misc: Make the remote GDB stub more resilient to bad connections.

Currently, if the remote gdb stub fails to read a byte from an incoming
packet because the connection has been dropped, the read call will return
anyway and the calling code will have no way to know something bad
happened. It might reattempt the read over and over again waiting for some
particular byte, doomed to never make forward progress.

This change modifies the remote GDB code so that if a read or write call
fails, it will instead detach from the debugger and continue. Before this
change, When simulating a port scan, ie connecting to the debugger port
and then immediately dropping the connection using this command:

nc -v -n -z -w 1 127.0.0.1 7000

gem5 would enter the previously described death spiral. After it, gem5
detaches from the bad connection and resumes execution. Subsequently
attaching with gdb was successful.

This code is written in a C centric style, and would benefit from some
refactoring.

Change-Id: Ie3c0bb35b9cfe3671d0f731e3907548bae0d292f
Reviewed-on: https://gem5-review.googlesource.com/3180
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

2 files changed, 151 insertions, 84 deletions

diff --git a/src/base/remote_gdb.cc b/src/base/remote_gdb.cc
index 665dba8c5..57fccd978 100644
--- a/src/base/remote_gdb.cc
+++ b/src/base/remote_gdb.cc
@@ -125,6 +125,7 @@
 #include <unistd.h>
 
 #include <csignal>
+#include <cstdint>
 #include <cstdio>
 #include <string>
 
@@ -368,24 +369,32 @@ BaseRemoteGDB::gdb_command(char cmd)
 //
 //
 
-uint8_t
-BaseRemoteGDB::getbyte()
+bool
+BaseRemoteGDB::getbyte(uint8_t &b)
 {
-    uint8_t b;
-    if (::read(fd, &b, 1) != 1)
-        warn("could not read byte from debugger");
-    return b;
+    if (::read(fd, &b, sizeof(b)) == sizeof(b)) {
+        return true;
+    } else {
+        warn("Couldn't read data from debugger, detaching.");
+        detach();
+        return false;
+    }
 }
 
-void
+bool
 BaseRemoteGDB::putbyte(uint8_t b)
 {
-    if (::write(fd, &b, 1) != 1)
-        warn("could not write byte to debugger");
+    if (::write(fd, &b, sizeof(b)) == sizeof(b)) {
+        return true;
+    } else {
+        warn("Couldn't write data to the debugger, detaching.");
+        detach();
+        return false;
+    }
 }
 
 // Send a packet to gdb
-void
+ssize_t
 BaseRemoteGDB::send(const char *bp)
 {
     const char *p;
@@ -396,20 +405,27 @@ BaseRemoteGDB::send(const char *bp)
     do {
         p = bp;
         //Start sending a packet
-        putbyte(GDBStart);
+        if (!putbyte(GDBStart))
+            return -1;
         //Send the contents, and also keep a check sum.
         for (csum = 0; (c = *p); p++) {
-            putbyte(c);
+            if (!putbyte(c))
+                return -1;
             csum += c;
         }
-        //Send the ending character.
-        putbyte(GDBEnd);
-        //Sent the checksum.
-        putbyte(i2digit(csum >> 4));
-        putbyte(i2digit(csum));
-        //Try transmitting over and over again until the other end doesn't send an
-        //error back.
-    } while ((c = getbyte() & 0x7f) == GDBBadP);
+        if (//Send the ending character.
+            !putbyte(GDBEnd) ||
+            //Sent the checksum.
+            !putbyte(i2digit(csum >> 4)) ||
+            !putbyte(i2digit(csum))) {
+            return -1;
+        }
+        //Try transmitting over and over again until the other end doesn't
+        //send an error back.
+        if (!getbyte(c))
+            return -1;
+    } while ((c & 0x7f) == GDBBadP);
+    return 0;
 }
 
