[lldb] Extend SWIG SBProcess interface with WriteMemoryAsCString method

[mib]

Summary

This patch tries to address an interoperability issue when writing
python string into the process memory.

Since the python string is not null-terminated, it would still be
written to memory however, when trying to read it again with
SBProcess::ReadCStringFromMemory, the memory read would fail, since
the read string doens't contain a null-terminator, and therefore is not
a valid C string.

To address that, this patch extends the SBProcess SWIG interface to
expose a new WriteMemoryAsCString method that is only exposed to the
SWIG target language. That method checks that the buffer to write is
null-terminated and otherwise, it appends a null byte at the end of it.

Signed-off-by: Med Ismail Bennani <[e-mail]>

[bulbazord]

I'm alright with the implementation, but I think you should add some documentation here. Also, what do you think of the name WriteMemoryAsCString? To make it clear that whatever you are passing will be written to memory as a cstring.

lldb/bindings/interface/SBProcessExtensions.i

5–9

[bulbazord]

LGTM. @jingham what do you think?

[labath]

lldb/bindings/interface/SBProcessExtensions.i

11

I think this will fail if the string is empty ("").

Somewhat relatedly, what if this appended the nul byte unconditionally (so it kinda behaved like std::string, which is always nul-terminated, but the nul byte is not counted as the length of the string)?

[jingham]

I see no tests. If there were tests, one of them could be for "" and then you would know whether Pavel was right or not...

[mib]

Handle empty string case and add tests

[bulbazord]

Thanks for adding those tests.

[mib]

Landed in e6cac17b563f

lldb/bindings/interface/SBProcessExtensions.i

 STRING_EXTENSION_OUTSIDE(SBProcess)
 %extend lldb::SBProcess {
 #ifdef SWIGPYTHON
     %pythoncode %{
+        def WriteMemoryAsCString(self, addr, str, error):
+            '''
+              WriteMemoryAsCString(self, addr, str, error):
+                This functions the same as `WriteMemory` except a null-terminator is appended
+                to the end of the buffer if it is not there already.
+            '''
+            if not str or len(str) == 0:
+                return 0
+            if not str[-1] == '\0':
+                str += '\0'
+            return self.WriteMemory(addr, str, error)
+
         def __get_is_alive__(self):
             '''Returns "True" if the process is currently alive, "False" otherwise'''
             s = self.GetState()
             if (s == eStateAttaching or
                 s == eStateLaunching or
                 s == eStateStopped or
                 s == eStateRunning or
                 s == eStateStepping or

lldb/test/API/python_api/process/TestProcessAPI.py

def test_write_memory(self):
             print("memory content:", content)
 
         self.expect(
             content,
             "Result from SBProcess.ReadMemory() matches our expected output: 'a'",
             exe=False,
             startstr=b'a')
 
+        # Get the SBValue for the global variable 'my_cstring'.
+        val = frame.FindValue("my_cstring", lldb.eValueTypeVariableGlobal)
+        self.DebugSBValue(val)
+
+        addr = val.AddressOf().GetValueAsUnsigned()
+
+        # Write an empty string to memory
+        bytes_written = process.WriteMemoryAsCString(addr, "", error)
+        self.assertEqual(bytes_written, 0)
+        if not error.Success():
+            self.fail("SBProcess.WriteMemoryAsCString() failed")
+
+        message = "Hello!"
+        bytes_written = process.WriteMemoryAsCString(addr, message, error)
+        self.assertEqual(bytes_written, len(message) + 1)
+        if not error.Success():
+            self.fail("SBProcess.WriteMemoryAsCString() failed")
+
+        cstring = process.ReadCStringFromMemory(
+            val.AddressOf().GetValueAsUnsigned(), 256, error)
+        if not error.Success():
+            self.fail("SBProcess.ReadCStringFromMemory() failed")
+
+        self.assertEqual(cstring, message)
+
+
     def test_access_my_int(self):
         """Test access 'my_int' using Python SBProcess.GetByteOrder() and other APIs."""
         self.build()
         exe = self.getBuildArtifact("a.out")
 
         target = self.dbg.CreateTarget(exe)
         self.assertTrue(target, VALID_TARGET)