x86_64, entry: Treat regs->ax the same in fastpath and slowpath syscalls

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 two-phase syscall
tracing, in which the first phase might modify the saved RAX without
leaving the fast path.  This change is just subtle enough that I'm
keeping it separate.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/01218b493f12ae2f98034b78c9ae085e38e94350.1409954077.git.luto@amacapital.net
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

2 files changed, 9 insertions, 10 deletions

diff --git a/arch/x86/include/asm/calling.h b/arch/x86/include/asm/calling.h
index cb4c73bfeb48..76659b67fd11 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 2fac1343a90b..0bd6d3c28064 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -404,8 +404,8 @@ 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)
@@ -417,7 +417,7 @@ 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)
@@ -476,10 +476,6 @@ 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.
@@ -519,7 +515,6 @@ 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
@@ -536,7 +531,7 @@ tracesys:
         andl $__SYSCALL_MASK,%eax
         cmpl $__NR_syscall_max,%eax
 #endif
-        ja   int_ret_from_sys_call      /* RAX(%rsp) set to -ENOSYS above */
+        ja   int_ret_from_sys_call      /* RAX(%rsp) is already set */
         movq %r10,%rcx  /* fixup for C */
         call *sys_call_table(,%rax,8)
         movq %rax,RAX-ARGOFFSET(%rsp)