Messages in this thread | | | Date | Fri, 25 Feb 2022 00:27:19 +0200 | Subject | Re: False positive kmemleak report for dtb properties names on powerpc | From | Ariel Marcovitch <> |
| |
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! >
| |