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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 as following:
(lldb) disassemble
example`fn:
0x55deb51866b0 <+0>: addb %al, (%rax)
0x55deb51866b2 <+2>: addb %al, (%rax)
0x55deb51866b4 <+4>: addb %al, (%rax)
0x55deb51866b6 <+6>: addb %al, (%rax)
....
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. If we are about to fill the buffer with all zeros, I guess
it is safe to just return zero, and to proceed with reading from file itself
(using the `ReadMemoryFromFileCache()`) (an alternative is to force/prefer
`ReadMemoryFromFileCache()` by settimg `prefer_file_cache` to true before
calling `ReadMemory()`).
Before this patch (simple example that has been attached into the test):
$ lldb
lldb/test/API/functionalities/postmortem/elf-core/linux-x86_64-for-disassemble.out
-c
lldb/test/API/functionalities/postmortem/elf-core/linux-x86_64-for-disassemble.core
(lldb) f 4
frame #4: 0x00007f1d2f862545 libc.so.6`__libc_start_main + 245
libc.so.6`__libc_start_main:
-> 0x7f1d2f862545 <+245>: addb %al, (%rax)
0x7f1d2f862547 <+247>: addb %al, (%rax)
0x7f1d2f862549 <+249>: addb %al, (%rax)
0x7f1d2f86254b <+251>: addb %al, (%rax)
After this patch applied:
(lldb) f 4
frame #4: 0x00007f1d2f862545 libc.so.6`__libc_start_main + 245
libc.so.6`__libc_start_main:
-> 0x7f1d2f862545 <+245>: movl %eax, %edi
0x7f1d2f862547 <+247>: callq 0x39d10 ; exit
0x7f1d2f86254c <+252>: xorl %edx, %edx
0x7f1d2f86254e <+254>: jmp 0x22489 ; <+57>
GDB was able to disassemble all the functions from core file.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D93939
Files:
lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp
lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py
lldb/test/API/functionalities/postmortem/elf-core/linux-x86_64-for-disassemble.core
lldb/test/API/functionalities/postmortem/elf-core/linux-x86_64-for-disassemble.out
Index: lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py
===================================================================
--- lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py
+++ lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py
@@ -155,6 +155,37 @@
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_frame_disassemble(self):
+ """Test that we are able to disassemble all the frames"""
+ disasmtarget =
self.dbg.CreateTarget("linux-x86_64-for-disassemble.out")
+ disasmprocess =
disasmtarget.LoadCore("linux-x86_64-for-disassemble.core")
+ self.assertTrue(disasmprocess, PROCESS_IS_VALID)
+
+ disasmthread = disasmprocess.GetSelectedThread()
+ framenum = disasmthread.GetNumFrames()
+ for i in range(framenum):
+ frame = disasmthread.GetFrameAtIndex(i)
+ disassembly = frame.Disassemble()
+ self.assertNotEqual(disassembly, "")
+ self.assertNotIn("error", disassembly)
+ # Make sure we don't have some dummy disassembly.
+ # Each function should start with:
+ # pushq %rbp
+ # ...
+ # Sometimes it just prints some dummy code as:
+ # addb %al, (%rax)
+ # addb %al, (%rax)
+ # ...
+ pushrbpinstr = disassembly.splitlines()[1]
+ self.assertNotIn("addb %al, (%rax)", pushrbpinstr)
+
+ self.dbg.DeleteTarget(disasmtarget)
+
@skipIf(triple='^mips')
@skipIfLLVMTargetMissing("X86")
def test_FPR_SSE(self):
Index: lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp
===================================================================
--- lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp
+++ lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp
@@ -384,6 +384,11 @@
if (zero_fill_size)
memset(((char *)buf) + bytes_copied, 0, zero_fill_size);
+ // No data found in the core file, so the buffer contains
+ // all zeros.
+ if (zero_fill_size == size)
+ return 0;
+
return bytes_copied + zero_fill_size;
}
Index: lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py
===================================================================
--- lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py
+++ lldb/test/API/functionalities/postmortem/elf-core/TestLinuxCore.py
@@ -155,6 +155,37 @@
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_frame_disassemble(self):
+ """Test that we are able to disassemble all the frames"""
+ disasmtarget = self.dbg.CreateTarget("linux-x86_64-for-disassemble.out")
+ disasmprocess = disasmtarget.LoadCore("linux-x86_64-for-disassemble.core")
+ self.assertTrue(disasmprocess, PROCESS_IS_VALID)
+
+ disasmthread = disasmprocess.GetSelectedThread()
+ framenum = disasmthread.GetNumFrames()
+ for i in range(framenum):
+ frame = disasmthread.GetFrameAtIndex(i)
+ disassembly = frame.Disassemble()
+ self.assertNotEqual(disassembly, "")
+ self.assertNotIn("error", disassembly)
+ # Make sure we don't have some dummy disassembly.
+ # Each function should start with:
+ # pushq %rbp
+ # ...
+ # Sometimes it just prints some dummy code as:
+ # addb %al, (%rax)
+ # addb %al, (%rax)
+ # ...
+ pushrbpinstr = disassembly.splitlines()[1]
+ self.assertNotIn("addb %al, (%rax)", pushrbpinstr)
+
+ self.dbg.DeleteTarget(disasmtarget)
+
@skipIf(triple='^mips')
@skipIfLLVMTargetMissing("X86")
def test_FPR_SSE(self):
Index: lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp
===================================================================
--- lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp
+++ lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp
@@ -384,6 +384,11 @@
if (zero_fill_size)
memset(((char *)buf) + bytes_copied, 0, zero_fill_size);
+ // No data found in the core file, so the buffer contains
+ // all zeros.
+ if (zero_fill_size == size)
+ return 0;
+
return bytes_copied + zero_fill_size;
}
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