x86_64: remove bogus optimization in sysret_signal

This short-circuit path in sysret_signal looks wrong to me.
AFAICT, in practice the branch is never taken--and if it were,
it would go wrong.  To wit, try loading a module whose init
function does set_thread_flag(TIF_IRET), and see insmod crash
(presumably with a wrong user stack pointer).

This is because the FIXUP_TOP_OF_STACK work hasn't been done yet
when we jump around the call to ptregscall_common and get to
int_with_check--where it expects the user RSP,SS,CS and EFLAGS to
have been stored by FIXUP_TOP_OF_STACK.

I don't think it's normally possible to get to sysret_signal with no
_TIF_DO_NOTIFY_MASK bits set anyway, so these two instructions are
already superfluous.  If it ever did happen, it is harmless to call
do_notify_resume with nothing for it to do.

Signed-off-by: Roland McGrath <roland@redhat.com>

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 8410e26f418337d7fc37d77dba6a1f0e60e7f525..a169225869ccfe62ed5c6780d7098e11c6680002 100644 (file)
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -402,16 +402,12 @@ sysret_careful:
 sysret_signal:
        TRACE_IRQS_ON
        ENABLE_INTERRUPTS(CLBR_NONE)
-       testl $_TIF_DO_NOTIFY_MASK,%edx
-       jz    1f
-
-       /* Really a signal */
        /* edx: work flags (arg3) */
        leaq do_notify_resume(%rip),%rax
        leaq -ARGOFFSET(%rsp),%rdi # &pt_regs -> arg1
        xorl %esi,%esi # oldset -> arg2
        call ptregscall_common
-1:     movl $_TIF_WORK_MASK,%edi
+       movl $_TIF_WORK_MASK,%edi
        /* Use IRET because user could have changed frame. This
           works because ptregscall_common has called FIXUP_TOP_OF_STACK. */
        DISABLE_INTERRUPTS(CLBR_NONE)