 // Receive a packet from gdb
@@ -417,19 +433,26 @@ int
 BaseRemoteGDB::recv(char *bp, int maxlen)
 {
     char *p;
-    int c, csum;
+    uint8_t c;
+    int csum;
     int len;
 
     do {
         p = bp;
         csum = len = 0;
         //Find the beginning of a packet
-        while ((c = getbyte()) != GDBStart)
-            ;
+        do {
+            if (!getbyte(c))
+                return -1;
+        } while (c != GDBStart);
 
         //Read until you find the end of the data in the packet, and keep
         //track of the check sum.
-        while ((c = getbyte()) != GDBEnd && len < maxlen) {
+        while (len < maxlen) {
+            if (!getbyte(c))
+                return -1;
+            if (c == GDBEnd)
+                break;
             c &= 0x7f;
             csum += c;
             *p++ = c;
@@ -442,29 +465,35 @@ BaseRemoteGDB::recv(char *bp, int maxlen)
 
         //If the command was too long, report an error.
         if (len >= maxlen) {
-            putbyte(GDBBadP);
+            if (!putbyte(GDBBadP))
+                return -1;
             continue;
         }
 
         //Bring in the checksum. If the check sum matches, csum will be 0.
-        csum -= digit2i(getbyte()) * 16;
-        csum -= digit2i(getbyte());
+        uint8_t csum1, csum2;
+        if (!getbyte(csum1) || !getbyte(csum2))
+            return -1;
+        csum -= digit2i(csum1) * 16;
+        csum -= digit2i(csum2);
 
         //If the check sum was correct
         if (csum == 0) {
             //Report that the packet was received correctly
-            putbyte(GDBGoodP);
+            if (!putbyte(GDBGoodP))
+                return -1;
             // Sequence present?
             if (bp[2] == ':') {
-                putbyte(bp[0]);
-                putbyte(bp[1]);
+                if (!putbyte(bp[0]) || !putbyte(bp[1]))
+                    return -1;
                 len -= 3;
                 memcpy(bp, bp+3, len);
             }
             break;
         }
         //Otherwise, report that there was a mistake.
-        putbyte(GDBBadP);
+        if (!putbyte(GDBBadP))
+            return -1;
     } while (1);
 
     DPRINTF(GDBRecv, "recv:  %s: %s\n", gdb_command(*bp), bp);
@@ -698,7 +727,6 @@ BaseRemoteGDB::trap(int type)
     uint64_t val;
     size_t datalen, len;
     char data[GDBPacketBufLen + 1];
-    char *buffer;
     size_t bufferSize;
     const char *p;
     char command, subcmd;
@@ -711,7 +739,8 @@ BaseRemoteGDB::trap(int type)
     unique_ptr<BaseRemoteGDB::BaseGdbRegCache> regCache(gdbRegs());
 
     bufferSize = regCache->size() * 2 + 256;
-    buffer = (char*)malloc(bufferSize);
+    unique_ptr<char[]> buffer_mem(new char[bufferSize]);
+    char *buffer = buffer_mem.get();
 
     DPRINTF(GDBMisc, "trap: PC=%s\n", context->pcState());
 
@@ -732,14 +761,18 @@ BaseRemoteGDB::trap(int type)
     } else {
         // Tell remote host that an exception has occurred.
         snprintf(buffer, bufferSize, "S%02x", type);
-        send(buffer);
+        if (send(buffer) < 0)
+            return true;
     }
 
     // Stick frame regs into our reg cache.
     regCache->getRegs(context);
 
     for (;;) {
-        datalen = recv(data, sizeof(data));
+        int recved = recv(data, sizeof(data));
+        if (recved < 0)
+            return true;
+        datalen = recved;
         data[sizeof(data) - 1] = 0; // Sentinel
         command = data[0];
         subcmd = 0;
@@ -752,7 +785,8 @@ BaseRemoteGDB::trap(int type)
             // of this loop when he issues a "remote-signal".
             snprintf(buffer, bufferSize,
                     "S%02x", type);
-            send(buffer);
+            if (send(buffer) < 0)
+                return true;
             continue;
 
