linux-user: SIGSEGV from sigreturn need not be fatal

If the sigreturn syscall fails to read memory then this causes a
SIGSEGV, but this is not necessarily a fatal signal -- the guest
process can catch it.

We don't implement this correctly because the behaviour of QEMU's
force_sig() function has drifted away from the kernel function of the
same name -- ours now does "always do a guest core dump and abort
execution", whereas the kernel version simply forces the guest to
take a signal, which may or may not eventually cause a core dump.

Rename our force_sig() to dump_core_and_abort(), and provide a
force_sig() which acts more like the kernel version as the sigreturn
implementations expect it to.  Since force_sig() now returns, we must
update all the callsites to return -TARGET_QEMU_ESIGRETURN so that
the main loop doesn't change the guest registers before the signal
handler is invoked.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/signal.c

@@ -512,6 +512,27 @@ void signal_init(void)
     }
 }
 
+#if !defined(TARGET_OPENRISC) && !defined(TARGET_UNICORE32) && \
+    !defined(TARGET_X86_64)
+/* Force a synchronously taken signal. The kernel force_sig() function
+ * also forces the signal to "not blocked, not ignored", but for QEMU
+ * that work is done in process_pending_signals().
+ */
+static void force_sig(int sig)
+{
+    CPUState *cpu = thread_cpu;
+    CPUArchState *env = cpu->env_ptr;
+    target_siginfo_t info;
+
+    info.si_signo = sig;
+    info.si_errno = 0;
+    info.si_code = TARGET_SI_KERNEL;
+    info._sifields._kill._pid = 0;
+    info._sifields._kill._uid = 0;
+    queue_signal(env, info.si_signo, QEMU_SI_KILL, &info);
+}
+#endif
+
 #if !(defined(TARGET_X86_64) || defined(TARGET_UNICORE32))
 
 /* Force a SIGSEGV if we couldn't write to memory trying to set
@@ -526,7 +547,7 @@ static void force_sigsegv(int oldsig)
 
     if (oldsig == SIGSEGV) {
         /* Make sure we don't try to deliver the signal again; this will
-         * end up with handle_pending_signal() calling force_sig().
+         * end up with handle_pending_signal() calling dump_core_and_abort().
          */
         sigact_table[oldsig - 1]._sa_handler = TARGET_SIG_DFL;
     }
@@ -540,7 +561,7 @@ static void force_sigsegv(int oldsig)
 #endif
 
 /* abort execution with signal */
-static void QEMU_NORETURN force_sig(int target_sig)
+static void QEMU_NORETURN dump_core_and_abort(int target_sig)
 {
     CPUState *cpu = thread_cpu;
     CPUArchState *env = cpu->env_ptr;
@@ -1181,7 +1202,7 @@ long do_sigreturn(CPUX86State *env)
 badframe:
     unlock_user_struct(frame, frame_addr, 0);
     force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 long do_rt_sigreturn(CPUX86State *env)
@@ -1212,7 +1233,7 @@ long do_rt_sigreturn(CPUX86State *env)
 badframe:
     unlock_user_struct(frame, frame_addr, 0);
     force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 #elif defined(TARGET_AARCH64)
@@ -1482,7 +1503,7 @@ long do_rt_sigreturn(CPUARMState *env)
  badframe:
     unlock_user_struct(frame, frame_addr, 0);
     force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 long do_sigreturn(CPUARMState *env)
@@ -2004,8 +2025,8 @@ static long do_sigreturn_v1(CPUARMState *env)
     return -TARGET_QEMU_ESIGRETURN;
 
 badframe:
-    force_sig(TARGET_SIGSEGV /* , current */);
+    force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 static abi_ulong *restore_sigframe_v2_vfp(CPUARMState *env, abi_ulong *regspace)
@@ -2131,8 +2152,8 @@ static long do_sigreturn_v2(CPUARMState *env)
 
 badframe:
     unlock_user_struct(frame, frame_addr, 0);
-    force_sig(TARGET_SIGSEGV /* , current */);
+    force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 long do_sigreturn(CPUARMState *env)
@@ -2185,8 +2206,8 @@ static long do_rt_sigreturn_v1(CPUARMState *env)
 
 badframe:
     unlock_user_struct(frame, frame_addr, 0);
-    force_sig(TARGET_SIGSEGV /* , current */);
+    force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 static long do_rt_sigreturn_v2(CPUARMState *env)
@@ -2218,8 +2239,8 @@ static long do_rt_sigreturn_v2(CPUARMState *env)
 
 badframe:
     unlock_user_struct(frame, frame_addr, 0);
-    force_sig(TARGET_SIGSEGV /* , current */);
+    force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 long do_rt_sigreturn(CPUARMState *env)
@@ -2553,6 +2574,7 @@ long do_sigreturn(CPUSPARCState *env)
 segv_and_exit:
     unlock_user_struct(sf, sf_addr, 0);
     force_sig(TARGET_SIGSEGV);
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 long do_rt_sigreturn(CPUSPARCState *env)
@@ -3110,8 +3132,8 @@ long do_sigreturn(CPUMIPSState *regs)
     return -TARGET_QEMU_ESIGRETURN;
 
 badframe:
-    force_sig(TARGET_SIGSEGV/*, current*/);
+    force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 # endif /* O32 */
 
