Re: [PATCH] pwm: core: Zero-initialize the temp state

[Munehisa Kamata]

On Sun, 2023-02-26 09:17:52 +0000, Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
>
> Hello,
> 
> On Sat, Feb 25, 2023 at 05:37:21PM -0800, Munehisa Kamata wrote:
> > Zero-initialize the on-stack structure to avoid unexpected behaviors. Some
> > drivers may not set or initialize all the values in pwm_state through their
> > .get_state() callback and therefore some random values may remain there and
> > be set into pwm->state eventually.
> > 
> > This actually caused regression on ODROID-N2+ as reported in [1]; kernel
> > fails to boot due to random panic or hang-up.
> > 
> > [1] https://forum.odroid.com/viewtopic.php?f=177&t=46360
> 
> Looking through the report I wonder what actually made the machine fail
> to boot. Doesn't this paper over a problem that should be fixed (also)
> somewhere else?

Yes, you're right and I think the commit message could have described more
details. This patch is for ensuring all drivers see zeroed state same as
before, but I still don't fully understand how it ends up such random-ish
crashes. There could be another or bigger problem that should be fixed.

> Which driver is the one that the problem occur for?

It's pwm-meson and seems to be caused by random polarity value somehow. If
meson_pwm_get_state() sets polarity to zero instead, I don't see the
problem. According the comment, looks like the driver does not set polarity
by design.

 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pwm/pwm-meson.c?h=v6.2&id=c9c3395d5e3dcc6daee66c6908354d47bf98cb0c#n9

Before commit c73a3107624d, the memory was kcalloc'ed and always zeroed,
but I don't know if the driver was (is) assuming that. I'm adding Meson SoC
people to CC.

Apart from how polarity should be handled in the driver, I'm very puzzled
by the crashes I've observed so far. There seems to be some patterns, but
they don't seem obviously related to PWM.

[    1.360542] soc soc0: Amlogic Meson G12B (S922X) Revision 29:c (40:2) Detected
[    1.363906] Insufficient stack space to handle exception!
[    1.363913] ESR: 0x0000000096000047 -- DABT (current EL)
[    1.363917] FAR: 0xffff800009a47ff0
[    1.363920] Task stack:     [0xffff800009a48000..0xffff800009a4c000]
[    1.363923] IRQ stack:      [0xffff8000099a8000..0xffff8000099ac000]
[    1.363927] Overflow stack: [0xffff000077b76100..0xffff000077b77100]
[    1.363931] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 6.2.0-odroid-arm64 #47
[    1.363938] Hardware name: Hardkernel ODROID-N2Plus (DT)
[    1.363941] pstate: 400000c5 (nZcv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[    1.363947] pc : __do_kernel_fault+0x4/0x180
[    1.363961] lr : do_page_fault+0xd0/0x3d0
[    1.363968] sp : ffff800009a48020
[    1.363970] x29: ffff800009a48020 x28: ffff000002948000 x27: 0000000000000000
[    1.363980] x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000
[    1.363987] x23: 0000000096000004 x22: ffff800009a48110 x21: 00000000ffffffff
[    1.363994] x20: ffff800009a48110 x19: 00000000ffffffff x18: 000000000000001c
[    1.364001] x17: 00000000863047f6 x16: ffff800009860a80 x15: 0000000000000003
[    1.364008] x14: 00000000000003ef x13: 0000000000000000 x12: 0000000000000279
[    1.364015] x11: 0000000000000001 x10: 0000000000000001 x9 : 0000000000000400
[    1.364021] x8 : ffff000077b7fc40 x7 : ffff000077b7fbc0 x6 : ffff8000094f7990
[    1.364028] x5 : 0000000000000000 x4 : ffff000002948000 x3 : 0001000000000000
[    1.364035] x2 : ffff800009a48110 x1 : 0000000096000004 x0 : 00000000ffffffff
[    1.364043] Kernel panic - not syncing: kernel stack overflow
[    1.364047] SMP: stopping secondary CPUs

Another example:

