[Lldb-commits] [lldb] 9abd8c1 - [elf-core] Improve reading memory from core file

[Djordje Todorovic via lldb-commits]

Author: Djordje Todorovic
Date: 2021-02-08T00:14:32-08:00
New Revision: 9abd8c1a4c3870f2831ee805cd3c0cec516a1c17

URL: https://github.com/llvm/llvm-project/commit/9abd8c1a4c3870f2831ee805cd3c0cec516a1c17
DIFF: https://github.com/llvm/llvm-project/commit/9abd8c1a4c3870f2831ee805cd3c0cec516a1c17.diff

LOG: [elf-core] Improve reading memory from core file

This patch tries to improve memory-read from core files
(in order to improve disassembly functionality).

I am using RHEL 7.7 (linux kernel 3.10) and for a lot of cases,
I was not able to disassemble some functions from backtrace when
debugging crashes from core files. It outputs some dummy code.

The cause of the problem was the fact we are returning all the zeros
from ProcessElfCore::ReadMemory() that is being called within
Disassembler::ParseInstructions() and it disassembles some dummy
opcodes from the buffer returned. Therefore, we are removing zero
bytes filling (padding) completely.

Differential Revision: https://reviews.llvm.org/D93939

Added: 
    

Modified: 
    lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp
    lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py

Removed: 
    


################################################################################
diff  --git a/lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp b/lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp
index ae19367ca3ae..9896638fa8ff 100644
--- a/lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp
+++ b/lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp
@@ -353,7 +353,6 @@ size_t ProcessElfCore::DoReadMemory(lldb::addr_t addr, void *buf, size_t size,
   const lldb::addr_t file_end = address_range->data.GetRangeEnd();
   size_t bytes_to_read = size; // Number of bytes to read from the core file
   size_t bytes_copied = 0;   // Number of bytes actually read from the core file
-  size_t zero_fill_size = 0; // Padding
   lldb::addr_t bytes_left =
       0; // Number of bytes available in the core file from the given address
 
@@ -367,24 +366,15 @@ size_t ProcessElfCore::DoReadMemory(lldb::addr_t addr, void *buf, size_t size,
   if (file_end > file_start + offset)
     bytes_left = file_end - (file_start + offset);
 
-  // Figure out how many bytes we need to zero-fill if we are reading more
-  // bytes than available in the on-disk segment
-  if (bytes_to_read > bytes_left) {
-    zero_fill_size = bytes_to_read - bytes_left;
+  if (bytes_to_read > bytes_left)
     bytes_to_read = bytes_left;
-  }
 
   // If there is data available on the core file read it
   if (bytes_to_read)
     bytes_copied =
         core_objfile->CopyData(offset + file_start, bytes_to_read, buf);
 
-  assert(zero_fill_size <= size);
-  // Pad remaining bytes
-  if (zero_fill_size)
-    memset(((char *)buf) + bytes_copied, 0, zero_fill_size);
-
-  return bytes_copied + zero_fill_size;
+  return bytes_copied;
 }
 
 void ProcessElfCore::Clear() {

diff  --git a/lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py b/lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py
index 162503c33d37..f3edf4b82bcd 100644
--- a/lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py
+++ b/lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py
@@ -155,6 +155,24 @@ def test_two_cores_same_pid(self):
         self.do_test("linux-x86_64", self._x86_64_pid, self._x86_64_regions,
                      "a.out")
 
+
+    @skipIf(triple='^mips')
+    @skipIfLLVMTargetMissing("X86")
+    @skipIfWindows
+    @skipIfReproducer
+    def test_read_memory(self):
+        """Test that we are able to read as many bytes as available"""
+        target = self.dbg.CreateTarget("linux-x86_64.out")
+        process = target.LoadCore("linux-x86_64.core")
+        self.assertTrue(process, PROCESS_IS_VALID)
+
+        error = lldb.SBError()
+        bytesread = process.ReadMemory(0x400ff0, 20, error)
+        
+        # read only 16 bytes without zero bytes filling
+        self.assertEqual(len(bytesread), 16)
+        self.dbg.DeleteTarget(target)
+
     @skipIf(triple='^mips')
     @skipIfLLVMTargetMissing("X86")
     def test_FPR_SSE(self):