@@ -3207,8 +3229,8 @@ long do_rt_sigreturn(CPUMIPSState *env)
     return -TARGET_QEMU_ESIGRETURN;
 
 badframe:
-    force_sig(TARGET_SIGSEGV/*, current*/);
+    force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 #elif defined(TARGET_SH4)
@@ -3474,7 +3496,7 @@ long do_sigreturn(CPUSH4State *regs)
 badframe:
     unlock_user_struct(frame, frame_addr, 0);
     force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 long do_rt_sigreturn(CPUSH4State *regs)
@@ -3506,7 +3528,7 @@ long do_rt_sigreturn(CPUSH4State *regs)
 badframe:
     unlock_user_struct(frame, frame_addr, 0);
     force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 #elif defined(TARGET_MICROBLAZE)
 
@@ -3725,6 +3747,7 @@ long do_sigreturn(CPUMBState *env)
     return -TARGET_QEMU_ESIGRETURN;
 badframe:
     force_sig(TARGET_SIGSEGV);
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 long do_rt_sigreturn(CPUMBState *env)
@@ -3892,6 +3915,7 @@ long do_sigreturn(CPUCRISState *env)
     return -TARGET_QEMU_ESIGRETURN;
 badframe:
     force_sig(TARGET_SIGSEGV);
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 long do_rt_sigreturn(CPUCRISState *env)
@@ -4383,7 +4407,7 @@ long do_sigreturn(CPUS390XState *env)
 
 badframe:
     force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 long do_rt_sigreturn(CPUS390XState *env)
@@ -4414,7 +4438,7 @@ long do_rt_sigreturn(CPUS390XState *env)
 badframe:
     unlock_user_struct(frame, frame_addr, 0);
     force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 #elif defined(TARGET_PPC)
@@ -4973,7 +4997,7 @@ sigsegv:
     unlock_user_struct(sr, sr_addr, 1);
     unlock_user_struct(sc, sc_addr, 1);
     force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 /* See arch/powerpc/kernel/signal_32.c.  */
@@ -5028,7 +5052,7 @@ long do_rt_sigreturn(CPUPPCState *env)
 sigsegv:
     unlock_user_struct(rt_sf, rt_sf_addr, 1);
     force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 #elif defined(TARGET_M68K)
@@ -5358,7 +5382,7 @@ long do_sigreturn(CPUM68KState *env)
 
 badframe:
     force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 long do_rt_sigreturn(CPUM68KState *env)
@@ -5391,7 +5415,7 @@ long do_rt_sigreturn(CPUM68KState *env)
 badframe:
     unlock_user_struct(frame, frame_addr, 0);
     force_sig(TARGET_SIGSEGV);
-    return 0;
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 #elif defined(TARGET_ALPHA)
@@ -5620,6 +5644,7 @@ long do_sigreturn(CPUAlphaState *env)
 
 badframe:
     force_sig(TARGET_SIGSEGV);
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 long do_rt_sigreturn(CPUAlphaState *env)
@@ -5649,6 +5674,7 @@ long do_rt_sigreturn(CPUAlphaState *env)
 badframe:
     unlock_user_struct(frame, frame_addr, 0);
     force_sig(TARGET_SIGSEGV);
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 #elif defined(TARGET_TILEGX)
@@ -5813,6 +5839,7 @@ long do_rt_sigreturn(CPUTLGState *env)
  badframe:
     unlock_user_struct(frame, frame_addr, 0);
     force_sig(TARGET_SIGSEGV);
+    return -TARGET_QEMU_ESIGRETURN;
 }
 
 #else
@@ -5879,12 +5906,12 @@ static void handle_pending_signal(CPUArchState *cpu_env, int sig,
                    sig != TARGET_SIGURG &&
                    sig != TARGET_SIGWINCH &&
                    sig != TARGET_SIGCONT) {
-            force_sig(sig);
+            dump_core_and_abort(sig);
         }
     } else if (handler == TARGET_SIG_IGN) {
         /* ignore sig */
     } else if (handler == TARGET_SIG_ERR) {
-        force_sig(sig);
+        dump_core_and_abort(sig);
     } else {
         /* compute the blocked signals during the handler execution */
         sigset_t *blocked_set;