Re: git: fa1d803c0f65 - main - epoch: replace hand coded assertion

  Brooks, John, Kostik,

On Fri, Jan 20, 2023 at 06:05:17PM +0000, Brooks Davis wrote:
B> The branch main has been updated by brooks:
B> 
B> URL: https://cgit.FreeBSD.org/src/commit/?id=fa1d803c0f652d72840a3c59139baf9d30792860
B> 
B> commit fa1d803c0f652d72840a3c59139baf9d30792860
B> Author:     Brooks Davis <brooks@FreeBSD.org>
B> AuthorDate: 2023-01-20 18:03:39 +0000
B> Commit:     Brooks Davis <brooks@FreeBSD.org>
B> CommitDate: 2023-01-20 18:04:40 +0000
B> 
B>     epoch: replace hand coded assertion
B>     
B>     The assertion is equivalent to kstack_contains() so use that rather
B>     than spelling it out.
B>     
B>     Suggested by:   jhb
B>     Reviewed by:    jhb
B>     MFC after:      1 week
B>     Sponsored by:   DARPA, AFRL
B>     Differential Revision:  https://reviews.freebsd.org/D38107
B> ---
B>  sys/kern/subr_epoch.c | 4 +---
B>  1 file changed, 1 insertion(+), 3 deletions(-)
B> 
B> diff --git a/sys/kern/subr_epoch.c b/sys/kern/subr_epoch.c
B> index 100221cd62f9..98a560e44c9d 100644
B> --- a/sys/kern/subr_epoch.c
B> +++ b/sys/kern/subr_epoch.c
B> @@ -468,9 +468,7 @@ _epoch_enter_preempt(epoch_t epoch, epoch_tracker_t et EPOCH_FILE_LINE)
B>  
B>  	MPASS(cold || epoch != NULL);
B>  	td = curthread;
B> -	MPASS((vm_offset_t)et >= td->td_kstack &&
B> -	    (vm_offset_t)et + sizeof(struct epoch_tracker) <=
B> -	    td->td_kstack + td->td_kstack_pages * PAGE_SIZE);
B> +	MPASS(kstack_contains(td, (vm_offset_t)et, sizeof(*et)));
B>  
B>  	INIT_CHECK(epoch);
B>  	MPASS(epoch->e_flags & EPOCH_PREEMPT);

At Netflix we are currently using more strict assertion:

    Check the epoch tracker not against the whole stack size, but against
    the legitimate part of the stack.  While here add the same check for
    the epoch exit.  The bug we are going to catch most likely is associated
    with incorrect exit.
    
diff --git a/FreeBSD/sys/kern/subr_epoch.c b/FreeBSD/sys/kern/subr_epoch.c
index b457b070cb65..b345719eb406 100644
--- a/FreeBSD/sys/kern/subr_epoch.c
+++ b/FreeBSD/sys/kern/subr_epoch.c
@@ -420,7 +420,8 @@ _epoch_enter_preempt(epoch_t epoch, epoch_tracker_t et EPOCH_FILE_LINE)
        MPASS(cold || epoch != NULL);
        MPASS(epoch->e_flags & EPOCH_PREEMPT);
        td = curthread;
-       MPASS((vm_offset_t)et >= td->td_kstack &&
+       MPASS((vm_offset_t)et + sizeof(struct epoch_tracker) >=
+           (vm_offset_t)__builtin_frame_address(0) &&
            (vm_offset_t)et + sizeof(struct epoch_tracker) <=
            td->td_kstack + td->td_kstack_pages * PAGE_SIZE);
 
@@ -457,8 +458,13 @@ _epoch_exit_preempt(epoch_t epoch, epoch_tracker_t et EPOCH_FILE_LINE)
        struct epoch_record *er;
        struct thread *td;
 
-       INIT_CHECK(epoch);
        td = curthread;
+       MPASS((vm_offset_t)et + sizeof(struct epoch_tracker) >=
+           (vm_offset_t)__builtin_frame_address(0) &&
+           (vm_offset_t)et + sizeof(struct epoch_tracker) <=
+           td->td_kstack + td->td_kstack_pages * PAGE_SIZE);
+
+       INIT_CHECK(epoch);
        critical_enter();
        sched_unpin();
        THREAD_SLEEPING_OK();


What do you guys think on legitimacy of the improved assertion that uses
compiler built to get the current top of stack and makes the assertion even
stricter?

I think in FreeBSD we should at least add the MPASS(kstack_contains(td, ...)
assertion to the epoch exit.

-- 
Gleb Smirnoff