Messages in this thread | | | Date | Wed, 25 Mar 2015 13:21:56 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86, entry: Check for syscall exit work with IRQs disabled |
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* Andy Lutomirski <luto@amacapital.net> wrote:
> On Tue, Mar 24, 2015 at 1:08 PM, Ingo Molnar <mingo@kernel.org> wrote: > > > > * Andy Lutomirski <luto@kernel.org> wrote: > > > >> We currently have a race: if we're preempted during syscall exit, we > >> can fail to process syscall return work that is queued up while > >> we're preempted in ret_from_sys_call after checking ti.flags. > >> > >> Fix it by disabling interrupts before checking ti.flags. > >> > >> Fixes: 96b6352c1271 x86_64, entry: Remove the syscall exit audit and schedule optimizations > >> Reported-by: Stefan Seyfried <stefan.seyfried@googlemail.com> > >> Reported-by: Takashi Iwai <tiwai@suse.de> > >> Signed-off-by: Andy Lutomirski <luto@kernel.org> > >> --- > >> > >> Ingo, I don't understand the LOCKDEP_SYS_EXIT stuff. Can you take a quick > >> look to confirm that it's okay to call it more than once? > > > > So the essence is that it wants to print this warning if we are > > holding a lock after a syscall: > > > > printk("[ BUG: lock held when returning to user space! ]\n"); > > > > it manipulates no state and is not sensitive to whether it's called > > before or after return-work processing. > > > >> arch/x86/kernel/entry_64.S | 18 ++++++++++++++---- > >> 1 file changed, 14 insertions(+), 4 deletions(-) > >> > >> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S > >> index 1d74d161687c..2babb393915e 100644 > >> --- a/arch/x86/kernel/entry_64.S > >> +++ b/arch/x86/kernel/entry_64.S > >> @@ -364,12 +364,21 @@ system_call_fastpath: > >> * Has incomplete stack frame and undefined top of stack. > >> */ > >> ret_from_sys_call: > >> - testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET) > >> - jnz int_ret_from_sys_call_fixup /* Go the the slow path */ > >> - > >> LOCKDEP_SYS_EXIT > >> DISABLE_INTERRUPTS(CLBR_NONE) > >> TRACE_IRQS_OFF > >> + > >> + /* > >> + * We must check ti flags with interrupts (or at least preemption) > >> + * off because we must *never* return to userspace without > >> + * processing exit work that is enqueued if we're preempted here. > >> + * In particular, returning to userspace with any of the one-shot > >> + * flags (TIF_NOTIFY_RESUME, TIF_USER_RETURN_NOTIFY, etc) set is > >> + * very bad. > >> + */ > >> + testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET) > >> + jnz int_ret_from_sys_call_fixup /* Go the the slow path */ > > > > Should be safe to call it once again after user-work processing has > > been finished. > > > > I've picked up your fix for tip:x86/urgent. > > FWIW, the tentative merge here: > > https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?h=tmp.tmp&id=a77dd1607ad88a601259a74ba4d646fa68b7cd9a > > looks funny. Why aren't you jumping to int_ret_from_sys_call_irqs_off?
Indeed - the orphaned label should have told me that. The mismerge is functionally harmless (causes extra overhead in the slowpath), that's why it passed testing.
Does:
06ab9c1ba6a1 Merge branch 'x86/urgent' into x86/asm, to resolve conflict
look better to you?
Thanks,
Ingo
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