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Commit 54eea995 authored by Andy Lutomirski's avatar Andy Lutomirski Committed by H. Peter Anvin
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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: default avatarAndy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/01218b493f12ae2f98034b78c9ae085e38e94350.1409954077.git.luto@amacapital.net


Signed-off-by: default avatarH. Peter Anvin <hpa@linux.intel.com>
parent e0ffbaab

arch/x86/include/asm/calling.h

+5 −1
Original line number Diff line number Diff line
@@ -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

arch/x86/kernel/entry_64.S

+4 −9
Original line number Diff line number Diff line
@@ -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)