[Qemu-devel] [PATCH 2/2] char: use a fixed idx for child muxed chr

["Marc-André Lureau" <marcandre.lureau@redhat.com>]

mux_chr_update_read_handler() is adding a new mux_cnt each time
mux_chr_update_read_handler() is called, it's not possible to actually
update the "child" chr callbacks that were set previously. This may lead
to crashes if the "child" chr is destroyed:

valgrind x86_64-softmmu/qemu-system-x86_64 -chardev
stdio,mux=on,id=char0 -mon chardev=char0,mode=control,default

when quitting:

==4306== Invalid read of size 8
==4306==    at 0x8061D3: json_lexer_destroy (json-lexer.c:385)
==4306==    by 0x7E39F8: json_message_parser_destroy (json-streamer.c:134)
==4306==    by 0x3447F6: monitor_qmp_event (monitor.c:3908)
==4306==    by 0x480153: mux_chr_send_event (qemu-char.c:630)
==4306==    by 0x480694: mux_chr_event (qemu-char.c:734)
==4306==    by 0x47F1E9: qemu_chr_be_event (qemu-char.c:205)
==4306==    by 0x481207: fd_chr_close (qemu-char.c:1114)
==4306==    by 0x481659: qemu_chr_close_stdio (qemu-char.c:1221)
==4306==    by 0x486F07: qemu_chr_free (qemu-char.c:4146)
==4306==    by 0x486F97: qemu_chr_delete (qemu-char.c:4154)
==4306==    by 0x487E66: qemu_chr_cleanup (qemu-char.c:4678)
==4306==    by 0x495A98: main (vl.c:4675)
==4306==  Address 0x28439e90 is 112 bytes inside a block of size 240 free'd
==4306==    at 0x4C2CD5A: free (vg_replace_malloc.c:530)
==4306==    by 0x1E4CBF2D: g_free (in /usr/lib64/libglib-2.0.so.0.4800.2)
==4306==    by 0x344DE9: monitor_cleanup (monitor.c:4058)
==4306==    by 0x495A93: main (vl.c:4674)
==4306==  Block was alloc'd at
==4306==    at 0x4C2BBAD: malloc (vg_replace_malloc.c:299)
==4306==    by 0x1E4CBE18: g_malloc (in /usr/lib64/libglib-2.0.so.0.4800.2)
==4306==    by 0x344BF8: monitor_init (monitor.c:4021)
==4306==    by 0x49063C: mon_init_func (vl.c:2417)
==4306==    by 0x7FC6DE: qemu_opts_foreach (qemu-option.c:1116)
==4306==    by 0x4954E0: main (vl.c:4473)

Instead, keep the "child" chr associated with a particular idx so its
handlers can be updated and removed to avoid the crash.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 qemu-char.c           | 22 +++++++++++++++-------
 include/sysemu/char.h |  1 +
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/qemu-char.c b/qemu-char.c
index f7a07e6..4b330ea 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -165,6 +165,7 @@ CharDriverState *qemu_chr_alloc(ChardevCommon *backend, Error **errp)
     CharDriverState *chr = g_malloc0(sizeof(CharDriverState));
     qemu_mutex_init(&chr->chr_write_lock);
 
+    chr->mux_idx = -1;
     if (backend->has_logfile) {
         int flags = O_WRONLY | O_CREAT;
         if (backend->has_logappend &&
@@ -738,17 +739,25 @@ static void mux_chr_update_read_handler(CharDriverState *chr,
                                         GMainContext *context)
 {
     MuxDriver *d = chr->opaque;
+    int idx;
 
     if (d->mux_cnt >= MAX_MUX) {
         fprintf(stderr, "Cannot add I/O handlers, MUX array is full\n");
         return;
     }
-    d->ext_opaque[d->mux_cnt] = chr->handler_opaque;
-    d->chr_can_read[d->mux_cnt] = chr->chr_can_read;
-    d->chr_read[d->mux_cnt] = chr->chr_read;
-    d->chr_event[d->mux_cnt] = chr->chr_event;
+
+    if (chr->mux_idx == -1) {
+        chr->mux_idx = d->mux_cnt++;
+    }
+
+    idx = chr->mux_idx;
+    d->ext_opaque[idx] = chr->handler_opaque;
+    d->chr_can_read[idx] = chr->chr_can_read;
+    d->chr_read[idx] = chr->chr_read;
+    d->chr_event[idx] = chr->chr_event;
+
     /* Fix up the real driver with mux routines */
-    if (d->mux_cnt == 0) {
+    if (d->mux_cnt == 1) {
         qemu_chr_add_handlers_full(d->drv, mux_chr_can_read,
                                    mux_chr_read,
                                    mux_chr_event,
@@ -757,8 +766,7 @@ static void mux_chr_update_read_handler(CharDriverState *chr,
     if (d->focus != -1) {
         mux_chr_send_event(d, d->focus, CHR_EVENT_MUX_OUT);
     }
-    d->focus = d->mux_cnt;
-    d->mux_cnt++;
+    d->focus = idx;
     mux_chr_send_event(d, d->focus, CHR_EVENT_MUX_IN);
 }
 
diff --git a/include/sysemu/char.h b/include/sysemu/char.h
index 0d0465a..4593576 100644
--- a/include/sysemu/char.h
+++ b/include/sysemu/char.h
@@ -92,6 +92,7 @@ struct CharDriverState {
     int explicit_be_open;
     int avail_connections;
     int is_mux;
+    int mux_idx;
     guint fd_in_tag;
     QemuOpts *opts;
     bool replay;
-- 
2.10.0