[PATCH] Fix signal handler to detect crashes in qemu-linux-user

[Helge Deller <deller@gmx.de>]

If there is an internal program error in the qemu source code which
triggers a SIGSEGV, qemu will currently assume this is a SIGSEGV of the
target and print:

(hppa-chroot)root@p100:/# cat /proc/self/maps
**
ERROR:../../home/cvs/qemu/qemu/accel/tcg/cpu-exec.c:532:cpu_exec_longjmp_cleanup: assertion failed: (cpu == current_cpu)
Bail out! ERROR:../../home/cvs/qemu/qemu/accel/tcg/cpu-exec.c:532:cpu_exec_longjmp_cleanup: assertion failed: (cpu == current_cpu)
**

This error message is very misleading for developers and end-users.

The attached patch will print instead:

(hppa-chroot)root@p100:/# cat /proc/self/maps
QEMU linux-user v8.0.93 for target parisc
QEMU internal error: signal=11, errno=0, code=1, addr=(nil)
while running: /usr/bin/cat
QEMU backtrace:
[0x7f70cd045115]
[0x7f70cd21b140]
[0x7f70cd04ec49]
[0x7f70cd04ec6b]
[0x7f70cd0597e2]
[0x7f70cd05d9ed]
[0x7f70cd064008]
[0x7f70ccffbd2d]
[0x7f70ccff57f8]
[0x7f70cd205868]
[0x7f70cd206f4f]
[0x7f70ccff60a5]

Note that glibc's backtrace() can not resolve the addresses to function
names for static binaries, which is why only addresses are show above.

Signed-off-by: Helge Deller <deller@gmx.de>

v2:
- Refined crash detection based on IP address, suggested by Richard
- More info in crash dump, e.g. qemu version and target
---
 linux-user/signal.c  | 36 ++++++++++++++++++++++++++++++++++++
 linux-user/syscall.c |  1 +
 2 files changed, 37 insertions(+)

diff --git a/linux-user/signal.c b/linux-user/signal.c
index 748a98f3e5..d445376f06 100644
--- a/linux-user/signal.c
+++ b/linux-user/signal.c
@@ -23,6 +23,7 @@

 #include <sys/ucontext.h>
 #include <sys/resource.h>
+#include <execinfo.h>

 #include "qemu.h"
 #include "user-internals.h"
@@ -781,6 +782,34 @@ static inline void rewind_if_in_safe_syscall(void *puc)
     }
 }

+static void qemu_show_backtrace(siginfo_t *info)
+{
+    void *array[20];
+    char **strings;
+    int size, i;
+
+    fprintf(stderr, "QEMU linux-user v" QEMU_VERSION " for target "
+                     UNAME_MACHINE "\n");
+    fprintf(stderr, "QEMU internal error: signal=%d, errno=%d, "
+                    "code=%d, addr=%p\n",
+                    info->si_signo, info->si_errno, info->si_code,
+                    info->si_addr);
+    fprintf(stderr, "while running: %s\n", exec_path);
+    size = backtrace(array, ARRAY_SIZE(array));
+    strings = backtrace_symbols(array, size);
+    if (strings) {
+        fprintf(stderr, "QEMU backtrace:\n");
+        for (i = 0; i < size; i++)
+            fprintf(stderr, "%s\n", strings[i]);
+    }
+    free (strings);
+    exit(info->si_code);
+}
+
+/* _init and _fini are provided by the linker */
+extern char _init;
+extern char _fini;
+
 static void host_signal_handler(int host_sig, siginfo_t *info, void *puc)
 {
     CPUArchState *env = thread_cpu->env_ptr;
@@ -819,6 +848,13 @@ static void host_signal_handler(int host_sig, siginfo_t *info, void *puc)
         if (host_sig == SIGSEGV) {
             bool maperr = true;

+            /* Did segfault happened in qemu source code? */
+            if ((pc >= (uintptr_t) &_init && pc < (uintptr_t) &_fini) ||
+                (TARGET_ABI_BITS == 32 && HOST_LONG_BITS == 64
+                 && !h2g_valid(host_addr))) {
+                qemu_show_backtrace(info);
+            }
+
             if (info->si_code == SEGV_ACCERR && h2g_valid(host_addr)) {
                 /* If this was a write to a TB protected page, restart. */
                 if (is_write &&
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 9353268cc1..da29d97816 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -8132,6 +8132,7 @@ static int open_self_maps_1(CPUArchState *cpu_env, int fd, bool smaps)
     IntervalTreeNode *s;
     int count;

+*(int*)NULL = 1;
     for (s = interval_tree_iter_first(map_info, 0, -1); s;
          s = interval_tree_iter_next(s, 0, -1)) {
         MapInfo *e = container_of(s, MapInfo, itree);
--
2.41.0