On Tue, May 28, 2019 at 04:32:24PM +0100, Will Deacon wrote:

On Tue, May 28, 2019 at 04:01:03PM +0200, Peter Zijlstra wrote:

quoted

On Tue, May 28, 2019 at 08:31:29PM +0800, Young Xiao wrote:

quoted

When a kthread calls call_usermodehelper() the steps are:
  1. allocate current->mm
  2. load_elf_binary()
  3. populate current->thread.regs

While doing this, interrupts are not disabled. If there is a perf
interrupt in the middle of this process (i.e. step 1 has completed
but not yet reached to step 3) and if perf tries to read userspace
regs, kernel oops.

This seems to be because pt_regs(current) gives NULL for kthreads on Power.

I think you mean task_pt_regs(current) here.

quoted

quoted

Fix it by setting abi to PERF_SAMPLE_REGS_ABI_NONE when userspace
pt_regs are not set.

See commit bf05fc25f268 ("powerpc/perf: Fix oops when kthread execs
user process") for details.

Why the hell do we set current->mm before it is complete? Note that
normally exec() builds the new mm before attaching it, see exec_mmap()
in flush_old_exec().

From the initial report [1], it doesn't look like the mm isn't initialised,
but rather than we're dereferencing a NULL pt_regs pointer somehow for the
current task (see previous comment). I don't see how that can happen on
arm64, given that we put the pt_regs on the kernel stack which is allocated
during fork.

Will

[1] https://git.kernel.org/linus/bf05fc25f268

One caveat is that for the idle threads, the initial SP overlaps the
task_pt_regs() area:

* __primary_switched starts SP at init_thread_union + THREAD_SIZE.

* __cpu_up() starts SP at task_stack_page(idle) + THREAD_SIZE.

... and in either case, sampling that would be bad.

For both arm, I believe similar holds true. AFAICT x86 seems to reserve
the regs area in its head_{64,32}.S, but I can't see what it does for
other threads.

Regardless, for arm, arm64, and x86, task_pt_regs(current) cannot be
NULL.

Thanks,
Mark.

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