x86/entry/32: Open-code return tracking from fork and kthreads

syscall_exit is going away, and return tracing is just a
function call now, so open-code the two non-syscall 32-bit
users.

While we're at it, update the big register layout comment.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/a6b3c472fda7cda0e368c3ccd553dea7447dfdd2.1444091585.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index a08ded481aba7250b354e4b4c2d2f1db38bf6850..36b6beb181f6ed30f9c737babc9f7d322328ba02 100644 (file)
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -3,7 +3,7 @@
  *
  * entry_32.S contains the system-call and low-level fault and trap handling routines.
  *
- * Stack layout in 'syscall_exit':
+ * Stack layout while running C code:
  *     ptrace needs to have all registers on the stack.
  *     If the order here is changed, it needs to be
  *     updated in fork.c:copy_process(), signal.c:do_signal(),
@@ -211,7 +211,11 @@ ENTRY(ret_from_fork)
        popl    %eax
        pushl   $0x0202                         # Reset kernel eflags
        popfl
-       jmp     syscall_exit
+
+       /* When we fork, we trace the syscall return in the child, too. */
+       movl    %esp, %eax
+       call    syscall_return_slowpath
+       jmp     restore_all
 END(ret_from_fork)
 
 ENTRY(ret_from_kernel_thread)
@@ -224,7 +228,15 @@ ENTRY(ret_from_kernel_thread)
        movl    PT_EBP(%esp), %eax
        call    *PT_EBX(%esp)
        movl    $0, PT_EAX(%esp)
-       jmp     syscall_exit
+
+       /*
+        * Kernel threads return to userspace as if returning from a syscall.
+        * We should check whether anything actually uses this path and, if so,
+        * consider switching it over to ret_from_fork.
+        */
+       movl    %esp, %eax
+       call    syscall_return_slowpath
+       jmp     restore_all
 ENDPROC(ret_from_kernel_thread)
 
 /*