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Date:   Sat, 30 Oct 2021 02:06:40 +0200
From:   "Erhard F." <erhard_f@mailbox.org>
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Cc:     devicetree@vger.kernel.org, erhard_f@mailbox.org,
        linux-kernel@vger.kernel.org, robh+dt@kernel.org
Subject: Re: Fwd: [Bug 214867] New: UBSAN: shift-out-of-bounds in
 drivers/of/unittest.c:1933:36
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(In reply to Arnd Bergmann from comment #2)
> My guess is that 'id' is negative here, which means it fails to tigger the
> WARN_ON() but ends up still being out of range.
> 
> Can you try changing it to 'unsigned int id'?
When I change it to static void of_unittest_untrack_overlay(unsigned int id) I get no UBSAN message but this warning:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 1 at drivers/of/unittest.c:1931 .of_unittest_untrack_overlay+0x20/0xac
Modules linked in:
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.15.0-rc7-TalosII+ #2
NIP:  c000000000ad9368 LR: c000000002099000 CTR: 0000000000000000
REGS: c000000004163700 TRAP: 0700   Not tainted  (5.15.0-rc7-TalosII+)
MSR:  9000000000029032 <SF,HV,EE,ME,IR,DR,RI>  CR: 44004242  XER: 20040000
CFAR: c000000002098ffc IRQMASK: 0 
GPR00: c000000002098f74 c0000000041639a0 c0000000023b2f00 ffffffffffffffff 
GPR04: c000000002270b90 0000000000000000 c000000004163794 c0000000027744d8 
GPR08: 00000007fdaf2000 0000000000000001 0000000000000000 00000000ffffffff 
GPR12: 0000000024004242 c0000000031d5000 b086ed6156fceb64 c0000000022718d8 
GPR16: d5cb61e74edefbbf 0000000000000000 c0000000010840b0 c000000001084078 
GPR20: c000000000e0df58 c0000000021d6120 0000000000000000 c000200004d579f0 
GPR24: c0000000020ad0a0 c0000000022713c8 c0000000031c9110 0000000000000000 
GPR28: c000000000e0d110 c0000000031c9158 7bba9c880526c811 c0000000041639a0 
NIP [c000000000ad9368] .of_unittest_untrack_overlay+0x20/0xac
LR [c000000002099000] .of_unittest+0x4c5c/0x5a04
Call Trace:
[c0000000041639a0] [c000000002098f74] .of_unittest+0x4bd0/0x5a04 (unreliable)
[c000000004163b60] [c000000000011b5c] .do_one_initcall+0x7c/0x4f0
[c000000004163c50] [c00000000200300c] .kernel_init_freeable+0x704/0x858
[c000000004163d90] [c000000000012730] .kernel_init+0x20/0x190
[c000000004163e10] [c00000000000ce78] .ret_from_kernel_thread+0x58/0x60
Instruction dump:
60000000 60000000 38600000 4e800020 3d22ffec e929e4ca 2c090000 4d800020 
7c691850 39200001 28030100 7d20481e <0b090000> 7c0802a6 fb81ffe0 fba1ffe8 
irq event stamp: 514764
hardirqs last  enabled at (514763): [<c0000000004398e4>] .__slab_free+0x394/0x590
hardirqs last disabled at (514764): [<c0000000000312bc>] .interrupt_enter_prepare.constprop.0+0xec/0x150
softirqs last  enabled at (509346): [<c000000000d845cc>] .__do_softirq+0x4cc/0x714
softirqs last disabled at (509339): [<c0000000000f6eb8>] .__irq_exit_rcu+0x148/0x1b0
---[ end trace 0c8618d488a1a13d ]---


In unittest.c id generally seems to be derived from a constant and counted downwards in loops.

[...]
#define MAX_UNITTEST_OVERLAYS   256
[...]
static void of_unittest_destroy_tracked_overlays(void)
{
        int id, ret, defers, ovcs_id;

        if (overlay_first_id < 0)
                return;

        /* try until no defers */
        do {
                defers = 0;
                /* remove in reverse order */
                for (id = MAX_UNITTEST_OVERLAYS - 1; id >= 0; id--) {
                        if (!of_unittest_overlay_tracked(id))
                                continue;

                        ovcs_id = id + overlay_first_id;
                        ret = of_overlay_remove(&ovcs_id);
                        if (ret == -ENODEV) {
                                pr_warn("%s: no overlay to destroy for #%d\n",
                                        __func__, id + overlay_first_id);
                                continue;
                        }
                        if (ret != 0) {
                                defers++;
                                pr_warn("%s: overlay destroy failed for #%d\n",
                                        __func__, id + overlay_first_id);
                                continue;
                        }

                        of_unittest_untrack_overlay(id);
                }
        } while (defers > 0);
}

It should not get negative with the "id >= 0" above in the for-loop. But from it's purpose it should be an unsigned int probably. It would need to be changed in the whole unittest.c though as it is used as int in several other places, e.g. 
static long of_unittest_overlay_tracked(int id)
static void of_unittest_track_overlay(int id)