Re: [PATCH] gdbstub: change GDBState.last_packet to GByteArray

[Luc Michel]

On 12/6/19 2:42 PM, Damien Hedde wrote:
> Remove the packet size upper limit by using a GByteArray
> instead of a statically allocated array for last_packet.
> Thus we can now send big packets.
> 
> Also remove the last_packet_len field and use last_packet->len
> instead.
> 
> Signed-off-by: Damien Hedde <address@hidden>
> ---
> 
> This patch is a follow-up of Alex's series about sve registers
> which introduces some GbyteArray/Gstring in the gdbstub.
> It removes the remaining barrier to send big packets.
> 
> Based-on <address@hidden>
> ---
>  gdbstub.c | 39 +++++++++++++++++++++------------------
>  1 file changed, 21 insertions(+), 18 deletions(-)
> 
> diff --git a/gdbstub.c b/gdbstub.c
> index 7b695bdebe..022edd6bdb 100644
> --- a/gdbstub.c
> +++ b/gdbstub.c
> @@ -351,8 +351,7 @@ typedef struct GDBState {
>      int line_buf_index;
>      int line_sum; /* running checksum */
>      int line_csum; /* checksum at the end of the packet */
> -    uint8_t last_packet[MAX_PACKET_LENGTH + 4];
> -    int last_packet_len;
> +    GByteArray *last_packet;
>      int signal;
>  #ifdef CONFIG_USER_ONLY
>      int fd;
> @@ -384,6 +383,7 @@ static void init_gdbserver_state(void)
>      gdbserver_state.init = true;
>      gdbserver_state.str_buf = g_string_new(NULL);
>      gdbserver_state.mem_buf = g_byte_array_sized_new(MAX_PACKET_LENGTH);
> +    gdbserver_state.last_packet = g_byte_array_sized_new(MAX_PACKET_LENGTH + 
> 4);
>  }
>  
>  #ifndef CONFIG_USER_ONLY
> @@ -626,28 +626,29 @@ static void hexdump(const char *buf, int len,
>  static int put_packet_binary(const char *buf, int len, bool dump)
>  {
>      int csum, i;
> -    uint8_t *p;
> -    uint8_t *ps = &gdbserver_state.last_packet[0];
> +    uint8_t footer[3];
>  
>      if (dump && 
> trace_event_get_state_backends(TRACE_GDBSTUB_IO_BINARYREPLY)) {
>          hexdump(buf, len, trace_gdbstub_io_binaryreply);
>      }
>  
>      for(;;) {
> -        p = ps;
> -        *(p++) = '$';
> -        memcpy(p, buf, len);
> -        p += len;
> +        g_byte_array_set_size(gdbserver_state.last_packet, 0);
> +        g_byte_array_append(gdbserver_state.last_packet,
> +                            (const uint8_t *) "$", 1);
> +        g_byte_array_append(gdbserver_state.last_packet,
> +                            (const uint8_t *) buf, len);
>          csum = 0;
>          for(i = 0; i < len; i++) {
>              csum += buf[i];
>          }
> -        *(p++) = '#';
> -        *(p++) = tohex((csum >> 4) & 0xf);
> -        *(p++) = tohex((csum) & 0xf);
> +        footer[0] = '#';
> +        footer[1] = tohex((csum >> 4) & 0xf);
> +        footer[2] = tohex((csum) & 0xf);
> +        g_byte_array_append(gdbserver_state.last_packet, footer, 3);
>  
> -        gdbserver_state.last_packet_len = p - ps;
> -        put_buffer(ps, gdbserver_state.last_packet_len);
> +        put_buffer(gdbserver_state.last_packet->data,
> +                   gdbserver_state.last_packet->len);
>  
>  #ifdef CONFIG_USER_ONLY
>          i = get_char();
> @@ -2812,20 +2813,22 @@ static void gdb_read_byte(GDBState *s, uint8_t ch)
>      uint8_t reply;
>  
>  #ifndef CONFIG_USER_ONLY
> -    if (gdbserver_state.last_packet_len) {
> +    if (gdbserver_state.last_packet->len) {
>          /* Waiting for a response to the last packet.  If we see the start
>             of a new command then abandon the previous response.  */
>          if (ch == '-') {
>              trace_gdbstub_err_got_nack();
> -            put_buffer((uint8_t *)gdbserver_state.last_packet, 
> gdbserver_state.last_packet_len);
> +            put_buffer(gdbserver_state.last_packet->data,
> +                       gdbserver_state.last_packet->len);
>          } else if (ch == '+') {
>              trace_gdbstub_io_got_ack();
>          } else {
>              trace_gdbstub_io_got_unexpected(ch);
>          }
>  
> -        if (ch == '+' || ch == '$')
> -            gdbserver_state.last_packet_len = 0;
> +        if (ch == '+' || ch == '$') {
> +            g_byte_array_set_size(gdbserver_state.last_packet, 0);
> +        }
>          if (ch != '$')
>              return;
>      }
> @@ -3209,7 +3212,7 @@ static int gdb_monitor_write(Chardev *chr, const 
> uint8_t *buf, int len)
>      const char *p = (const char *)buf;
>      int max_sz;
>  
> -    max_sz = (sizeof(gdbserver_state.last_packet) - 2) / 2;
> +    max_sz = MAX_PACKET_LENGTH / 2;
I think max_sz is not the same as before here.

Before we had (sizeof(gdbserver_state.last_packet) - 2) / 2
           -> ((MAX_PACKET_LENGTH + 4) - 2) / 2
           -> (MAX_PACKET_LENGTH / 2) + 1

Now we have MAX_PACKET_LENGTH / 2

But looking at the gdb_monitor_output() function, it performs the
following check:
    if (len > (MAX_PACKET_LENGTH/2) - 1)
        len = (MAX_PACKET_LENGTH/2) - 1;

So both versions seem actually wrong and can lead to characters being
dropped.

However gdb_monitor_output() is still using a MAX_PACKET_LENGTH sized
local buffer. With this patch it can probably be removed entirely so
that last_packet is written directly (with a g_byte_array aware version
of memtohex?).

-- 
Luc

>      for (;;) {
>          if (len <= max_sz) {
>              gdb_monitor_output(&gdbserver_state, p, len);
>