seccomp: Improve performace by optimizing rmb()

According to Kees's suggest, we started with the patch that just replaces
rmb() with smp_rmb() and did a performance test with UnixBench. The
results showed the overhead about 2.53% in rmb() test compared to the
smp_rmb() one, in a x86-64 kernel with CONFIG_SMP enabled running inside a
qemu-kvm vm. The test is a "syscall" testcase in UnixBench, which executes
5 syscalls in a loop during a certain timeout (100 second in our test) and
counts the total number of executions of this 5-syscall sequence. We set
a seccomp filter with all allow rule for all used syscalls in this test
(which will go bitmap path) to make sure the rmb() will be executed. The
details for the test:

with rmb():
/txm # ./syscall_allow_min 100
COUNT|35861159|1|lps
/txm # ./syscall_allow_min 100
COUNT|35545501|1|lps
/txm # ./syscall_allow_min 100
COUNT|35664495|1|lps

with smp_rmb():
/txm # ./syscall_allow_min 100
COUNT|36552771|1|lps
/txm # ./syscall_allow_min 100
COUNT|36491247|1|lps
/txm # ./syscall_allow_min 100
COUNT|36504746|1|lps

For a x86-64 kernel with CONFIG_SMP enabled, the smp_rmb() is just a
compiler barrier() which have no impact in runtime, while rmb() is a
lfence which will prevent all memory access operations (not just load
according the recently claim by Intel) behind itself. We can also figure
it out in disassembly:

with rmb():
0000000000001430 <__seccomp_filter>:
    1430:   41 57                   push   %r15
    1432:   41 56                   push   %r14
    1434:   41 55                   push   %r13
    1436:   41 54                   push   %r12
    1438:   55                      push   %rbp
    1439:   53                      push   %rbx
    143a:   48 81 ec 90 00 00 00    sub    $0x90,%rsp
    1441:   89 7c 24 10             mov    %edi,0x10(%rsp)
    1445:   89 54 24 14             mov    %edx,0x14(%rsp)
    1449:   65 48 8b 04 25 28 00    mov    %gs:0x28,%rax
    1450:   00 00
    1452:   48 89 84 24 88 00 00    mov    %rax,0x88(%rsp)
    1459:   00
    145a:   31 c0                   xor    %eax,%eax
*   145c:   0f ae e8                lfence
    145f:   48 85 f6                test   %rsi,%rsi
    1462:   49 89 f4                mov    %rsi,%r12
    1465:   0f 84 42 03 00 00       je     17ad <__seccomp_filter+0x37d>
    146b:   65 48 8b 04 25 00 00    mov    %gs:0x0,%rax
    1472:   00 00
    1474:   48 8b 98 80 07 00 00    mov    0x780(%rax),%rbx
    147b:   48 85 db                test   %rbx,%rbx

with smp_rmb();
0000000000001430 <__seccomp_filter>:
    1430:   41 57                   push   %r15
    1432:   41 56                   push   %r14
    1434:   41 55                   push   %r13
    1436:   41 54                   push   %r12
    1438:   55                      push   %rbp
    1439:   53                      push   %rbx
    143a:   48 81 ec 90 00 00 00    sub    $0x90,%rsp
    1441:   89 7c 24 10             mov    %edi,0x10(%rsp)
    1445:   89 54 24 14             mov    %edx,0x14(%rsp)
    1449:   65 48 8b 04 25 28 00    mov    %gs:0x28,%rax
    1450:   00 00
    1452:   48 89 84 24 88 00 00    mov    %rax,0x88(%rsp)
    1459:   00
    145a:   31 c0                   xor    %eax,%eax
    145c:   48 85 f6                test   %rsi,%rsi
    145f:   49 89 f4                mov    %rsi,%r12
    1462:   0f 84 42 03 00 00       je     17aa <__seccomp_filter+0x37a>
    1468:   65 48 8b 04 25 00 00    mov    %gs:0x0,%rax
    146f:   00 00
    1471:   48 8b 98 80 07 00 00    mov    0x780(%rax),%rbx
    1478:   48 85 db                test   %rbx,%rbx
Signed-off-by: wanghongzhe <wanghongzhe@huawei.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/1612496049-32507-1-git-send-email-wanghongzhe@huawei.com

kernel/seccomp.c

@@ -1164,7 +1164,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 	 * Make sure that any changes to mode from another thread have
 	 * been seen after SYSCALL_WORK_SECCOMP was seen.
 	 */
-	rmb();
+	smp_rmb();
 
 	if (!sd) {
 		populate_seccomp_data(&sd_local);