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

[Claudio Imbrenda]

Hi,

I noticed that this patch kills the Qemu monitor for me. If I start a
text-mode guest with -nographic, then I can't switch to the monitor any
longer with Ctrl+a c . The guest works otherwise, e.g. I can login from
the console.

Tested on s390, but I think it's a more general issue, since the patch
is not arch-dependent.


On 03/10/16 11:47, Marc-André Lureau wrote:
> 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 <address@hidden>
> ---
>  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;
>