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Subject[tip:x86/asm] x86/asm/entry/32: Do not use R9 in SYSCALL32 entry point
Commit-ID:  53e9accf0f7682d717c7b578b6e01fd297ba6630
Gitweb: http://git.kernel.org/tip/53e9accf0f7682d717c7b578b6e01fd297ba6630
Author: Denys Vlasenko <dvlasenk@redhat.com>
AuthorDate: Wed, 3 Jun 2015 14:56:09 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 5 Jun 2015 13:22:22 +0200

x86/asm/entry/32: Do not use R9 in SYSCALL32 entry point

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 <dvlasenk@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1433336169-18964-1-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/entry/ia32entry.S | 19 ++++++++-----------
1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/arch/x86/entry/ia32entry.S b/arch/x86/entry/ia32entry.S
index 56f819e..6321915 100644
--- a/arch/x86/entry/ia32entry.S
+++ b/arch/x86/entry/ia32entry.S
@@ -323,7 +323,7 @@ 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)
@@ -333,7 +333,7 @@ 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) */
@@ -349,6 +349,7 @@ 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
@@ -375,9 +376,8 @@ 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:
@@ -389,16 +389,14 @@ 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 /* do not leak kernel information */
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. (see sysenter_tracesys) */
movl RCX(%rsp), %ecx
@@ -408,8 +406,7 @@ cstar_tracesys:
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:

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