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Discard the correct thread event reported for a ptrace stop.

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When multiple threads wish to report a tracing event to a debugger,
both threads call ptracestop() and one thread will win the race to be
the reporting thread (p->p_xthread).  The debugger uses PT_LWPINFO
with the process ID to determine which thread / LWP is reporting an
event and the details of that event.  This event is cleared as a side
effect of the subsequent ptrace event that resumed the process
(PT_CONTINUE, PT_STEP, etc.).  However, ptrace() was clearing the
event identified by the LWP ID passed to the resume request even if
that wasn't the 'p_xthread'.  This could result in clearing an event
that had not yet been observed by the debugger and leaving the
existing event for 'p_thread' pending so that it was reported a second
time.

Specifically, if the debugger stopped due to a software breakpoint in
one thread, but then switched to another thread that was used to
resume (e.g. if the user switched to a different thread and issued a
step), the resume request (PT_STEP) cleared a pending event (if any)
for the thread being stepped.  However, the process immediately
stopped and the first thread reported it's breakpoint event a second
time.  The debugger decremented the PC for "both" breakpoint events
which resulted in the PC now pointing into the middle of an
instruction (on x86) and a SIGILL fault when the process was resumed a
second time.

To fix, always clear the pending event for 'p_xthread' when resuming a
process.  ptrace() still honors the requested LWP ID when enabling
single-stepping (PT_STEP) or setting a different PC (PT_CONTINUE).

Reported by:	GDB testsuite (gdb.threads/continue-pending-status.exp)
Reviewed by:	kib
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D12794
This commit is contained in:
jhb 2017-10-27 03:16:19 +00:00
parent 2793077fcd
commit 0472a49b7f
3 changed files with 179 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
sys/kern/sys_process.c
View File

@ -1129,6 +1129,13 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
}
sendsig:
/*
* Clear the pending event for the thread that just
* reported its event (p_xthread). This may not be
* the thread passed to PT_CONTINUE, PT_STEP, etc. if
* the debugger is resuming a different thread.
*/
td2 = p->p_xthread;
if (proctree_locked) {
sx_xunlock(&proctree_lock);
proctree_locked = 0;

2
sys/sys/param.h
View File

@ -58,7 +58,7 @@
* in the range 5 to 9.
*/
#undef __FreeBSD_version
#define __FreeBSD_version 1200051 /* Master, propagated to newvers */
#define __FreeBSD_version 1200052 /* Master, propagated to newvers */
/*
* __FreeBSD_kernel__ indicates that this system uses the kernel of FreeBSD,