           case GDBRegR:
@@ -760,36 +794,43 @@ BaseRemoteGDB::trap(int type)
                 panic("buffer too small");
 
             mem2hex(buffer, regCache->data(), regCache->size());
-            send(buffer);
+            if (send(buffer) < 0)
+                return true;
             continue;
 
           case GDBRegW:
             p = hex2mem(regCache->data(), p, regCache->size());
-            if (p == NULL || *p != '\0')
-                send("E01");
-            else {
+            if (p == NULL || *p != '\0') {
+                if (send("E01") < 0)
+                    return true;
+            } else {
                 regCache->setRegs(context);
-                send("OK");
+                if (send("OK") < 0)
+                    return true;
             }
             continue;
 
           case GDBMemR:
             val = hex2i(&p);
             if (*p++ != ',') {
-                send("E02");
+                if (send("E02") < 0)
+                    return true;
                 continue;
             }
             len = hex2i(&p);
             if (*p != '\0') {
-                send("E03");
+                if (send("E03") < 0)
+                    return true;
                 continue;
             }
             if (len > bufferSize) {
-                send("E04");
+                if (send("E04") < 0)
+                    return true;
                 continue;
             }
             if (!acc(val, len)) {
-                send("E05");
+                if (send("E05") < 0)
+                    return true;
                 continue;
             }
 
@@ -798,50 +839,65 @@ BaseRemoteGDB::trap(int type)
                // officially support those...
                char *temp = new char[2*len+1];
                mem2hex(temp, buffer, len);
-               send(temp);
+               if (send(temp) < 0) {
+                   delete [] temp;
+                   return true;
+               }
                delete [] temp;
             } else {
-               send("E05");
+               if (send("E05") < 0)
+                   return true;
             }
             continue;
 
           case GDBMemW:
             val = hex2i(&p);
             if (*p++ != ',') {
-                send("E06");
+                if (send("E06") < 0)
+                    return true;
                 continue;
             }
             len = hex2i(&p);
             if (*p++ != ':') {
-                send("E07");
+                if (send("E07") < 0)
+                    return true;
                 continue;
             }
             if (len > datalen - (p - data)) {
-                send("E08");
+                if (send("E08") < 0)
+                    return true;
                 continue;
             }
             p = hex2mem(buffer, p, bufferSize);
             if (p == NULL) {
-                send("E09");
+                if (send("E09") < 0)
+                    return true;
                 continue;
             }
             if (!acc(val, len)) {
-                send("E0A");
+                if (send("E0A") < 0)
+                    return true;
                 continue;
             }
-            if (write(val, (size_t)len, buffer))
-              send("OK");
-            else
-              send("E0B");
+            if (write(val, (size_t)len, buffer)) {
+                if (send("OK") < 0)
+                    return true;
+            } else {
+                if (send("E0B") < 0)
+                    return true;
+            }
             continue;
 
           case GDBSetThread:
             subcmd = *p++;
             val = hex2i(&p);
-            if (val == 0)
-                send("OK");
-            else
-                send("E01");
+            if (val == 0) {
+                if (send("OK") < 0)
+                    return true;
+            } else {
+                if (send("E01") < 0)
+                    return true;
+            }
             continue;
 
           case GDBDetach:
@@ -849,7 +905,7 @@ BaseRemoteGDB::trap(int type)
             active = false;
             clearSingleStep();
             detach();
-            goto out;
+            return true;
 
           case GDBAsyncCont:
             subcmd = hex2i(&p);
@@ -858,7 +914,7 @@ BaseRemoteGDB::trap(int type)
                 context->pcState(val);
             }
             clearSingleStep();
-            goto out;
+            return true;
 
           case GDBCont:
             if (p - data < (ptrdiff_t)datalen) {
@@ -866,7 +922,7 @@ BaseRemoteGDB::trap(int type)
                 context->pcState(val);
             }
             clearSingleStep();
-            goto out;
+            return true;
 
