x86/asm/entry/64: Clean up and document various entry code details

This patch does a lot of cleanup in comments and formatting,
but it does not change any code:

 - Rename 'save_paranoid' to 'paranoid_entry': this makes naming
   similar to its "non-paranoid" sibling, 'error_entry',
   and to its counterpart, 'paranoid_exit'.

 - Use the same CFI annotation atop 'paranoid_entry' and 'error_entry'.

 - Fix irregular indentation of assembler operands.

 - Add/fix comments on top of 'paranoid_entry' and 'error_entry'.

 - Remove stale comment about "oldrax".

 - Make comments about "no swapgs" flag in ebx more prominent.

 - Deindent wrongly indented top-level comment atop 'paranoid_exit'.

 - Indent wrongly deindented comment inside 'error_entry'.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/4640f9fcd5ea46eb299b1cd6d3f5da3167d2f78d.1424989793.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 9e33d492ace398d9920ef0803f989f7e492d6309..466947770648389f4aaf66ca122b70b85f664d4f 100644 (file)
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -982,10 +982,11 @@ ENTRY(\sym)
        testl $3, CS(%rsp)              /* If coming from userspace, switch */
        jnz 1f                          /* stacks. */
        .endif
-       call save_paranoid
+       call paranoid_entry
        .else
        call error_entry
        .endif
+       /* returned flag: ebx=0: need swapgs on exit, ebx=1: don't need it */
 
        DEFAULT_FRAME 0
 
@@ -1016,10 +1017,11 @@ ENTRY(\sym)
        addq $EXCEPTION_STKSZ, INIT_TSS_IST(\shift_ist)
        .endif
 
+       /* these procedures expect "no swapgs" flag in ebx */
        .if \paranoid
-       jmp paranoid_exit               /* %ebx: no swapgs flag */
+       jmp paranoid_exit
        .else
-       jmp error_exit                  /* %ebx: no swapgs flag */
+       jmp error_exit
        .endif
 
        .if \paranoid == 1
@@ -1257,8 +1259,13 @@ idtentry async_page_fault do_async_page_fault has_error_code=1
 idtentry machine_check has_error_code=0 paranoid=1 do_sym=*machine_check_vector(%rip)
 #endif
 
-ENTRY(save_paranoid)
-       XCPT_FRAME 1 RDI+8
+/*
+ * Save all registers in pt_regs, and switch gs if needed.
+ * Use slow, but surefire "are we in kernel?" check.
+ * Return: ebx=0: need swapgs on exit, ebx=1: otherwise
+ */
+ENTRY(paranoid_entry)
+       XCPT_FRAME 1 15*8
        cld
        SAVE_C_REGS 8
        SAVE_EXTRA_REGS 8
@@ -1271,20 +1278,19 @@ ENTRY(save_paranoid)
        xorl %ebx,%ebx
 1:     ret
        CFI_ENDPROC
-END(save_paranoid)
-
-       /*
-        * "Paranoid" exit path from exception stack.  This is invoked
-        * only on return from non-NMI IST interrupts that came
-        * from kernel space.
-        *
-        * We may be returning to very strange contexts (e.g. very early
-        * in syscall entry), so checking for preemption here would
-        * be complicated.  Fortunately, we there's no good reason
-        * to try to handle preemption here.
-        */
+END(paranoid_entry)
 
-       /* ebx: no swapgs flag */
+/*
+ * "Paranoid" exit path from exception stack.  This is invoked
+ * only on return from non-NMI IST interrupts that came
+ * from kernel space.
+ *
+ * We may be returning to very strange contexts (e.g. very early
+ * in syscall entry), so checking for preemption here would
+ * be complicated.  Fortunately, we there's no good reason
+ * to try to handle preemption here.
+ */
+/* On entry, ebx is "no swapgs" flag (1: don't need swapgs, 0: need it) */
 ENTRY(paranoid_exit)
        DEFAULT_FRAME
        DISABLE_INTERRUPTS(CLBR_NONE)
@@ -1305,13 +1311,11 @@ paranoid_exit_restore:
 END(paranoid_exit)
 
 /*
- * Exception entry point. This expects an error code/orig_rax on the stack.
- * returns in "no swapgs flag" in %ebx.
+ * Save all registers in pt_regs, and switch gs if needed.
+ * Return: ebx=0: need swapgs on exit, ebx=1: otherwise
  */
 ENTRY(error_entry)
-       XCPT_FRAME
-       CFI_ADJUST_CFA_OFFSET 15*8
-       /* oldrax contains error code */
+       XCPT_FRAME 1 15*8
        cld
        SAVE_C_REGS 8
        SAVE_EXTRA_REGS 8
@@ -1324,12 +1328,12 @@ error_sti:
        TRACE_IRQS_OFF
        ret
 
-/*
- * There are two places in the kernel that can potentially fault with
- * usergs. Handle them here.  B stepping K8s sometimes report a
- * truncated RIP for IRET exceptions returning to compat mode. Check
- * for these here too.
- */
+       /*
+        * There are two places in the kernel that can potentially fault with
+        * usergs. Handle them here.  B stepping K8s sometimes report a
+        * truncated RIP for IRET exceptions returning to compat mode. Check
+        * for these here too.
+        */
 error_kernelspace:
        CFI_REL_OFFSET rcx, RCX+8
        incl %ebx
@@ -1359,7 +1363,7 @@ error_bad_iret:
 END(error_entry)
 
 
-/* ebx:        no swapgs flag (1: don't need swapgs, 0: need it) */
+/* On entry, ebx is "no swapgs" flag (1: don't need swapgs, 0: need it) */
 ENTRY(error_exit)
        DEFAULT_FRAME
        movl %ebx,%eax
@@ -1585,13 +1589,13 @@ end_repeat_nmi:
        ALLOC_PT_GPREGS_ON_STACK
 
        /*
-        * Use save_paranoid to handle SWAPGS, but no need to use paranoid_exit
+        * Use paranoid_entry to handle SWAPGS, but no need to use paranoid_exit
         * as we should not be calling schedule in NMI context.
         * Even with normal interrupts enabled. An NMI should not be
         * setting NEED_RESCHED or anything that normal interrupts and
         * exceptions might do.
         */
-       call save_paranoid
+       call paranoid_entry
        DEFAULT_FRAME 0
 
        /*