Peter Zijlstra <[email protected]> writes: > 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?
We just assumed it was our bug, 'cause we have plenty of those :) cheers