           case GDBAsyncStep:
             subcmd = hex2i(&p);
@@ -875,7 +931,7 @@ BaseRemoteGDB::trap(int type)
                 context->pcState(val);
             }
             setSingleStep();
-            goto out;
+            return true;
 
           case GDBStep:
             if (p - data < (ptrdiff_t)datalen) {
@@ -883,13 +939,15 @@ BaseRemoteGDB::trap(int type)
                 context->pcState(val);
             }
             setSingleStep();
-            goto out;
+            return true;
 
           case GDBClrHwBkpt:
             subcmd = *p++;
-            if (*p++ != ',') send("E0D");
+            if (*p++ != ',' && send("E0D") < 0)
+                return true;
             val = hex2i(&p);
-            if (*p++ != ',') send("E0D");
+            if (*p++ != ',' && send("E0D") < 0)
+                return true;
             len = hex2i(&p);
 
             DPRINTF(GDBMisc, "clear %s, addr=%#x, len=%d\n",
@@ -910,18 +968,22 @@ BaseRemoteGDB::trap(int type)
               case '3': // read watchpoint
               case '4': // access watchpoint
               default: // unknown
-                send("");
+                if (send("") < 0)
+                    return true;
                 break;
             }
 
-            send(ret ? "OK" : "E0C");
+            if (send(ret ? "OK" : "E0C") < 0)
+                return true;
             continue;
 
           case GDBSetHwBkpt:
             subcmd = *p++;
-            if (*p++ != ',') send("E0D");
+            if (*p++ != ',' && send("E0D") < 0)
+                return true;
             val = hex2i(&p);
-            if (*p++ != ',') send("E0D");
+            if (*p++ != ',' && send("E0D") < 0)
+                return true;
             len = hex2i(&p);
 
             DPRINTF(GDBMisc, "set %s, addr=%#x, len=%d\n",
@@ -942,19 +1004,24 @@ BaseRemoteGDB::trap(int type)
               case '3': // read watchpoint
               case '4': // access watchpoint
               default: // unknown
-                send("");
+                if (send("") < 0)
+                    return true;
                 break;
             }
 
-            send(ret ? "OK" : "E0C");
+            if (send(ret ? "OK" : "E0C") < 0)
+                return true;
             continue;
 
           case GDBQueryVar:
             var = string(p, datalen - 1);
-            if (var == "C")
-                send("QC0");
-            else
-                send("");
+            if (var == "C") {
+                if (send("QC0") < 0)
+                    return true;
+            } else {
+                if (send("") < 0)
+                    return true;
+            }
             continue;
 
           case GDBSetBaud:
@@ -972,22 +1039,22 @@ BaseRemoteGDB::trap(int type)
             DPRINTF(GDBMisc, "Unsupported command: %s\n",
                     gdb_command(command));
             DDUMP(GDBMisc, (uint8_t *)data, datalen);
-            send("");
+            if (send("") < 0)
+                return true;
             continue;
 
           default:
             // Unknown command.
             DPRINTF(GDBMisc, "Unknown command: %c(%#x)\n",
                     command, command);
-            send("");
+            if (send("") < 0)
+                return true;
             continue;
 
 
         }
     }
 
-  out:
-    free(buffer);
     return true;
 }
 
diff --git a/src/base/remote_gdb.hh b/src/base/remote_gdb.hh
index 2ab7a84dd..b3a2b5c60 100644
--- a/src/base/remote_gdb.hh
+++ b/src/base/remote_gdb.hh
@@ -203,11 +203,11 @@ class BaseRemoteGDB
     };
 
   protected:
-    uint8_t getbyte();
-    void putbyte(uint8_t b);
+    bool getbyte(uint8_t &b);
+    bool putbyte(uint8_t b);
 
     int recv(char *data, int len);
-    void send(const char *data);
+    ssize_t send(const char *data);
 
   protected:
     // Machine memory