git: dbec10e08808 - main - Fetch the sigfastblock value in syscalls that wait for signals

[Jonathan T. Looney]

The branch main has been updated by jtl:

URL: https://cgit.FreeBSD.org/src/commit/?id=dbec10e08808e375365fb2a2462f306e0cdfda32

commit dbec10e08808e375365fb2a2462f306e0cdfda32
Author:     Jonathan T. Looney <jtl at FreeBSD.org>
AuthorDate: 2021-03-12 18:14:17 +0000
Commit:     Jonathan T. Looney <jtl at FreeBSD.org>
CommitDate: 2021-03-12 18:14:17 +0000

    Fetch the sigfastblock value in syscalls that wait for signals
    
    We have seen several cases of processes which have become "stuck" in
    kern_sigsuspend(). When this occurs, the kernel's td_sigblock_val
    is set to 0x10 (one block outstanding) and the userspace copy of the
    word is set to 0 (unblocked). Because the kernel's cached value
    shows that signals are blocked, kern_sigsuspend() blocks almost all
    signals, which means the process hangs indefinitely in sigsuspend().
    
    It is not entirely clear what is causing this condition to occur.
    However, it seems to make sense to add some protection against this
    case by fetching the latest sigfastblock value from userspace for
    syscalls which will sleep waiting for signals. Here, the change is
    applied to kern_sigsuspend() and kern_sigtimedwait().
    
    Reviewed by:    kib
    Sponsored by:   Netflix
    Differential Revision:  https://reviews.freebsd.org/D29225
---
 sys/kern/kern_sig.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/sys/kern/kern_sig.c b/sys/kern/kern_sig.c
index 7884b5be9f91..3d55405d3151 100644
--- a/sys/kern/kern_sig.c
+++ b/sys/kern/kern_sig.c
@@ -1268,6 +1268,9 @@ kern_sigtimedwait(struct thread *td, sigset_t waitset, ksiginfo_t *ksi,
 	ets.tv_nsec = 0;
 	traced = false;
 
+	/* Ensure the sigfastblock value is up to date. */
+	sigfastblock_fetch(td);
+
 	if (timeout != NULL) {
 		if (timeout->tv_nsec >= 0 && timeout->tv_nsec < 1000000000) {
 			timevalid = 1;
@@ -1527,6 +1530,9 @@ kern_sigsuspend(struct thread *td, sigset_t mask)
 	struct proc *p = td->td_proc;
 	int has_sig, sig;
 
+	/* Ensure the sigfastblock value is up to date. */
+	sigfastblock_fetch(td);
+
 	/*
 	 * When returning from sigsuspend, we want
 	 * the old mask to be restored after the