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 <l...@amacapital.net>
---
arch/x86/include/asm/calling.h | 6 +++++-
arch/x86/kernel/entry_64.S | 13 ++++---------
2 files changed, 9 insertions(+), 10 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..1eb3094 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)
movq %rcx,RIP-ARGOFFSET(%rsp)
CFI_REL_OFFSET rip,RIP-ARGOFFSET
testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
@@ -418,7 +418,7 @@ 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)
@@ -477,10 +477,6 @@ 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.
@@ -520,7 +516,6 @@ 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
@@ -537,7 +532,7 @@ 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)
--
1.9.3
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
