[lldb-dev] [PATCH] Handle escape characters in binary gdbserver output

[Steve Pucci]

I'm attaching a patch that fixes handling of gdbserver binary packets
received.  We were not properly handling the escape character 0x7d.
https://sourceware.org/gdb/onlinedocs/gdb/Overview.html#Overview says

The binary data representation uses 7d (ascii ‘}’) as an escape character.
Any escaped byte is transmitted as the escape character followed by the
original character XORed with 0x20. For example, the byte 0x7d would be
transmitted as the two bytes 0x7d 0x5d. The bytes 0x23 (ascii ‘#’), 0x24
(ascii ‘$’), and 0x7d (ascii ‘}’) must always be escaped. Responses sent by
the stub must also escape 0x2a (ascii ‘*’), so that it is not interpreted
as the start of a run-length encoded sequence (described next).


I've verified that this fixes a problem with the reading of the auxv table
on 64-bit Ubuntu 12.04 via the qXfer:auxv:read packet response; prior to
this fix, the fields of the auxv table were obviously wrong, and now they
look reasonable (though there is another problem which is preventing the
correct handling of the table).

Thanks,
  Steve

diff --git a/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp
b/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp
index 1ec75a4..72600d8 100644
--- a/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp
+++ b/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp
@@ -497,6 +497,13 @@ GDBRemoteCommunication::CheckForPacket (const uint8_t
*src, size_t src_len, Stri
                     for (int i = 0; i < repeat_count; ++i)
                         packet_str.push_back(char_to_repeat);
                 }
+                else if (*c == 0x7d)
+                {
+                    // 0x7d is the escape character.  The next character
is to
+                    // be XOR'd with 0x20.
+                    char escapee = *++c ^ 0x20;
+                    packet_str.push_back(escapee);
+                }
                 else
                 {
                     packet_str.push_back(*c);
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diff --git a/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp b/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp
index 1ec75a4..72600d8 100644
--- a/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp
+++ b/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp
@@ -497,6 +497,13 @@ GDBRemoteCommunication::CheckForPacket (const uint8_t *src, size_t src_len, Stri
                     for (int i = 0; i < repeat_count; ++i)
                         packet_str.push_back(char_to_repeat);
                 }
+                else if (*c == 0x7d)
+                {
+                    // 0x7d is the escape character.  The next character is to
+                    // be XOR'd with 0x20.
+                    char escapee = *++c ^ 0x20;
+                    packet_str.push_back(escapee);
+                }
                 else
                 {
                     packet_str.push_back(*c);