Re: False positive kmemleak report for dtb properties names on powerpc

[Ariel Marcovitch]

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!