Subject: Re: powerpc/mm: Don't report PUDs as memory leaks when using kmemleak

From: Michael Ellerman <[email-masked]>
Date: Wed, 1 Aug 2018 15:24:13 +1000 (AEST)

On Thu, 2018-07-19 at 14:33:16 UTC, Michael Ellerman wrote:
> Paul Menzel reported that kmemleak was producing reports such as:
> 
>   unreferenced object 0xc0000000f8b80000 (size 16384):
>     comm "init", pid 1, jiffies 4294937416 (age 312.240s)
>     hex dump (first 32 bytes):
>       00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>       00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>     backtrace:
>       [<00000000d997deb7>] __pud_alloc+0x80/0x190
>       [<0000000087f2e8a3>] move_page_tables+0xbac/0xdc0
>       [<00000000091e51c2>] shift_arg_pages+0xc0/0x210
>       [<00000000ab88670c>] setup_arg_pages+0x22c/0x2a0
>       [<0000000060871529>] load_elf_binary+0x41c/0x1648
>       [<00000000ecd9d2d4>] search_binary_handler.part.11+0xbc/0x280
>       [<0000000034e0cdd7>] __do_execve_file.isra.13+0x73c/0x940
>       [<000000005f953a6e>] sys_execve+0x58/0x70
>       [<000000009700a858>] system_call+0x5c/0x70
> 
> Indicating that a PUD was being leaked.
> 
> However what's really happening is that kmemleak is not able to
> recognise the references from the PGD to the PUD, because they are not
> fully qualified pointers.
> 
> We can confirm that in xmon, eg:
> 
> Find the task struct for pid 1 "init":
>   0:mon> P
>        task_struct     ->thread.ksp    PID   PPID S  P CMD
>   c0000001fe7c0000 c0000001fe803960      1      0 S 13 systemd
> 
> Dump virtual address 0 to find the PGD:
>   0:mon> dv 0 c0000001fe7c0000
>   pgd  @ 0xc0000000f8b01000
> 
> Dump the memory of the PGD:
>   0:mon> d c0000000f8b01000
>   c0000000f8b01000 00000000f8b90000 0000000000000000  |................|
>   c0000000f8b01010 0000000000000000 0000000000000000  |................|
>   c0000000f8b01020 0000000000000000 0000000000000000  |................|
>   c0000000f8b01030 0000000000000000 00000000f8b80000  |................|
>                                     ^^^^^^^^^^^^^^^^
> 
> There we can see the reference to our supposedly leaked PUD. But
> because it's missing the leading 0xc, kmemleak won't recognise it.
> 
> We can confirm it's still in use by translating an address that is
> mapped via it:
>   0:mon> dv 7fff94000000 c0000001fe7c0000
>   pgd  @ 0xc0000000f8b01000
>   pgdp @ 0xc0000000f8b01038 = 0x00000000f8b80000 <--
>   pudp @ 0xc0000000f8b81ff8 = 0x00000000037c4000
>   pmdp @ 0xc0000000037c5ca0 = 0x00000000fbd89000
>   ptep @ 0xc0000000fbd89000 = 0xc0800001d5ce0386
>   Maps physical address = 0x00000001d5ce0000
>   Flags = Accessed Dirty Read Write
> 
> The fix is fairly simple. We need to tell kmemleak to ignore PUD
> allocations and never report them as leaks. We can also tell it not to
> scan the PGD, because it will never find pointers in there. However it
> will still notice if we allocate a PGD and then leak it.
> 
> Reported-by: Paul Menzel <[email-masked]>
> Signed-off-by: Michael Ellerman <[email-masked]>
> Tested-by: Paul Menzel <[email-masked]> on IBM S822LC

Applied to powerpc next.

https://git.kernel.org/powerpc/c/a984506c542e26b31cbb446438f843

cheers