On 06/19/2018 07:22 AM, Alexey Brodkin wrote: > From: Peter Zijlstra <peterz at infradead.org> > > arc_usr_cmpxchg syscall is supposed to be used on platforms > that lack support of Load-Locked/Store-Conditional instructions > in hardware. And in that case we mimic missing hardware features > with help of kernel's sycall that "atomically" checks current > value in memory and then if it matches caller expectation new > value is written to that same location. > > What's important in the description above: > - Check-and-exchange must be "atomical" which means > preemption must be disabled during entire "transaction" > - Data accessed is from user-space, i.e. we're dealing > with virtual addresses > > And in current implementation we have a couple of problems: > > 1. We do disable preemprion around __get_user() & __put_user() > but that in its turn disables page fault handler. > That means if a pointer to user's data has no mapping in > the TLB we won't be able to access required data. > Instead software "exception handling" code from __get_user_fn() > will return -EFAULT. > > 2. What's worse if we're dealing with data from not yet allocated > page (think of pre-copy-on-write state) we'll successfully > read data but on write we'll silently return to user-space > with correct result (which we really read just before). That leads > to very strange problems in user-space app further down the line > because new value was never written to the destination. > > 3. Regardless of what went wrong we'll return from syscall > and user-space application will continue to execute. > Even if user's pointer was completely bogus. > In case of hardware LL/SC that app would have been killed > by the kernel. > > With that change we attempt to imrove on all 3 items above: > > 1. We still disable preemption around write of user's data but > if we happen to fail with write we're enabling preemption > and try to fix-up page fault so that we have a correct permission > for writing user's data. Then re-try again in "atomic" context. > > 2. If real page fault fails or even access_ok() returns false > we send SIGSEGV to the user-space process so if something goes > seriously wrong we'll know about it much earlier. > > Signed-off-by: Peter Zijlstra <peterz at infradead.org> > Signed-off-by: Alexey Brodkin <abrodkin at synopsys.com> > Cc: Vineet Gupta <vgupta at synopsys.com> > Cc: Max Filippov <jcmvbkbc at gmail.com> > Cc: linux-arch at vger.kernel.org > --- > > Changes v1 -> v2: > > * Peter's almost clean-room reimplmentation with less paranoid checks > and direct invocation of fixup_user_fault() for in-place update of > write permissions. > I don't like the changelog - it is way too verbose and doesn't say the exact problem we are trying to solve. How about something like below ? -----> ??? ARC: Improve cmpxchg syscall implementation ??? ??? This is used in configs lacking hardware atomics to emulate atomic r-m-w ??? for user space, implemented by disabling preemption in kernel. ??? ??? However there are issues in current implementation: ??? ??? 1. Process not terminated if invalid user pointer passed: ?????? i.e. __get_user() failed. ??? ??? 2. The reason for this patch was __put_user() failure not being handled, ?????? for COW break scenario. The zero page is initially wired up and ?????? read by __get_user() succeeds. However a write by __put_user() ?????? doesn't complete the page fault handling due to the page fault ?????? disabling from preempt disable. And what's worse is we silently return ?????? the stale zero value from __get_user() to user space. So the fix ?????? handles the specific case by re-enabling preemption and explicitly ?????? fixing up the fault and retrying the whole sequence over. OK ? -Vineet
