SYSENTER and SYSCALL 32-bit entry points differ in handling of arg2
and arg6.
SYSENTER:
* ecx arg2
* ebp user stack
* 0(%ebp) arg6
SYSCALL:
* ebp arg2
* esp user stack
* 0(%esp) arg6
Sysenter code loads 0(%ebp) to %ebp right away.
(This destroys %ebp. It means we do not preserve it on return.
It's not causing problems since userspace VDSO code does not depend on it,
and SYSENTER insn can't be sanely used outside of VDSO).
Syscall code loads 0(%ebp) to %r9. This allows to eliminate one MOV insn
(r9 is a register where arg6 should be for 64-bit ABI), but on audit/ptrace
code paths this requires juggling of r9 and ebp:
(1) ptrace expects arg6 to be in pt_regs->bp;
(2) r9 is callee-clobbered register and needs to be saved/restored
around calls to C functions.
This patch changes syscall code to load 0(%ebp) to %ebp, making it more
similar to sysenter code. It's a bit smaller:
text data bss dec hex filename
1407 0 0 1407 57f ia32entry.o.before
1391 0 0 1391 56f ia32entry.o
To preserve ABI compat, we restore ebp on exit.
Run-tested.
Signed-off-by: Denys Vlasenko <dvlas...@redhat.com>
CC: Linus Torvalds <torva...@linux-foundation.org>
CC: Steven Rostedt <rost...@goodmis.org>
CC: Ingo Molnar <mi...@kernel.org>
CC: Borislav Petkov <b...@alien8.de>
CC: "H. Peter Anvin" <h...@zytor.com>
CC: Andy Lutomirski <l...@amacapital.net>
CC: Oleg Nesterov <o...@redhat.com>
CC: Frederic Weisbecker <fweis...@gmail.com>
CC: Alexei Starovoitov <a...@plumgrid.com>
CC: Will Drewry <w...@chromium.org>
CC: Kees Cook <keesc...@chromium.org>
CC: x...@kernel.org
CC: linux-kernel@vger.kernel.org
---
This patch is on top of:
x86/asm/entry/32: Simplify zeroing of pt_regs->r8..r11 on int80 code path
x86/asm/entry/32: Open-code CLEAR_RREGS. No code changes.
x86/asm/entry/32: Open-code LOAD_ARGS32. No code changes.
arch/x86/ia32/ia32entry.S | 19 ++++++++-----------
1 file changed, 8 insertions(+), 11 deletions(-)
diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index 0ff676a..6f29f1f 100644
--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -327,7 +327,7 @@ ENTRY(ia32_cstar_target)
* 32bit zero extended
*/
ASM_STAC
-1: movl (%r8),%r9d
+1: movl (%r8),%ebp
_ASM_EXTABLE(1b,ia32_badarg)
ASM_CLAC
orl $TS_COMPAT, ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS)
@@ -337,7 +337,7 @@ ENTRY(ia32_cstar_target)
cstar_do_call:
/* 32bit syscall -> 64bit C ABI argument conversion */
movl %edi,%r8d /* arg5 */
- /* r9 already loaded */ /* arg6 */
+ movl %ebp,%r9d /* arg6 */
xchg %ecx,%esi /* rsi:arg2, rcx:arg4 */
movl %ebx,%edi /* arg1 */
movl %edx,%edx /* arg3 (zero extension) */
@@ -353,6 +353,7 @@ cstar_dispatch:
jnz sysretl_audit
sysretl_from_sys_call:
andl $~TS_COMPAT, ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS)
+ movl RCX(%rsp), %ebp
RESTORE_RSI_RDI_RDX
movl RIP(%rsp),%ecx
movl EFLAGS(%rsp),%r11d
@@ -379,9 +380,8 @@ sysretl_from_sys_call:
#ifdef CONFIG_AUDITSYSCALL
cstar_auditsys:
- movl %r9d,R9(%rsp) /* register to be clobbered by call */
auditsys_entry_common
- movl R9(%rsp),%r9d /* reload 6th syscall arg */
+ movl %ebp, %r9d /* reload 6th syscall arg */
jmp cstar_dispatch
sysretl_audit:
@@ -393,16 +393,14 @@ cstar_tracesys:
testl $(_TIF_WORK_SYSCALL_ENTRY & ~_TIF_SYSCALL_AUDIT),
ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
jz cstar_auditsys
#endif
- xchgl %r9d,%ebp
SAVE_EXTRA_REGS
xorl %eax, %eax
movq %rax, R11(%rsp)
movq %rax, R10(%rsp)
- movq %r9, R9(%rsp)
+ movq %rax, R9(%rsp)
movq %rax, R8(%rsp)
- movq %rsp,%rdi /* &pt_regs -> arg1 */
- call syscall_trace_enter
- movl R9(%rsp),%r9d
+ movq %rsp, %rdi /* &pt_regs -> arg1 */
+ call syscall_trace_enter
/*
* Reload arg registers from stack in case ptrace changed them.
* Don't reload %eax because syscall_trace_enter() returned
@@ -416,8 +414,7 @@ cstar_tracesys:
movl RDI(%rsp), %edi
movl %eax, %eax /* zero extension */
RESTORE_EXTRA_REGS
- xchgl %ebp,%r9d
- jmp cstar_do_call
+ jmp cstar_do_call
END(ia32_cstar_target)
ia32_badarg:
--
1.8.1.4
--
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