[PATCH] XArray: fix kmemleak false positive in xas_shrink()

[Jared Kangas <jkangas@xxxxxxxxxx>]

Kmemleak periodically produces a false positive report that resembles
the following:

unreferenced object 0xffff0000c105ed08 (size 576):
  comm "swapper/0", pid 1, jiffies 4294937478
  hex dump (first 32 bytes):
    00 00 03 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    d8 e7 0a 8b 00 80 ff ff 20 ed 05 c1 00 00 ff ff  ........ .......
  backtrace (crc 69e99671):
    kmemleak_alloc+0xb4/0xc4
    kmem_cache_alloc_lru+0x1f0/0x244
    xas_alloc+0x2a0/0x3a0
    xas_expand.constprop.0+0x144/0x4dc
    xas_create+0x2b0/0x484
    xas_store+0x60/0xa00
    __xa_alloc+0x194/0x280
    __xa_alloc_cyclic+0x104/0x2e0
    dev_index_reserve+0xd8/0x18c
    register_netdevice+0x5e8/0xf90
    register_netdev+0x28/0x50
    loopback_net_init+0x68/0x114
    ops_init+0x90/0x2c0
    register_pernet_operations+0x20c/0x554
    register_pernet_device+0x3c/0x8c
    net_dev_init+0x5cc/0x7d8

This transient leak can be traced to xas_shrink(): when the xarray's
head is reassigned, kmemleak may have already started scanning the
xarray. When this happens, if kmemleak fails to scan the new xa_head
before it moves, kmemleak will see it as a leak until the xarray is
scanned again.

The report can be reproduced by running the xdp_bonding BPF selftest,
although it doesn't appear consistently due to the bug's transience.
In my testing, the following script has reliably triggered the report in
under an hour on a debug kernel with kmemleak enabled, where KSELFTESTS
is set to the install path for the kernel selftests:

        #!/bin/sh
        set -eu

        echo 1 >/sys/module/kmemleak/parameters/verbose
        echo scan=1 >/sys/kernel/debug/kmemleak

        while :; do
                $KSELFTESTS/bpf/test_progs -t xdp_bonding
        done

To prevent this false positive report, mark the new xa_head in
xas_shrink() as a transient leak.

Signed-off-by: Jared Kangas <jkangas@xxxxxxxxxx>
---
 lib/xarray.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/xarray.c b/lib/xarray.c
index 9644b18af18d1..51314fa157b31 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -8,6 +8,7 @@
 
 #include <linux/bitmap.h>
 #include <linux/export.h>
+#include <linux/kmemleak.h>
 #include <linux/list.h>
 #include <linux/slab.h>
 #include <linux/xarray.h>
@@ -476,6 +477,7 @@ static void xas_shrink(struct xa_state *xas)
 			break;
 		node = xa_to_node(entry);
 		node->parent = NULL;
+		kmemleak_transient_leak(node);
 	}
 }
 
-- 
2.49.0