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fork: avoid endless wait with PTRACE_FORK and RFSTOPPED.

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An RFSTOPPED thread can't clean TDB_STOPATFORK, which is done in the
fork_return() in its context, so parent is stuck forever.  Triggered
when trying to ptrace linux process.  Instead of waiting for the new
thread to clear TDB_STOPATFORK, tag it as traced and reparent to the
debugger in do_fork(), and let it only notify the debugger when run.

Submitted by:	Yanko Yankulov <yanko.yankulov@gmail.com>
Reviewed by:	jhb
MFC after:	1 week
X-MFC-Note:	keep p_dbgwait placeholder intact
Differential revision:	https://reviews.freebsd.org/D15857
This commit is contained in:
Konstantin Belousov 2018-06-21 21:12:49 +00:00
parent ac4bc0c171
commit 6e22bbf66e
4 changed files with 39 additions and 36 deletions
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71
sys/kern/kern_fork.c
View File

@ -721,18 +721,6 @@ do_fork(struct thread *td, struct fork_req *fr, struct proc *p2, struct thread *
* but before we wait for the debugger.
*/
_PHOLD(p2);
if (p1->p_ptevents & PTRACE_FORK) {
/*
* Arrange for debugger to receive the fork event.
*
* We can report PL_FLAG_FORKED regardless of
* P_FOLLOWFORK settings, but it does not make a sense
* for runaway child.
*/
td->td_dbgflags |= TDB_FORK;
td->td_dbg_forked = p2->p_pid;
td2->td_dbgflags |= TDB_STOPATFORK;
}
if (fr->fr_flags & RFPPWAIT) {
td->td_pflags |= TDP_RFPPWAIT;
td->td_rfppwait_p = p2;
@ -756,7 +744,42 @@ do_fork(struct thread *td, struct fork_req *fr, struct proc *p2, struct thread *
procdesc_finit(p2->p_procdesc, fp_procdesc);
fdrop(fp_procdesc, td);
}
/*
* Speculative check for PTRACE_FORK. PTRACE_FORK is not
* synced with forks in progress so it is OK if we miss it
* if being set atm.
*/
if ((p1->p_ptevents & PTRACE_FORK) != 0) {
sx_xlock(&proctree_lock);
PROC_LOCK(p2);
/*
* p1->p_ptevents & p1->p_pptr are protected by both
* process and proctree locks for modifications,
* so owning proctree_lock allows the race-free read.
*/
if ((p1->p_ptevents & PTRACE_FORK) != 0) {
/*
* Arrange for debugger to receive the fork event.
*
* We can report PL_FLAG_FORKED regardless of
* P_FOLLOWFORK settings, but it does not make a sense
* for runaway child.
*/
td->td_dbgflags |= TDB_FORK;
td->td_dbg_forked = p2->p_pid;
td2->td_dbgflags |= TDB_STOPATFORK;
proc_set_traced(p2, true);
CTR2(KTR_PTRACE,
"do_fork: attaching to new child pid %d: oppid %d",
p2->p_pid, p2->p_oppid);
proc_reparent(p2, p1->p_pptr);
}
PROC_UNLOCK(p2);
sx_xunlock(&proctree_lock);
}
if ((fr->fr_flags & RFSTOPPED) == 0) {
/*
* If RFSTOPPED not requested, make child runnable and
@ -773,11 +796,6 @@ do_fork(struct thread *td, struct fork_req *fr, struct proc *p2, struct thread *
}
PROC_LOCK(p2);
/*
* Wait until debugger is attached to child.
*/
while (td2->td_proc == p2 && (td2->td_dbgflags & TDB_STOPATFORK) != 0)
cv_wait(&p2->p_dbgwait, &p2->p_mtx);
_PRELE(p2);
racct_proc_fork_done(p2);
PROC_UNLOCK(p2);
@ -1063,24 +1081,15 @@ fork_exit(void (*callout)(void *, struct trapframe *), void *arg,
void
fork_return(struct thread *td, struct trapframe *frame)
{
struct proc *p, *dbg;
struct proc *p;
p = td->td_proc;
if (td->td_dbgflags & TDB_STOPATFORK) {
sx_xlock(&proctree_lock);
PROC_LOCK(p);
if (p->p_pptr->p_ptevents & PTRACE_FORK) {
if ((p->p_flag & P_TRACED) != 0) {
/*
* If debugger still wants auto-attach for the
* parent's children, do it now.
* Inform the debugger if one is still present.
*/
dbg = p->p_pptr->p_pptr;
proc_set_traced(p, true);
CTR2(KTR_PTRACE,
"fork_return: attaching to new child pid %d: oppid %d",
p->p_pid, p->p_oppid);
proc_reparent(p, dbg);
sx_xunlock(&proctree_lock);
td->td_dbgflags |= TDB_CHILD | TDB_SCX | TDB_FSTP;
ptracestop(td, SIGSTOP, NULL);
td->td_dbgflags &= ~(TDB_CHILD | TDB_SCX);
@ -1088,9 +1097,7 @@ fork_return(struct thread *td, struct trapframe *frame)
/*
* ... otherwise clear the request.
*/
sx_xunlock(&proctree_lock);
td->td_dbgflags &= ~TDB_STOPATFORK;
cv_broadcast(&p->p_dbgwait);
}
PROC_UNLOCK(p);
} else if (p->p_flag & P_TRACED || td->td_dbgflags & TDB_BORN) {

1
sys/kern/kern_proc.c
View File

@ -266,7 +266,6 @@ proc_init(void *mem, int size, int flags)
mtx_init(&p->p_itimmtx, "pitiml", NULL, MTX_SPIN | MTX_NEW);
mtx_init(&p->p_profmtx, "pprofl", NULL, MTX_SPIN | MTX_NEW);
cv_init(&p->p_pwait, "ppwait");
cv_init(&p->p_dbgwait, "dbgwait");
TAILQ_INIT(&p->p_threads); /* all threads in proc */
EVENTHANDLER_DIRECT_INVOKE(process_init, p);
p->p_stats = pstats_alloc();

1
sys/kern/kern_sig.c
View File

@ -2581,7 +2581,6 @@ ptracestop(struct thread *td, int sig, ksiginfo_t *si)
}
if ((td->td_dbgflags & TDB_STOPATFORK) != 0) {
td->td_dbgflags &= ~TDB_STOPATFORK;
cv_broadcast(&p->p_dbgwait);
}
stopme:
thread_suspend_switch(td, p);

2
sys/sys/proc.h
View File

@ -685,8 +685,6 @@ struct proc {
LIST_HEAD(, mqueue_notifier) p_mqnotifier; /* (c) mqueue notifiers.*/
struct kdtrace_proc *p_dtrace; /* (*) DTrace-specific data. */
struct cv p_pwait; /* (*) wait cv for exit/exec. */
struct cv p_dbgwait; /* (*) wait cv for debugger attach
after fork. */
uint64_t p_prev_runtime; /* (c) Resource usage accounting. */
struct racct *p_racct; /* (b) Resource accounting. */
int p_throttled; /* (c) Flag for racct pcpu throttling */