Subject: Re: [PATCH] powerpc: booke: fix boot crash due to null hugepd

From: Laurentiu Tudor <[email-masked]>
Date: Fri, 17 Feb 2017 12:37:30 +0000

On 02/17/2017 02:18 PM, Aneesh Kumar K.V wrote:
> [email-masked] writes:
>
>> From: Laurentiu Tudor <[email-masked]>
>>
>> On 32-bit book-e machines, hugepd_ok() does not take
>> into account null hugepd values, causing this crash at boot:
>>
>> Unable to handle kernel paging request for data at address 0x80000000
>> Faulting instruction address: 0xc00182a8
>> Oops: Kernel access of bad area, sig: 11 [#1]
>> SMP NR_CPUS=24
>> CoreNet Generic
>> Modules linked in:
>> CPU: 1 PID: 1 Comm: swapper/0 Tainted: G        W       4.10.0-rc8-00016-g69b1f87 #11
>> task: e5050000 task.stack: e5058000
>> NIP: c00182a8 LR: c001829c CTR: 00007ffe
>> REGS: e5059c50 TRAP: 0300   Tainted: G        W        (4.10.0-rc8-00016-g69b1f87)
>> MSR: 00021002 <CE,ME>
>>    CR: 88428e82  XER: 00000000
>> DEAR: 80000000 ESR: 00000000
>> GPR00: c0107510 e5059d00 e5050000 80000000 bffffff1 e5059d0c e5059d08 00002017
>> GPR08: 00000000 00000000 00000000 00000000 28428e82 00000000 c00027d0 00000000
>> GPR16: 00000000 00000000 88a28e82 20000000 48422e82 00000000 88a28e84 dd004000
>> GPR24: e5059e38 00000000 00000000 bffffff1 dd004000 00000001 00029002 bffffff1
>> NIP [c00182a8] follow_huge_addr+0x38/0xf0
>> LR [c001829c] follow_huge_addr+0x2c/0xf0
>> Call Trace:
>> [e5059d00] [e5059d00] 0xe5059d00 (unreliable)
>> [e5059d20] [c0107510] follow_page_mask+0x40/0x3c0
>> [e5059d80] [c0107958] __get_user_pages+0xc8/0x420
>> [e5059de0] [c010817c] get_user_pages_remote+0x8c/0x230
>> [e5059e30] [c013f170] copy_strings+0x110/0x3a0
>> [e5059ea0] [c013f42c] copy_strings_kernel+0x2c/0x50
>> [e5059ec0] [c0141324] do_execveat_common+0x474/0x620
>> [e5059f10] [c01414fc] do_execve+0x2c/0x40
>> [e5059f20] [c0001f68] try_to_run_init_process+0x18/0x60
>> [e5059f30] [c000289c] kernel_init+0xcc/0x120
>> [e5059f40] [c000f1e8] ret_from_kernel_thread+0x5c/0x64
>> Instruction dump:
>> bfc10018 7c9f2378 90010024 7fc000a6 7c000146 80630020 38a1000c 38c10008
>> 4bfff869 2c030000 41c20090 81210008 <81430000> 81630004 3860ffea 2f890000
>> ---[ end trace 4bf94e15fd9fa824 ]---
>
>
> Which code path is that. That null should be filtered by the if
> (pmd_none(pmd)) check in find_linux_pte_or_hugepte right ?
>

I haven't characterized the issue in detail as i wanted to get the patch 
out ASAP.
I only noticed that the previous check, that is:

	"signed hpd_val > 0"

vs the new one, that is:

	"unsigned hpd_val & PD_HUGE == 0"

evaluate differently for a value of zero: old expression evaluates
to false and the new one to true.

---
Best Regards, Laurentiu