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[PATCH] Add gdbarch_sizeof_g_packet to "struct remote_arch_state" to handle the issue that get error "Remote 'g' packet reply is too long" with qemu-system-x86_64


If qemu-system-x86_64 start with -S, gdb will get error "Remote 'g' packet reply
is too long", for example:
sudo qemu-system-x86_64 -nographic -k en-us -kernel /boot/vmlinuz-3.13.0-34-generic -gdb tcp::12345 -append "console=ttyS0,115200 root=/dev/sda rw kmemleak=off" -S -m 512 -smp 1
gdb /usr/lib/debug/boot/vmlinux-$(uname -r)
(gdb) set debug remote 1
(gdb) target remote :12345
...
...
Sending packet: $g#67...Ack
Packet received: 0000000000000000630600000000000000000000000000000000000
000000000f0ff00000200000000f00000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000000000000
000000000000000007f03000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000801f0000
(gdb) b start_kernel
(gdb) c
Continuing.
...
...
Sending packet: $g#67...Ack
Packet received: 0000000000000000d381ffffffff000040000000000000000010060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Remote 'g' packet reply is too long: 0000000000000000d381ffffffff0000400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(gdb) c
Continuing.
Cannot execute this command while the selected thread is running.

The root cause of this issue is:
When GDB load a debug binary, function init_remote_state will be called and
rsa->sizeof_g_packet will be initialized.
  /* Record the maximum possible size of the g packet - it may turn out
     to be smaller.  */
  rsa->sizeof_g_packet = map_regcache_remote_table (gdbarch, rsa->regs);
When remote.c get first reply of g that its size is smaller than
rsa->sizeof_g_packet in function process_g_packet, sizeof_g_packet will be
updated to this smaller size.
      rsa->sizeof_g_packet = buf_len / 2;
When inferior is breaked by breakpoint, because some reason architecture reason,
QEMU send a packet for "g" that is more bigger than previous one.  Then GDB will
get error in function process_g_packet.
  /* Further sanity checks, with knowledge of the architecture.  */
  if (buf_len > 2 * rsa->sizeof_g_packet)
    error (_("Remote 'g' packet reply is too long: %s"), rs->buf);

To handle this issue:
This patch add "gdbarch_sizeof_g_packet" to "struct remote_arch_state", it will
keep the size of g packet that get from gdbarch.  And function process_g_packet
use it do the check for the reply of "g".  And update sizeof_g_packet if it is
not same with reply of "g".

2014-08-21  Hui Zhu  <teawater@gmail.com>

	* remote.c (struct remote_arch_state): Add gdbarch_sizeof_g_packet.
	(init_remote_state): Initialize rsa->gdbarch_sizeof_g_packet.
	(process_g_packet): Check buf_len with rsa->gdbarch_sizeof_g_packet.
	Update code to update rsa->sizeof_g_packet if buf_len is not for it.
---
 gdb/remote.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/gdb/remote.c b/gdb/remote.c
index 357e9f2..9727915 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -439,6 +439,11 @@ struct remote_arch_state
   /* Description of the remote protocol registers.  */
   long sizeof_g_packet;
 
+  /* The sizeof_g_packet will be changed in some GDBstub (QEMU) when it executes.
+     Description of the remote protocol registers that get form current
+     gdbarch.  */
+  long gdbarch_sizeof_g_packet;
+
   /* Description of the remote protocol registers indexed by REGNUM
      (making an array gdbarch_num_regs in size).  */
   struct packet_reg *regs;
@@ -673,7 +678,8 @@ init_remote_state (struct gdbarch *gdbarch)
 
   /* Record the maximum possible size of the g packet - it may turn out
      to be smaller.  */
-  rsa->sizeof_g_packet = map_regcache_remote_table (gdbarch, rsa->regs);
+  rsa->gdbarch_sizeof_g_packet = map_regcache_remote_table (gdbarch, rsa->regs);
+  rsa->sizeof_g_packet = rsa->gdbarch_sizeof_g_packet;
 
   /* Default maximum number of characters in a packet body.  Many
      remote stubs have a hardwired buffer size of 400 bytes
@@ -693,8 +699,8 @@ init_remote_state (struct gdbarch *gdbarch)
      header / footer.  NOTE: cagney/1999-10-26: I suspect that 8
      (``$NN:G...#NN'') is a better guess, the below has been padded a
      little.  */
-  if (rsa->sizeof_g_packet > ((rsa->remote_packet_size - 32) / 2))
-    rsa->remote_packet_size = (rsa->sizeof_g_packet * 2 + 32);
+  if (rsa->gdbarch_sizeof_g_packet > ((rsa->remote_packet_size - 32) / 2))
+    rsa->remote_packet_size = (rsa->gdbarch_sizeof_g_packet * 2 + 32);
 
   /* Make sure that the packet buffer is plenty big enough for
      this architecture.  */
@@ -6070,7 +6076,7 @@ process_g_packet (struct regcache *regcache)
   buf_len = strlen (rs->buf);
 
   /* Further sanity checks, with knowledge of the architecture.  */
-  if (buf_len > 2 * rsa->sizeof_g_packet)
+  if (buf_len > 2 * rsa->gdbarch_sizeof_g_packet)
     error (_("Remote 'g' packet reply is too long: %s"), rs->buf);
 
   /* Save the size of the packet sent to us by the target.  It is used
@@ -6079,11 +6085,11 @@ process_g_packet (struct regcache *regcache)
   if (rsa->actual_register_packet_size == 0)
     rsa->actual_register_packet_size = buf_len;
 
-  /* If this is smaller than we guessed the 'g' packet would be,
+  /* If this is not same with we guessed the 'g' packet would be,
      update our records.  A 'g' reply that doesn't include a register's
      value implies either that the register is not available, or that
      the 'p' packet must be used.  */
-  if (buf_len < 2 * rsa->sizeof_g_packet)
+  if (buf_len != 2 * rsa->sizeof_g_packet)
     {
       rsa->sizeof_g_packet = buf_len / 2;
 
-- 
1.9.3


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