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@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 b25ca96..432c190 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S

@@ -405,8 +405,8 @@ 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)