171
tests/sys/kern/ptrace_test.c
View File

@ -3463,6 +3463,174 @@ ATF_TC_BODY(ptrace__PT_STEP_with_signal, tc)
ATF_REQUIRE(errno == ECHILD);
}
#if defined(__amd64__) || defined(__i386__)
/*
* Only x86 both define breakpoint() and have a PC after breakpoint so
* that restarting doesn't retrigger the breakpoint.
*/
static void *
continue_thread(void *arg)
{
breakpoint();
return (NULL);
}
static __dead2 void
continue_thread_main(void)
{
pthread_t threads[2];
CHILD_REQUIRE(pthread_create(&threads[0], NULL, continue_thread,
NULL) == 0);
CHILD_REQUIRE(pthread_create(&threads[1], NULL, continue_thread,
NULL) == 0);
CHILD_REQUIRE(pthread_join(threads[0], NULL) == 0);
CHILD_REQUIRE(pthread_join(threads[1], NULL) == 0);
exit(1);
}
/*
* Ensure that PT_CONTINUE clears the status of the thread that
* triggered the stop even if a different thread's LWP was passed to
* PT_CONTINUE.
*/
ATF_TC_WITHOUT_HEAD(ptrace__PT_CONTINUE_different_thread);
ATF_TC_BODY(ptrace__PT_CONTINUE_different_thread, tc)
{
struct ptrace_lwpinfo pl;
pid_t fpid, wpid;
lwpid_t lwps[2];
bool hit_break[2];
int i, j, status;
ATF_REQUIRE((fpid = fork()) != -1);
if (fpid == 0) {
trace_me();
continue_thread_main();
}
/* The first wait() should report the stop from SIGSTOP. */
wpid = waitpid(fpid, &status, 0);
ATF_REQUIRE(wpid == fpid);
ATF_REQUIRE(WIFSTOPPED(status));
ATF_REQUIRE(WSTOPSIG(status) == SIGSTOP);
ATF_REQUIRE(ptrace(PT_LWPINFO, wpid, (caddr_t)&pl,
sizeof(pl)) != -1);
ATF_REQUIRE(ptrace(PT_LWP_EVENTS, wpid, NULL, 1) == 0);
/* Continue the child ignoring the SIGSTOP. */
ATF_REQUIRE(ptrace(PT_CONTINUE, fpid, (caddr_t)1, 0) == 0);
/* One of the new threads should report it's birth. */
wpid = waitpid(fpid, &status, 0);
ATF_REQUIRE(wpid == fpid);
ATF_REQUIRE(WIFSTOPPED(status));
ATF_REQUIRE(WSTOPSIG(status) == SIGTRAP);
ATF_REQUIRE(ptrace(PT_LWPINFO, wpid, (caddr_t)&pl, sizeof(pl)) != -1);
ATF_REQUIRE((pl.pl_flags & (PL_FLAG_BORN | PL_FLAG_SCX)) ==
(PL_FLAG_BORN | PL_FLAG_SCX));
lwps[0] = pl.pl_lwpid;
/*
* Suspend this thread to ensure both threads are alive before
* hitting the breakpoint.
*/
ATF_REQUIRE(ptrace(PT_SUSPEND, lwps[0], NULL, 0) != -1);
ATF_REQUIRE(ptrace(PT_CONTINUE, fpid, (caddr_t)1, 0) == 0);
/* Second thread should report it's birth. */
wpid = waitpid(fpid, &status, 0);
ATF_REQUIRE(wpid == fpid);
ATF_REQUIRE(WIFSTOPPED(status));
ATF_REQUIRE(WSTOPSIG(status) == SIGTRAP);
ATF_REQUIRE(ptrace(PT_LWPINFO, wpid, (caddr_t)&pl, sizeof(pl)) != -1);
ATF_REQUIRE((pl.pl_flags & (PL_FLAG_BORN | PL_FLAG_SCX)) ==
(PL_FLAG_BORN | PL_FLAG_SCX));
ATF_REQUIRE(pl.pl_lwpid != lwps[0]);
lwps[1] = pl.pl_lwpid;
/* Resume both threads waiting for breakpoint events. */
hit_break[0] = hit_break[1] = false;
ATF_REQUIRE(ptrace(PT_RESUME, lwps[0], NULL, 0) != -1);
ATF_REQUIRE(ptrace(PT_CONTINUE, fpid, (caddr_t)1, 0) == 0);
/* One thread should report a breakpoint. */
wpid = waitpid(fpid, &status, 0);
ATF_REQUIRE(wpid == fpid);
ATF_REQUIRE(WIFSTOPPED(status));
ATF_REQUIRE(WSTOPSIG(status) == SIGTRAP);
ATF_REQUIRE(ptrace(PT_LWPINFO, wpid, (caddr_t)&pl, sizeof(pl)) != -1);
ATF_REQUIRE((pl.pl_flags & PL_FLAG_SI) != 0);
ATF_REQUIRE(pl.pl_siginfo.si_signo == SIGTRAP &&
pl.pl_siginfo.si_code == TRAP_BRKPT);
if (pl.pl_lwpid == lwps[0])
i = 0;
else
i = 1;
hit_break[i] = true;
/*
* Resume both threads but pass the other thread's LWPID to
* PT_CONTINUE.
*/
ATF_REQUIRE(ptrace(PT_CONTINUE, lwps[i ^ 1], (caddr_t)1, 0) == 0);
/*
* Will now get two thread exit events and one more breakpoint
* event.
*/
for (j = 0; j < 3; j++) {
wpid = waitpid(fpid, &status, 0);
ATF_REQUIRE(wpid == fpid);
ATF_REQUIRE(WIFSTOPPED(status));
ATF_REQUIRE(WSTOPSIG(status) == SIGTRAP);
ATF_REQUIRE(ptrace(PT_LWPINFO, wpid, (caddr_t)&pl,
sizeof(pl)) != -1);
if (pl.pl_lwpid == lwps[0])
i = 0;
else
i = 1;
ATF_REQUIRE_MSG(lwps[i] != 0, "event for exited thread");
if (pl.pl_flags & PL_FLAG_EXITED) {
ATF_REQUIRE_MSG(hit_break[i],
"exited thread did not report breakpoint");
lwps[i] = 0;
} else {
ATF_REQUIRE((pl.pl_flags & PL_FLAG_SI) != 0);
ATF_REQUIRE(pl.pl_siginfo.si_signo == SIGTRAP &&
pl.pl_siginfo.si_code == TRAP_BRKPT);
ATF_REQUIRE_MSG(!hit_break[i],
"double breakpoint event");
hit_break[i] = true;
}
ATF_REQUIRE(ptrace(PT_CONTINUE, fpid, (caddr_t)1, 0) == 0);
}
/* Both threads should have exited. */
ATF_REQUIRE(lwps[0] == 0);
ATF_REQUIRE(lwps[1] == 0);
/* The last event should be for the child process's exit. */
wpid = waitpid(fpid, &status, 0);
ATF_REQUIRE(WIFEXITED(status));
ATF_REQUIRE(WEXITSTATUS(status) == 1);
wpid = wait(&status);
ATF_REQUIRE(wpid == -1);
ATF_REQUIRE(errno == ECHILD);
}
#endif
ATF_TP_ADD_TCS(tp)
{
@ -3515,6 +3683,9 @@ ATF_TP_ADD_TCS(tp)
ATF_TP_ADD_TC(tp, ptrace__event_mask_sigkill_discard);
ATF_TP_ADD_TC(tp, ptrace__PT_ATTACH_with_SBDRY_thread);
ATF_TP_ADD_TC(tp, ptrace__PT_STEP_with_signal);
#if defined(__amd64__) || defined(__i386__)
ATF_TP_ADD_TC(tp, ptrace__PT_CONTINUE_different_thread);
#endif
return (atf_no_error());
}