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From: Ilya Leoshkevich <iii@linux.ibm.com>
To: Warner Losh <imp@bsdimp.com>, Riku Voipio <riku.voipio@iki.fi>,
Laurent Vivier <laurent@vivier.eu>, Paolo Bonzini <pbonzini@redhat.com>,
Richard Henderson <richard.henderson@linaro.org>
Cc: Kyle Evans <kevans@freebsd.org>,
=?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= <philmd@linaro.org>,
qemu-devel@nongnu.org, Ilya Leoshkevich <iii@linux.ibm.com>
Subject: [PATCH v3 4/8] user: Introduce host_interrupt_signal
Date: Mon, 16 Dec 2024 13:33:22 +0100
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Attaching to the gdbstub of a running process requires stopping its
threads. For threads that run on a CPU, cpu_exit() is enough, but the
only way to grab attention of a thread that is stuck in a long-running
syscall is to interrupt it with a signal.
Reserve a host realtime signal for this, just like it's already done
for TARGET_SIGABRT on Linux. This may reduce the number of available
guest realtime signals by one, but this is acceptable, since there are
quite a lot of them, and it's unlikely that there are apps that need
them all.
Set signal_pending for the safe_sycall machinery to prevent invoking
the syscall. This is a lie, since we don't queue a guest signal, but
process_pending_signals() can handle the absence of pending signals.
The syscall returns with QEMU_ERESTARTSYS errno, which arranges for
the automatic restart. This is important, because it helps avoiding
disturbing poorly written guests.
Reviewed-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
bsd-user/signal.c | 12 ++++++++++++
include/user/signal.h | 2 ++
linux-user/signal.c | 25 ++++++++++++++++++++-----
3 files changed, 34 insertions(+), 5 deletions(-)
diff --git a/bsd-user/signal.c b/bsd-user/signal.c
index a2b11a97131..992736df5c5 100644
--- a/bsd-user/signal.c
+++ b/bsd-user/signal.c
@@ -49,6 +49,8 @@ static inline int sas_ss_flags(TaskState *ts, unsigned lo=
ng sp)
on_sig_stack(ts, sp) ? SS_ONSTACK : 0;
}
=20
+int host_interrupt_signal =3D SIGRTMAX;
+
/*
* The BSD ABIs use the same signal numbers across all the CPU architectur=
es, so
* (unlike Linux) these functions are just the identity mapping. This migh=
t not
@@ -489,6 +491,12 @@ static void host_signal_handler(int host_sig, siginfo_=
t *info, void *puc)
uintptr_t pc =3D 0;
bool sync_sig =3D false;
=20
+ if (host_sig =3D=3D host_interrupt_signal) {
+ ts->signal_pending =3D 1;
+ cpu_exit(thread_cpu);
+ return;
+ }
+
/*
* Non-spoofed SIGSEGV and SIGBUS are synchronous, and need special
* handling wrt signal blocking and unwinding.
@@ -852,6 +860,9 @@ void signal_init(void)
=20
for (i =3D 1; i <=3D TARGET_NSIG; i++) {
host_sig =3D target_to_host_signal(i);
+ if (host_sig =3D=3D host_interrupt_signal) {
+ continue;
+ }
sigaction(host_sig, NULL, &oact);
if (oact.sa_sigaction =3D=3D (void *)SIG_IGN) {
sigact_table[i - 1]._sa_handler =3D TARGET_SIG_IGN;
@@ -870,6 +881,7 @@ void signal_init(void)
sigaction(host_sig, &act, NULL);
}
}
+ sigaction(host_interrupt_signal, &act, NULL);
}
=20
static void handle_pending_signal(CPUArchState *env, int sig,
diff --git a/include/user/signal.h b/include/user/signal.h
index 19b6b9e5ddc..7fa33b05d91 100644
--- a/include/user/signal.h
+++ b/include/user/signal.h
@@ -20,4 +20,6 @@
*/
int target_to_host_signal(int sig);
=20
+extern int host_interrupt_signal;
+
#endif
diff --git a/linux-user/signal.c b/linux-user/signal.c
index b1c468cf5c3..551c6bb1448 100644
--- a/linux-user/signal.c
+++ b/linux-user/signal.c
@@ -515,6 +515,8 @@ static int core_dump_signal(int sig)
}
}
=20
+int host_interrupt_signal;
+
static void signal_table_init(const char *rtsig_map)
{
int hsig, tsig, count;
@@ -578,10 +580,10 @@ static void signal_table_init(const char *rtsig_map)
* Attempts for configure "missing" signals via sigaction will be
* silently ignored.
*
- * Reserve one signal for internal usage (see below).
+ * Reserve two signals for internal usage (see below).
*/
=20
- hsig =3D SIGRTMIN + 1;
+ hsig =3D SIGRTMIN + 2;
for (tsig =3D TARGET_SIGRTMIN;
hsig <=3D SIGRTMAX && tsig <=3D TARGET_NSIG;
hsig++, tsig++) {
@@ -602,12 +604,17 @@ static void signal_table_init(const char *rtsig_map)
host_to_target_signal_table[SIGABRT] =3D 0;
for (hsig =3D SIGRTMIN; hsig <=3D SIGRTMAX; hsig++) {
if (!host_to_target_signal_table[hsig]) {
- host_to_target_signal_table[hsig] =3D TARGET_SIGABRT;
- break;
+ if (host_interrupt_signal) {
+ host_to_target_signal_table[hsig] =3D TARGET_SIGABRT;
+ break;
+ } else {
+ host_interrupt_signal =3D hsig;
+ }
}
}
if (hsig > SIGRTMAX) {
- fprintf(stderr, "No rt signals left for SIGABRT mapping\n");
+ fprintf(stderr,
+ "No rt signals left for interrupt and SIGABRT mapping\n");
exit(EXIT_FAILURE);
}
=20
@@ -687,6 +694,8 @@ void signal_init(const char *rtsig_map)
}
sigact_table[tsig - 1]._sa_handler =3D thand;
}
+
+ sigaction(host_interrupt_signal, &act, NULL);
}
=20
/* Force a synchronously taken signal. The kernel force_sig() function
@@ -1034,6 +1043,12 @@ static void host_signal_handler(int host_sig, siginf=
o_t *info, void *puc)
bool sync_sig =3D false;
void *sigmask;
=20
+ if (host_sig =3D=3D host_interrupt_signal) {
+ ts->signal_pending =3D 1;
+ cpu_exit(thread_cpu);
+ return;
+ }
+
/*
* Non-spoofed SIGSEGV and SIGBUS are synchronous, and need special
* handling wrt signal blocking and unwinding. Non-spoofed SIGILL,
--=20
2.47.0