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Date:   Fri, 18 Mar 2022 21:44:38 +0200
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Subject: Re: False positive kmemleak report for dtb properties names on
 powerpc
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From:   Ariel Marcovitch <arielmarcovitch@gmail.com>
To:     catalin.marinas@arm.com, akpm@linux-foundation.org,
        mpe@ellerman.id.au, benh@kernel.crashing.org, paulus@samba.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org
References: <1e976df3-2925-f6c6-6723-67f127b9e544@gmail.com>
Cc:     christophe.leroy@csgroup.eu
In-Reply-To: <1e976df3-2925-f6c6-6723-67f127b9e544@gmail.com>
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Pinging again :)

On 25/02/2022 0:27, Ariel Marcovitch wrote:
> Ping :)
>
> On 18/02/2022 21:45, Ariel Marcovitch wrote:
>> Hello!
>>
>> I was running a powerpc 32bit kernel (built using 
>> qemu_ppc_mpc8544ds_defconfig
>> buildroot config, with enabling DEBUGFS+KMEMLEAK+HIGHMEM in the 
>> kernel config)
>> on qemu and invoked the kmemleak scan (twice. for some reason the 
>> first time wasn't enough).
>>
>> (Actually the problem will probably reproduce on every ppc kernel with
>> HIGHMEM enabled, but I only checked this config)
>>
>> I got 97 leak reports, all similar to the following:
>>
>> ```
>>
>> unreferenced object 0xc1803840 (size 16):
>>   comm "swapper", pid 1, jiffies 4294892303 (age 39.320s)
>>   hex dump (first 16 bytes):
>>     64 65 76 69 63 65 5f 74 79 70 65 00 00 00 00 00 device_type.....
>>   backtrace:
>>     [<(ptrval)>] kstrdup+0x40/0x98
>>     [<(ptrval)>] __of_add_property_sysfs+0xa4/0x10c
>>     [<(ptrval)>] __of_attach_node_sysfs+0xc0/0x110
>>     [<(ptrval)>] of_core_init+0xa8/0x15c
>>     [<(ptrval)>] driver_init+0x24/0x3c
>>     [<(ptrval)>] kernel_init_freeable+0xb8/0x23c
>>     [<(ptrval)>] kernel_init+0x24/0x14c
>>     [<(ptrval)>] ret_from_kernel_thread+0x5c/0x64
>> ```
>>
>> The objects in the reports are the names of the sysfs files created 
>> for the dtb
>> nodes and properties.
>>
>> These are definitely not leaked, as they are even visible to the user 
>> as the sysfs file names.
>>
>> These strings (for dtb properties, in the case of the shown report, 
>> but the case with dtb nodes is very similar) are created in 
>> __of_add_property_sysfs() and the pointer to them is stored in 
>> pp->attr.attr.name (so, actually stored in the memory pointed by pp)
>>
>> pp is one of the dtb property objects which are allocated in 
>> early_init_dt_alloc_memory_arch() in of/fdt.c using memblock_alloc. 
>> This happens very early, in setup_arch()->unflatten_device_tree().
>>
>> memblock_alloc lets kmemleak know about the allocated memory using 
>> kmemleak_alloc_phys (in mm/memblock.c:memblock_alloc_range_nid()).
>>
>> The problem is with the following code (mm/kmemleak.c):
>>
>> ```c
>>
>> void __ref kmemleak_alloc_phys(phys_addr_t phys, size_t size, int 
>> min_count,
>>                                gfp_t gfp)
>> {
>>         if (!IS_ENABLED(CONFIG_HIGHMEM) || PHYS_PFN(phys) < max_low_pfn)
>>                 kmemleak_alloc(__va(phys), size, min_count, gfp);
>> }
>>
>> ```
>>
>> When CONFIG_HIGHMEM is enabled, the pfn of the allocated memory is 
>> checked against max_low_pfn, to make sure it is not in the HIGHMEM zone.
>>
>> However, when called through unflatten_device_tree(), max_low_pfn is 
>> not yet initialized in powerpc.
>>
>> max_low_pfn is initialized (when NUMA is disabled) in 
>> arch/powerpc/mm/mem.c:mem_topology_setup() which is called only after 
>> unflatten_device_tree() is called in the same function (setup_arch()).
>>
>> Because max_low_pfn is global it is 0 before initialization, so as 
>> far as kmemleak_alloc_phys() is concerned, every memory is HIGHMEM (: 
>> and the allocated memory is not tracked by kmemleak, causing 
>> references to objects allocated later with kmalloc() to be ignored 
>> and these objects are marked as leaked.
>>
>> I actually tried to find out whether this happen on other arches as 
>> well, and it seems like arm64 also have this problem when dtb is used 
>> instead of acpi, although I haven't had the chance to confirm this.
>>
>> I don't suppose I can just shuffle the calls in setup_arch() around, 
>> so I wanted to hear your opinions first
>>
>> Thanks!
>>