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Add a knob to not drop signal with default ignored or ignored actions

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Traditionally, BSD drops signals with the default action during send,
not even putting them to the destination process queue.  This semantic
is not shared with other operating systems (Linux), which do queue
such signals.  In particular, sigtimedwait(2) and related syscalls can
observe the delivery.

Add a global knob kern.sig_discard_ign which can be set to false to force
enqueuing of the signals with default action.  Also add an ABI flag to
indicate that signals should be queued.

Note that it is not practical to run with the knob turned on, because almost
all software that care about the delivery of such signals, is aware of the
difference, and misbehaves if the signals are actually queued.  The purpose
of the knob as is is to allow for easier diagnostic of the programs that
need the adjustments, to confirm the cause of problem.

Reported by:	dchagin
Reviewed by:	dchagin, markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D30675
This commit is contained in:
Konstantin Belousov 2021-06-05 15:42:27 +03:00
parent acced8b043
commit bc38762474
3 changed files with 40 additions and 27 deletions
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65
sys/kern/kern_sig.c
View File

@ -163,6 +163,12 @@ SYSCTL_BOOL(_kern, OID_AUTO, sigfastblock_fetch_always, CTLFLAG_RWTUN,
"Fetch sigfastblock word on each syscall entry for proper "
"blocking semantic");
static bool kern_sig_discard_ign = true;
SYSCTL_BOOL(_kern, OID_AUTO, sig_discard_ign, CTLFLAG_RWTUN,
&kern_sig_discard_ign, 0,
"Discard ignored signals on delivery, otherwise queue them to "
"the target queue");
SYSINIT(signal, SI_SUB_P1003_1B, SI_ORDER_FIRST+3, sigqueue_start, NULL);
/*
@ -1290,6 +1296,9 @@ kern_sigtimedwait(struct thread *td, sigset_t waitset, ksiginfo_t *ksi,
PROC_LOCK(p);
saved_mask = td->td_sigmask;
SIGSETNAND(td->td_sigmask, waitset);
if ((p->p_sysent->sv_flags & SV_SIG_DISCIGN) != 0 ||
!kern_sig_discard_ign)
td->td_pflags2 |= TDP2_SIGWAIT;
for (;;) {
mtx_lock(&ps->ps_mtx);
sig = cursig(td);
@ -1352,6 +1361,7 @@ kern_sigtimedwait(struct thread *td, sigset_t waitset, ksiginfo_t *ksi,
if (error == 0 && (p->p_ptevents & PTRACE_SYSCALL) != 0)
traced = true;
}
td->td_pflags2 &= ~TDP2_SIGWAIT;
new_block = saved_mask;
SIGSETNAND(new_block, td->td_sigmask);
@ -2200,22 +2210,25 @@ tdsendsignal(struct proc *p, struct thread *td, int sig, ksiginfo_t *ksi)
SDT_PROBE3(proc, , , signal__send, td, p, sig);
/*
* If the signal is being ignored,
* then we forget about it immediately.
* (Note: we don't set SIGCONT in ps_sigignore,
* and if it is set to SIG_IGN,
* action will be SIG_DFL here.)
* If the signal is being ignored, then we forget about it
* immediately, except when the target process executes
* sigwait(). (Note: we don't set SIGCONT in ps_sigignore,
* and if it is set to SIG_IGN, action will be SIG_DFL here.)
*/
mtx_lock(&ps->ps_mtx);
if (SIGISMEMBER(ps->ps_sigignore, sig)) {
SDT_PROBE3(proc, , , signal__discard, td, p, sig);
if (kern_sig_discard_ign &&
(p->p_sysent->sv_flags & SV_SIG_DISCIGN) == 0) {
SDT_PROBE3(proc, , , signal__discard, td, p, sig);
mtx_unlock(&ps->ps_mtx);
if (ksi && (ksi->ksi_flags & KSI_INS))
ksiginfo_tryfree(ksi);
return (ret);
}
if (SIGISMEMBER(td->td_sigmask, sig))
mtx_unlock(&ps->ps_mtx);
if (ksi && (ksi->ksi_flags & KSI_INS))
ksiginfo_tryfree(ksi);
return (ret);
} else {
action = SIG_CATCH;
}
} else if (SIGISMEMBER(td->td_sigmask, sig))
action = SIG_HOLD;
else if (SIGISMEMBER(ps->ps_sigcatch, sig))
action = SIG_CATCH;
@ -2950,11 +2963,13 @@ issignal(struct thread *td)
}
/*
* We should see pending but ignored signals
* only if P_TRACED was on when they were posted.
* We should allow pending but ignored signals below
* only if there is sigwait() active, or P_TRACED was
* on when they were posted.
*/
if (SIGISMEMBER(ps->ps_sigignore, sig) &&
(p->p_flag & P_TRACED) == 0) {
(p->p_flag & P_TRACED) == 0 &&
(td->td_pflags2 & TDP2_SIGWAIT) == 0) {
sigqueue_delete(&td->td_sigqueue, sig);
sigqueue_delete(&p->p_sigqueue, sig);
continue;
@ -3066,10 +3081,11 @@ issignal(struct thread *td)
PROC_SUNLOCK(p);
mtx_lock(&ps->ps_mtx);
goto next;
} else if (prop & SIGPROP_IGNORE) {
} else if ((prop & SIGPROP_IGNORE) != 0 &&
(td->td_pflags2 & TDP2_SIGWAIT) == 0) {
/*
* Except for SIGCONT, shouldn't get here.
* Default action is to ignore; drop it.
* Default action is to ignore; drop it if
* not in kern_sigtimedwait().
*/
break; /* == ignore */
} else
@ -3077,15 +3093,10 @@ issignal(struct thread *td)
/*NOTREACHED*/
case (intptr_t)SIG_IGN:
/*
* Masking above should prevent us ever trying
* to take action on an ignored signal other
* than SIGCONT, unless process is traced.
*/
if ((prop & SIGPROP_CONT) == 0 &&
(p->p_flag & P_TRACED) == 0)
printf("issignal\n");
break; /* == ignore */
if ((td->td_pflags2 & TDP2_SIGWAIT) == 0)
break; /* == ignore */
else
return (sig);
default:
/*

1
sys/sys/proc.h
View File

@ -529,6 +529,7 @@ do { \
#define TDP2_SBPAGES 0x00000001 /* Owns sbusy on some pages */
#define TDP2_COMPAT32RB 0x00000002 /* compat32 ABI for robust lists */
#define TDP2_ACCT 0x00000004 /* Doing accounting */
#define TDP2_SIGWAIT 0x00000008 /* Ignore ignored signals */
/*
* Reasons that the current thread can not be run yet.

1
sys/sys/sysent.h
View File

@ -161,6 +161,7 @@ struct sysentvec {
#define SV_TIMEKEEP 0x040000 /* Shared page timehands. */
#define SV_ASLR 0x080000 /* ASLR allowed. */
#define SV_RNG_SEED_VER 0x100000 /* random(4) reseed generation. */
#define SV_SIG_DISCIGN 0x200000 /* Do not discard ignored signals */
#define SV_ABI_MASK 0xff
#define SV_PROC_FLAG(p, x) ((p)->p_sysent->sv_flags & (x))