[    1.360997] soc soc0: Amlogic Meson G12B (S922X) Revision 29:c (40:2) Detected
[    1.364333] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
[    1.364435] Unable to handle kernel paging request at virtual address 0000000008003388
[    1.367470] Internal error: Oops - Undefined instruction: 0000000002000000 [#1] PREEMPT SMP
[    1.367483] Modules linked in:
[    1.367490] CPU: 2 PID: 66 Comm: kworker/u12:1 Not tainted 6.2.0-odroid-arm64 #47
[    1.367498] Hardware name: Hardkernel ODROID-N2Plus (DT)
[    1.367502] Workqueue: events_unbound async_run_entry_fn
[    1.367516] pstate: 200000c5 (nzCv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[    1.367523] pc : arch_timer_handler_phys+0x0/0x44
[    1.367535] lr : handle_percpu_devid_irq+0x84/0x140
[    1.367543] sp : ffff8000099abf70
[    1.367546] x29: ffff8000099abf70 x28: ffff000002a40000 x27: 0000000000000006
[    1.367556] x26: ffff800009d208a4 x25: ffff800009d20b44 x24: 0000000000000008
[    1.367565] x23: ffff8000094f8b50 x22: 000000000000000b x21: ffff000002829000
[    1.367573] x20: ffff800008f89fb0 x19: ffff00000282a600 x18: ffff0000021792e8
[    1.367581] x17: ffff80006e685000 x16: ffff8000099ac000 x15: 0000000000004000
[    1.367589] x14: ffff800009d27bde x13: ffff800009d2887b x12: 0000000000000308
[    1.367597] x11: 0000000000000a92 x10: 0000000000000068 x9 : ef01a5948f440b31
[    1.367606] x8 : 0000000000003273 x7 : ffff800009d262a8 x6 : 000000003754d375
[    1.367614] x5 : ffff80006e685000 x4 : ffff8000099abf70 x3 : ffff80006e685000
[    1.367622] x2 : ffff800008c26c30 x1 : ffff000077b83a00 x0 : 000000000000000b
[    1.367630] Call trace:
[    1.367633]  arch_timer_handler_phys+0x0/0x44
[    1.367641]  generic_handle_domain_irq+0x2c/0x44
[    1.367650]  gic_handle_irq+0x44/0xc4
[    1.367659]  call_on_irq_stack+0x2c/0x5c
[    1.367666]  do_interrupt_handler+0x80/0x84
[    1.367672]  el1_interrupt+0x34/0x70
[    1.367682]  el1h_64_irq_handler+0x18/0x2c
[    1.367686]  el1h_64_irq+0x64/0x68
[    1.367690]  HUF_decompress4X1_usingDTable_internal_default+0x2fc/0xd60
[    1.367702]  HUF_decompress4X_hufOnly_wksp_bmi2+0xec/0x140
[    1.367711]  ZSTD_decodeLiteralsBlock+0x580/0x630
[    1.367717]  ZSTD_decompressBlock_internal.part.0+0x5c/0x1b4
[    1.367723]  ZSTD_decompressBlock_internal+0x1c/0x30
[    1.367729]  ZSTD_decompressContinue.part.0+0x364/0x444
[    1.367734]  ZSTD_decompressContinueStream+0x98/0x180
[    1.367738]  ZSTD_decompressStream+0x5b0/0x8c0
[    1.367743]  zstd_decompress_stream+0x10/0x20
[    1.367751]  unzstd+0x290/0x37c
[    1.367760]  unpack_to_rootfs+0x174/0x298
[    1.367767]  do_populate_rootfs+0x84/0x1dc
[    1.367773]  async_run_entry_fn+0x34/0x150
[    1.367778]  process_one_work+0x1d0/0x320
[    1.367785]  worker_thread+0x14c/0x444

Perhaps, the driver or the PWM core could do something wrong based on the
invalid polarity and corrupt certain memory location, but I'm still not
able to relate the random value to these crashes.


Regards,
Munehisa

> Best regards
> Uwe
> 
> -- 
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | https://www.pengutronix.de/ |
>