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

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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>
This commit is contained in:
Andy Lutomirski 2014-09-05 15:13:55 -07:00 committed by H. Peter Anvin
parent e0ffbaabc4
commit 54eea9957f
2 changed files with 9 additions and 10 deletions
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6
arch/x86/include/asm/calling.h
View File

@ -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

13
arch/x86/kernel/entry_64.S
View File

@ -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)