" to check whether the
> > bits are dynamically allocated. However, the check is incorrect and may
> > cause a kmemleak as shown below:
> >
> > unreferenced object 0xffff88812628c8c0 (size 32):
> >   comm "swapper/0", pid 1, jiffies 4294727320
> >   hex dump (first 32 bytes):
> >     b0 c1 55 f5 81 88 ff ff f0 f0 f0 f0 f0 f0 f0 f0  ..U.............
> >     f0 f0 f0 f0 f0 f0 f0 f0 00 00 00 00 00 00 00 00  ................
> >   backtrace (crc 781e32cc):
> >     [<00000000c452b4ab>] kmemleak_alloc+0x4b/0x80
> >     [<0000000004e09f80>] __kmalloc_node_noprof+0x480/0x5c0
> >     [<00000000597124d6>] __alloc.isra.0+0x89/0xb0
> >     [<000000004ebfffcd>] alloc_bulk+0x2af/0x720
> >     [<00000000d9c10145>] prefill_mem_cache+0x7f/0xb0
> >     [<00000000ff9738ff>] bpf_mem_alloc_init+0x3e2/0x610
> >     [<000000008b616eac>] bpf_global_ma_init+0x19/0x30
> >     [<00000000fc473efc>] do_one_initcall+0xd3/0x3c0
> >     [<00000000ec81498c>] kernel_init_freeable+0x66a/0x940
> >     [<00000000b119f72f>] kernel_init+0x20/0x160
> >     [<00000000f11ac9a7>] ret_from_fork+0x3c/0x70
> >     [<0000000004671da4>] ret_from_fork_asm+0x1a/0x30
> >
> > That is because nr_bits will be set as zero in bpf_iter_bits_next()
> > after all bits have been iterated.
> >
>
> so maybe don't touch nr_bits and just use `kit->bit >= kit->nr_bits`
> condition to know when iterator is done?

No, we can't do that. The iterator may only process a few bits, which
would result in `kit->bit < kit->nr_bits`. Wouldn't it be better to
simply remove the line `kit->nr_bits = 0;`?

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 1a43d06eab28..7fcd3163cf68 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2939,10 +2939,8 @@ __bpf_kfunc int *bpf_iter_bits_next(struct
bpf_iter_bits *it)

        bits = nr_bits == 64 ? &kit->bits_copy : kit->bits;
        bit = find_next_bit(bits, nr_bits, kit->bit + 1);
-       if (bit >= nr_bits) {
-               kit->nr_bits = 0;
+       if (bit >= nr_bits)
                return NULL;
-       }

        kit->bit = bit;
        return &kit->bit;

--
Regards

Yafang