Re: [PATCH v4 00/10] klp-convert livepatch build tooling

Joe Lawrence
2019-06-14T14:20:24Z

On 6/14/19 4:34 AM, Petr Mladek wrote:
> On Thu 2019-06-13 16:48:02, Joe Lawrence wrote:
>> On 6/13/19 9:15 AM, Joe Lawrence wrote:
>>> On 6/13/19 9:00 AM, Miroslav Benes wrote:
>>>> Hi Joe,
>>>>
>>>> first, I'm sorry for the lack of response so far.
>>>>
>>>> Maybe you've already noticed but the selftests fail. Well, at least in
>>>> my VM. When test_klp_convert1.ko is loaded, the process is killed with
>>>>
>>>> [ 518.041826] BUG: kernel NULL pointer dereference, address: 0000000000000000
>>>> [ 518.042816] #PF: supervisor read access in kernel mode
>>>> [ 518.043393] #PF: error_code(0x0000) - not-present page
>>>> [ 518.043981] PGD 0 P4D 0
>>>> [ 518.044185] Oops: 0000 [#1] SMP PTI
>>>> [ 518.044518] CPU: 2 PID: 2255 Comm: insmod Tainted: G O K 5.1.0-klp_convert_v4-193435-g67748576637e #2
>>>> [ 518.045784] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58-prebuilt.qemu.org 04/01/2014
>>>> [ 518.046940] RIP: 0010:test_klp_convert_init+0x1c/0x40 [test_klp_convert1]
>>>> [ 518.047611] Code: 1b a0 48 89 c6 e9 a8 c0 f4 e0 0f 1f 40 00 0f 1f 44 00 00 53 48 c7 c7 00 30 1b a0 e8 5e 33 f6 e0 85 c0 89 c3 74 04 89 d8 5b c3 <48> 8b 35 5d ef e4 5f 48 c7 c7 28 20 1b a0 e8 75 c0 f4 e0 e8 6c ff
>>>> [ 518.049779] RSP: 0018:ffffc90000f37cc8 EFLAGS: 00010246
>>>> [ 518.050243] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000027de0
>>>> [ 518.050922] RDX: 0000000000000000 RSI: 0000000000000006 RDI: ffff88807ab54f40
>>>> [ 518.051619] RBP: ffffffffa01b1080 R08: 0000000096efde7a R09: 0000000000000001
>>>> [ 518.052332] R10: 0000000000000000 R11: 0000000000000000 R12: 00000000ffffffff
>>>> [ 518.053012] R13: 0000000000000000 R14: ffff888078b55000 R15: ffffc90000f37ea0
>>>> [ 518.053714] FS: 00007febece1fb80(0000) GS:ffff88807d400000(0000) knlGS:0000000000000000
>>>> [ 518.054514] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>> [ 518.055078] CR2: 0000000000000000 CR3: 000000007a56a000 CR4: 00000000000006e0
>>>> [ 518.055818] Call Trace:
>>>> [ 518.056007] do_one_initcall+0x6a/0x2da
>>>> [ 518.056340] ? do_init_module+0x22/0x230
>>>> [ 518.056702] ? rcu_read_lock_sched_held+0x96/0xa0
>>>> [ 518.057125] ? kmem_cache_alloc_trace+0x284/0x2e0
>>>> [ 518.057493] do_init_module+0x5a/0x230
>>>> [ 518.057900] load_module+0x17bc/0x1f50
>>>> [ 518.058214] ? __symbol_put+0x40/0x40
>>>> [ 518.058499] ? vfs_read+0x12d/0x160
>>>> [ 518.058766] __do_sys_finit_module+0x83/0xc0
>>>> [ 518.059122] do_syscall_64+0x57/0x190
>>>> [ 518.059407] entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>>> ...
>>>>
>>>> It crashes right in test_klp_convert_init() when print_*() using
>>>> supposed-to-be-converted symbols are called. I'll debug it next week. Can
>>>> you reproduce it too?
>>>
>>> Hey, thanks for the report..
>>>
>>> I don't recall the tests crashing, but I had put this patchset on the
>>> side for a few weeks now. I'll try to fire up a VM and see what happens
>>> today.
>>>
>>
>> Hmm, I haven't been able to reproduce using my original base (Linux 5.1-rc6)
>> or when rebased ontop of livepatching.git/master + 997a55f3fb6d("stacktrace: Unbreak stack_trace_save_tsk_reliable()")
>
> I stared into the code a bit but I did not find any bug. Let's hope
> that it was just some pre-vacation last minute mistake (system
> inconsistency or so ;-)
>
> Anyway, I am curious about one thing. I saw:
>
> function __load_mod() {
> local mod="$1"; shift
>
> local msg="% modprobe $mod $*"
> log "${msg%% }"
> ret=$(modprobe "$mod" "$@" 2>&1)
> if [[ "$ret" != "" ]]; then
> die "$ret"
> fi
>
> # Wait for module in sysfs ...
> loop_until '[[ -e "/sys/module/$mod" ]]' ||
> die "failed to load module $mod"
> }
>
> Is the waiting for sysfs really necessary here?
>
> Note that it is /sys/module and not /sys/kernel/livepatch/.

I can't remember if that was just paranoid-protective-bash coding or
actually required. Libor provided great feedback on the initial patch
series that introduced the self-tests, perhaps he remembers.

> My understanding is that modprobe waits until the module succesfully
> loaded. mod_sysfs_setup() is called before the module init callback.
> Therefore the sysfs interface should be read before modprobe returns.
> Do I miss something?
>
> If it works different way then there might be some races because
> mod_sysfs_setup() is called before the module is alive.

All of this is called from a single bash script function, so in a call
stack fashion, something like this would occur when loading a livepatch
module:

[ mod_sysfs_setup() ]
modprobe waits for: .init complete, MODULE_STATE_LIVE
__load_mod() waits for: /sys/module/$mod
load_lp_nowait() waits for: /sys/kernel/livepatch/$mod
load_lp() waits for: /sys/kernel/livepatch/$mod/transition = 0
test-script.sh

So I would think that by calling modprobe, we ensure that the module
code is ready to go. The /sys/module/$mod check might be redundant as
you say, but because modprobe completed, we should be safe, no?

The only "nowait" function we have is load_lp_nowait(), which would let
us march onward before the livepatch transition may have completed.

-- Joe