[RFC 2/5] x86_64, entry: Treat regs->ax the same in fastpath and slowpath syscalls

[Andy Lutomirski]

For slowpath syscalls, we initialize regs->ax to -ENOSYS and stick
the syscall number into regs->orig_ax prior to any possible tracing
and syscall execution.  This is user-visible ABI used by ptrace
syscall emulation and seccomp.

For fastpath syscalls, there's no good reason not to do the same
thing.  It's even slightly simpler than what we're currently doing.
It probably has no measureable performance impact.  It should have
no user-visible effect.

The purpose of this patch is to prepare for seccomp-based syscall
emulation in the fast path.  This change is just subtle enough that
I'm keeping it separate.

Signed-off-by: Andy Lutomirski <luto at amacapital.net>
---
 arch/x86/include/asm/calling.h |  6 +++++-
 arch/x86/kernel/entry_64.S     | 11 +++--------
 2 files changed, 8 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/calling.h b/arch/x86/include/asm/calling.h
index cb4c73b..76659b6 100644
--- a/arch/x86/include/asm/calling.h
+++ b/arch/x86/include/asm/calling.h
@@ -85,7 +85,7 @@ For 32-bit we have the following conventions - kernel is built with
 #define ARGOFFSET	R11
 #define SWFRAME		ORIG_RAX
 
-	.macro SAVE_ARGS addskip=0, save_rcx=1, save_r891011=1
+	.macro SAVE_ARGS addskip=0, save_rcx=1, save_r891011=1, rax_enosys=0
 	subq  $9*8+\addskip, %rsp
 	CFI_ADJUST_CFA_OFFSET	9*8+\addskip
 	movq_cfi rdi, 8*8
@@ -96,7 +96,11 @@ For 32-bit we have the following conventions - kernel is built with
 	movq_cfi rcx, 5*8
 	.endif
 
+	.if \rax_enosys
+	movq $-ENOSYS, 4*8(%rsp)
+	.else
 	movq_cfi rax, 4*8
+	.endif
 
 	.if \save_r891011
 	movq_cfi r8,  3*8
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 1e96c36..f9e713a 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -611,8 +611,8 @@ GLOBAL(system_call_after_swapgs)
 	 * and short:
 	 */
 	ENABLE_INTERRUPTS(CLBR_NONE)
-	SAVE_ARGS 8,0
-	movq  %rax,ORIG_RAX-ARGOFFSET(%rsp)
+	SAVE_ARGS 8, 0, rax_enosys=1
+	movq_cfi rax,(ORIG_RAX-ARGOFFSET)
 	movq  %rcx,RIP-ARGOFFSET(%rsp)
 	CFI_REL_OFFSET rip,RIP-ARGOFFSET
 	testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
@@ -624,7 +624,7 @@ system_call_fastpath:
 	andl $__SYSCALL_MASK,%eax
 	cmpl $__NR_syscall_max,%eax
 #endif
-	ja badsys
+	ja ret_from_sys_call  /* and return regs->ax */
 	movq %r10,%rcx
 	call *sys_call_table(,%rax,8)  # XXX:	 rip relative
 	movq %rax,RAX-ARGOFFSET(%rsp)
@@ -683,10 +683,6 @@ sysret_signal:
 	FIXUP_TOP_OF_STACK %r11, -ARGOFFSET
 	jmp int_check_syscall_exit_work
 
-badsys:
-	movq $-ENOSYS,RAX-ARGOFFSET(%rsp)
-	jmp ret_from_sys_call
-
 #ifdef CONFIG_AUDITSYSCALL
 	/*
 	 * Fast path for syscall audit without full syscall trace.
@@ -726,7 +722,6 @@ tracesys:
 	jz auditsys
 #endif
 	SAVE_REST
-	movq $-ENOSYS,RAX(%rsp) /* ptrace can change this for a bad syscall */
 	FIXUP_TOP_OF_STACK %rdi
 	movq %rsp,%rdi
 	call syscall_trace_enter
-- 
1.9.3