[PATCH 1/2 v2] x86/asm/entry/64: better label name, fix comments

[Denys Vlasenko <dvlasenk@redhat.com>]

A named label "ret_from_sys_call" implies that there are jumps
to this location from elsewhere, as happens with many other labels
in this file.
But this label is used only by the JMP a few insns above.
To make that obvious, use local numeric label instead.

Do the same in the second copy of the syscall table dispatch code,
it has a similar JMP which skips the CALL.

Improve comments:

"and return regs->ax" isn't too informative. We always return regs->ax.

The comment suggesting that it'd be cool to use rip relative addressing for CALL
is deleted. It's unclear why that would be an improvement - we aren't striving
to use position-independent code here. PIC code here would require something like
LEA sys_call_table(%rip),reg + CALL *(reg,%rax*8)...

"iret frame is also incomplete" is no longer true, fix that too.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@kernel.org>
CC: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
CC: Will Drewry <wad@chromium.org>
CC: Kees Cook <keescook@chromium.org>
CC: x86@kernel.org
CC: linux-kernel@vger.kernel.org
---

Changes in v2: gave "jmp 1f" treatment to the second jump.

 arch/x86/kernel/entry_64.S | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index bf9afad..9c8661c 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -258,16 +258,16 @@ system_call_fastpath:
 	andl $__SYSCALL_MASK,%eax
 	cmpl $__NR_syscall_max,%eax
 #endif
-	ja ret_from_sys_call  /* and return regs->ax */
+	ja	1f	/* return -ENOSYS (already in pt_regs->ax) */
 	movq %r10,%rcx
-	call *sys_call_table(,%rax,8)  # XXX:	 rip relative
+	call *sys_call_table(,%rax,8)
 	movq %rax,RAX(%rsp)
+1:
+
 /*
- * Syscall return path ending with SYSRET (fast path)
- * Has incompletely filled pt_regs, iret frame is also incomplete.
+ * Syscall return path ending with SYSRET (fast path).
+ * Has incompletely filled pt_regs.
  */
-ret_from_sys_call:
-
 	LOCKDEP_SYS_EXIT
 	DISABLE_INTERRUPTS(CLBR_NONE)
 	TRACE_IRQS_OFF
@@ -334,10 +334,11 @@ tracesys_phase2:
 	andl $__SYSCALL_MASK,%eax
 	cmpl $__NR_syscall_max,%eax
 #endif
-	ja   int_ret_from_sys_call	/* RAX(%rsp) is already set */
+	ja	1f	/* return -ENOSYS (already in pt_regs->ax) */
 	movq %r10,%rcx	/* fixup for C */
 	call *sys_call_table(,%rax,8)
 	movq %rax,RAX(%rsp)
+1:
 	/* Use IRET because user could have changed pt_regs->foo */
 
 /*
-- 
1.8.1.4