Re: [PATCH v3] seccomp: Improve performace by optimizing rmb()
From: Andy Lutomirski <luto@amacapital.net>
Date: 2021-02-24 16:21:35
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lkml, netdev
On Feb 24, 2021, at 12:03 AM, wanghongzhe [off-list ref] wrote: As Kees haved accepted the v2 patch at a381b70a1 which just replaced rmb() with smp_rmb(), this patch will base on that and just adjust the smp_rmb() to the correct position. As the original comment shown (and indeed it should be): /* * Make sure that any changes to mode from another thread have * been seen after SYSCALL_WORK_SECCOMP was seen. */ the smp_rmb() should be put between reading SYSCALL_WORK_SECCOMP and reading seccomp.mode to make sure that any changes to mode from another thread have been seen after SYSCALL_WORK_SECCOMP was seen, for TSYNC situation. However, it is misplaced between reading seccomp.mode and seccomp->filter. This issue seems to be misintroduced at 13aa72f0fd0a9f98a41cefb662487269e2f1ad65 which aims to refactor the filter callback and the API. So let's just adjust the smp_rmb() to the correct position. A next optimization patch will be provided if this ajustment is appropriate.
Would it be better to make the syscall work read be smp_load_acquire()?
quoted hunk ↗ jump to hunk
v2 -> v3: - move the smp_rmb() to the correct position v1 -> v2: - only replace rmb() with smp_rmb() - provide the performance test number RFC -> v1: - replace rmb() with smp_rmb() - move the smp_rmb() logic to the middle between TIF_SECCOMP and mode Signed-off-by: wanghongzhe <redacted> --- kernel/seccomp.c | 15 +++++++-------- 1 file changed, 7 insertions(+), 8 deletions(-)diff --git a/kernel/seccomp.c b/kernel/seccomp.c index 1d60fc2c9987..64b236cb8a7f 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c@@ -1160,12 +1160,6 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd, int data; struct seccomp_data sd_local; - /* - * Make sure that any changes to mode from another thread have - * been seen after SYSCALL_WORK_SECCOMP was seen. - */ - smp_rmb(); - if (!sd) { populate_seccomp_data(&sd_local); sd = &sd_local;@@ -1291,7 +1285,6 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,int __secure_computing(const struct seccomp_data *sd) { - int mode = current->seccomp.mode; int this_syscall; if (IS_ENABLED(CONFIG_CHECKPOINT_RESTORE) &&@@ -1301,7 +1294,13 @@ int __secure_computing(const struct seccomp_data *sd) this_syscall = sd ? sd->nr : syscall_get_nr(current, current_pt_regs()); - switch (mode) { + /* + * Make sure that any changes to mode from another thread have + * been seen after SYSCALL_WORK_SECCOMP was seen. + */ + smp_rmb(); + + switch (current->seccomp.mode) { case SECCOMP_MODE_STRICT: __secure_computing_strict(this_syscall); /* may call do_exit */ return 0;-- 2.19.1