On 7/15/24 13:59, Mateusz Guzik wrote:
On Mon, Jul 15, 2024 at 6:22 PM John Baldwin <j...@freebsd.org> wrote:
On 7/11/24 07:07, Mateusz Guzik wrote:
The branch main has been updated by mjg:
URL:
https://cgit.FreeBSD.org/src/commit/?id=87ee63bac69dc49291f55590b8baa57cad6c7d85
commit 87ee63bac69dc49291f55590b8baa57cad6c7d85
Author: Mateusz Guzik <m...@freebsd.org>
AuthorDate: 2024-07-11 00:17:27 +0000
Commit: Mateusz Guzik <m...@freebsd.org>
CommitDate: 2024-07-11 11:06:52 +0000
locks: add a runtime check for missing turnstile
There are sometimes bugs which result in the unlock fast path failing,
which in turns causes a not-helpful crash report when dereferencing a
NULL turnstile. Help debugging such cases by pointing out what happened
along with some debug.
Sponsored by: Rubicon Communications, LLC ("Netgate")
---
sys/kern/kern_mutex.c | 4 +++-
sys/kern/kern_rwlock.c | 16 ++++++++++++----
2 files changed, 15 insertions(+), 5 deletions(-)
diff --git a/sys/kern/kern_mutex.c b/sys/kern/kern_mutex.c
index 90361b23c09a..0fa624cc4bb1 100644
--- a/sys/kern/kern_mutex.c
+++ b/sys/kern/kern_mutex.c
@@ -1053,7 +1053,9 @@ __mtx_unlock_sleep(volatile uintptr_t *c, uintptr_t v)
turnstile_chain_lock(&m->lock_object);
_mtx_release_lock_quick(m);
ts = turnstile_lookup(&m->lock_object);
- MPASS(ts != NULL);
+ if (__predict_false(ts == NULL)) {
+ panic("got NULL turnstile on mutex %p v %zx", m, v);
+ }
Hmm, this is just an expanded KASSERT() but always on rather than conditional
on INVARIANTS?
Do you have examples of the type of bugs that cause this? (Is it unlocking a
freed mutex
or the like?) We generally hide all these types of checks under INVARIANTS
rather than
shipping them in release kernels.
Use-after-free, overflow, underflow, bitflip or what have you all can
fail the fast path.
Once that happens and the kernel crashes with a null pointer deref,
here is a crash at netgate which prodded this:
calltrap() at calltrap+0x8/frame 0xfffffe0106720920
--- trap 0xc, rip = 0xffffffff80d5ab70, rsp = 0xfffffe01067209f0, rbp
= 0xfffffe0106720a00 ---
turnstile_broadcast() at turnstile_broadcast+0x40/frame 0xfffffe0106720a00
__rw_wunlock_hard() at __rw_wunlock_hard+0x9e/frame 0xfffffe0106720a30
nd6_resolve_slow() at nd6_resolve_slow+0x2d7/frame 0xfffffe0106720aa0
nd6_resolve() at nd6_resolve+0x125/frame 0xfffffe0106720b10
ether_output() at ether_output+0x4e7/frame 0xfffffe0106720ba0
ip_output_send() at ip_output_send+0xdc/frame 0xfffffe0106720be0
ip_output() at ip_output+0x1295/frame 0xfffffe0106720ce0
ip_forward() at ip_forward+0x3c2/frame 0xfffffe0106720d90
ip_input() at ip_input+0x705/frame 0xfffffe0106720df0
swi_net() at swi_net+0x138/frame 0xfffffe0106720e60
ithread_loop() at ithread_loop+0x257/frame 0xfffffe0106720ef0
fork_exit() at fork_exit+0x7f/frame 0xfffffe0106720f30
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe0106720f30
--- trap 0, rip = 0, rsp = 0, rbp = 0 ---
Neither the register dump nor anything in the backtrace indicate what happened.
Since the kernel is going down anyway, one may as well get some debug from it.
If you don't mind the extra branches for sanity checks, why not just run with
INVARIANTS? That is, what makes these particular assertions different from
other assertions such that they should be on unconditionally? The last line
below
applies to pretty much every other assertion in the tree.
--
John Baldwin
