Re: [PATCH] perf: Fix oops when kthread execs user process

[Peter Zijlstra]

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

Also, why did those PPC folks 'fix' this in isolation? And why didn't
you Cc them?