[RFC PATCH v2] x86_64: Check for AST when exiting a syscall

[Sergey Bugaev]

...like it's already done when exiting a trap. This is required, since
handing a syscall can result in an AST; in particular this happens when
the current thread is being terminated, which sets AST_TERMINATE and
expects the thread to never return to userspace.

Fixes a kernel crash upon calling exit () or pthread_exit () in glibc.
---

Differences to v1:
* Remove the TODO entry about checking for AST
* Attempt to save the syscall return value

Are we actually aligning the stack correctly before the call to syscall
implementation?

 x86_64/locore.S | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/x86_64/locore.S b/x86_64/locore.S
index 366ef292..2b8e4c44 100644
--- a/x86_64/locore.S
+++ b/x86_64/locore.S
@@ -1341,7 +1341,6 @@ END(syscall)
      - for now we assume the return address is canonical, but apparently there
        can be cases where it's not (see how Linux handles this). Does it apply
        here?
-     - do we need to check for ast on syscalls? Maybe on interrupts is enough
      - check that the case where a task is suspended, and later returns via
        iretq from return_from_trap, works fine in all combinations
  */
@@ -1428,10 +1427,33 @@ _syscall64_args_stack:
 
 _syscall64_call:
        call    *EXT(mach_trap_table)+8(%rax)  /* call procedure */
-       // XXX: check ast on exit?
 
-       /* Restore thread state and return to user using sysret. */
+_syscall64_check_for_ast:
+       /* Check for ast. */
        CPU_NUMBER(%r11)
+       cmpl    $0,CX(EXT(need_ast),%r11)
+       jz      _syscall64_restore_state
+
+       /* Save the syscall return value, both on our stack, for the case
+        * i386_astintr returns normally, and in the PCB stack, in case it
+        * instead calls thread_block(thread_exception_return).
+        */
+       pushq   %rax                            /* save the return value on our 
stack */
+       pushq   $0                              /* dummy value to keep the 
stack aligned */
+
+       /* Find the PCB stack. */
+       movq    %rsp,%rcx
+       or      $(KERNEL_STACK_SIZE-1),%rcx
+       movq    -7-IKS_SIZE(%rcx),%rcx
+
+       movq    %rax,R_EAX(%rcx)                /* save the return value in the 
PCB stack */
+       call    EXT(i386_astintr)
+       popq    %rax
+       popq    %rax                            /* restore the return value */
+       jmp     _syscall64_check_for_ast        /* check again */
+
+_syscall64_restore_state:
+       /* Restore thread state and return to user using sysret. */
        movq    CX(EXT(active_threads),%r11),%r11 /* point to current thread */
        movq    TH_PCB(%r11),%r11               /* point to pcb */
        addq    $ PCB_ISS,%r11                  /* point to saved state */
-- 
2.40.1