Re: [syzbot] BUG: soft lockup in do_wp_page (4)

On Thu, 25 Mar 2021 at 20:29, Will Deacon [off-list ref] wrote:

On Thu, Mar 25, 2021 at 08:24:53PM +0100, Dmitry Vyukov wrote:

quoted

On Thu, Mar 25, 2021 at 8:10 PM Will Deacon [off-list ref] wrote:

quoted

On Thu, Mar 25, 2021 at 07:34:54PM +0100, Dmitry Vyukov wrote:

quoted

On Thu, Mar 25, 2021 at 7:20 PM Will Deacon [off-list ref] wrote:

quoted

On Thu, Mar 18, 2021 at 08:34:16PM +0100, Dmitry Vyukov wrote:

quoted

On Thu, Mar 18, 2021 at 8:31 PM syzbot
[off-list ref] wrote:
commit cae118b6acc309539b33339e846cbb19187c164c
Author: Will Deacon
Date:   Wed Mar 3 13:49:27 2021 +0000
    arm64: Drop support for CMDLINE_EXTEND

syzbot passes lots of critical things in CONFIG_CMDLINE:
https://github.com/google/syzkaller/blob/c3c81c94865791469d376eba84f4a2d7763d3f71/dashboard/config/linux/upstream-arm64-kasan.config#L495
but also wants the bootloader args to be appended.
What is the way to do it now?

For now, there isn't a way to do it with CONFIG_CMDLINE, so I think you can
either:

  * Revert my patch for your kernels
  * Pass the arguments via QEMU's -append option
  * Take a look at one of the series which should hopefully add this
    functionality back (but with well-defined semantics) [1] [2]

Unfortunately none of these work for syzbot (and I assume other
testing environments).

syzbot does not support custom patches by design:
http://bit.do/syzbot#no-custom-patches
As any testing system, it tests the official trees.

It's not humans who start these VMs, so it's not as easy as changing
the command line after typing...
There is no support for passing args specifically to qemu, syzkaller
support not just qemu, so these things are specifically localized in
the config. Additionally there is an issue of communicating all these
scattered details to developers in bug reports. Currently syzbot
reports the kernel config and it as well captures command line.

Could you revert the patch? Is there any point in removing the
currently supported feature before the new feature lands?

Well, we only just merged it (in 5.10 I think?), and the semantics of the
new version will be different, so I really don't see the value in supporting
both (even worse, Android has its own implementation which is different
again). The timeline was: we merged CMDLINE_EXTEND, then we noticed it was
broken, my fixes were rejected, so we removed the feature rather than
support the broken version. In the relatively small window while it was
merged, syzbot started using it :(

I didn't realize it was just introduced :)
We used CMDLINE_EXTEND on x86, and I looked for a similar option for
arm64 and found it.

quoted

So I really think the best bet is to wait until the patches are sorted out.
I think Christophe is about to spin a new version, and I reviewed his last
copy, so I don't see this being far off,

If it's expected to be merged soon, let's wait.

Thanks, and knowing that we have a keen user helps to prioritise the review
:)

Any updates on proper CMDLINE support for arm64? This is still broken
for syzbot and it periodically reports false positives due to
incorrect cmdline.

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