[Lldb-commits] [lldb] r360354 - [JITLoaderGDB] Set eTypeJIT for objects read from JIT descriptors

Stefan Granitz via lldb-commits lldb-commits at lists.llvm.org
Thu May 9 09:40:58 PDT 2019

Author: stefan.graenitz
Date: Thu May  9 09:40:57 2019
New Revision: 360354

URL: http://llvm.org/viewvc/llvm-project?rev=360354&view=rev
[JITLoaderGDB] Set eTypeJIT for objects read from JIT descriptors

First part of a fix for JITed code debugging. This has been a regression from 5.0 to 6.0 and it's is still reproducible on current master: https://bugs.llvm.org/show_bug.cgi?id=36209

The address of the breakpoint site is corrupt: the 0x4 value we end up with, looks like an offset on a zero base address. When we parse the ELF section headers from the JIT descriptor, the load address for the text section we find in `header.sh_addr` is correct.

The bug manifests in `VMAddressProvider::GetVMRange(const ELFSectionHeader &)` (follow it from `ObjectFileELF::CreateSections()`). Here we think the object type was `eTypeObjectFile` and unleash some extra logic [1] which essentially overwrites the address with a zero value.

The object type is deduced from the ELF header's `e_type` in `ObjectFileELF::CalculateType()`. It never returns `eTypeJIT`, because the ELF header has no representation for it [2]. Instead the in-memory ELF object states `ET_REL`, which leads to `eTypeObjectFile`. This is what we get from `lli` at least since 3.x. (Might it be better to write `ET_EXEC` on the JIT side instead? In fact, relocations were already applied at this point, so "Relocatable" is not quite exact.)

So, this patch proposes to set `eTypeJIT` explicitly whenever we read from a JIT descriptor. In `ObjectFileELF::CreateSections()` we can then call `GetType()`, which returns the explicit value or otherwise falls back to `CalculateType()`.

LLDB then sets the breakpoint successfully. Next step: debug info.
Process 1056 stopped
* thread #1, name = 'lli', stop reason = breakpoint 1.2
    frame #0: 0x00007ffff7ff7000 JIT(0x3ba2030)`jitbp()
->  0x7ffff7ff7000 <+0>:  pushq  %rbp
    0x7ffff7ff7001 <+1>:  movq   %rsp, %rbp
    0x7ffff7ff7004 <+4>:  movabsq $0x7ffff7ff6000, %rdi     ; imm = 0x7FFFF7FF6000
    0x7ffff7ff700e <+14>: movabsq $0x7ffff6697e80, %rcx     ; imm = 0x7FFFF6697E80

[1] It was first introduced with https://reviews.llvm.org/D38142#change-lF6csxV8HdlL, which has also been the original breaking change. The code has changed a lot since then.

[2] ELF object types: https://github.com/llvm/llvm-project/blob/2d2277f5/llvm/include/llvm/BinaryFormat/ELF.h#L110

Reviewers: labath, JDevlieghere, bkoropoff, clayborg, espindola, alexshap, stella.stamenova

Reviewed By: labath, clayborg

Subscribers: probinson, emaste, aprantl, arichardson, MaskRay, AlexDenisov, yurydelendik, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D61611


Added: lldb/trunk/lit/Breakpoint/Inputs/jitbp.cpp
URL: http://llvm.org/viewvc/llvm-project/lldb/trunk/lit/Breakpoint/Inputs/jitbp.cpp?rev=360354&view=auto
--- lldb/trunk/lit/Breakpoint/Inputs/jitbp.cpp (added)
+++ lldb/trunk/lit/Breakpoint/Inputs/jitbp.cpp Thu May  9 09:40:57 2019
@@ -0,0 +1,2 @@
+int jitbp() { return 0; }
+int main() { return jitbp(); }

Added: lldb/trunk/lit/Breakpoint/jitbp_elf.test
URL: http://llvm.org/viewvc/llvm-project/lldb/trunk/lit/Breakpoint/jitbp_elf.test?rev=360354&view=auto
--- lldb/trunk/lit/Breakpoint/jitbp_elf.test (added)
+++ lldb/trunk/lit/Breakpoint/jitbp_elf.test Thu May  9 09:40:57 2019
@@ -0,0 +1,12 @@
+# REQUIRES: target-x86_64
+# XFAIL: system-windows
+# RUN: %clang -g -S -emit-llvm --target=x86_64-unknown-unknown-elf -o %t.ll %p/Inputs/jitbp.cpp
+# RUN: %lldb -b -o 'settings set plugin.jit-loader.gdb.enable on' -o 'b jitbp' -o 'run -jit-kind=mcjit %t.ll' lli | FileCheck %s
+# CHECK: Breakpoint 1: no locations (pending).
+# CHECK: (lldb) run -jit-kind=mcjit {{.*}}/jitbp_elf.test.tmp.ll
+# CHECK: 1 location added to breakpoint 1
+# CHECK: Process {{.*}} stopped
+# CHECK: JIT(0x{{.*}})`jitbp:
+# CHECK: Process {{.*}} launched: {{.*}}

Modified: lldb/trunk/lit/helper/toolchain.py
URL: http://llvm.org/viewvc/llvm-project/lldb/trunk/lit/helper/toolchain.py?rev=360354&r1=360353&r2=360354&view=diff
--- lldb/trunk/lit/helper/toolchain.py (original)
+++ lldb/trunk/lit/helper/toolchain.py Thu May  9 09:40:57 2019
@@ -134,6 +134,6 @@ def use_support_substitutions(config):
     support_tools = ['yaml2obj', 'obj2yaml', 'llvm-pdbutil',
                      'llvm-mc', 'llvm-readobj', 'llvm-objdump',
-                     'llvm-objcopy']
+                     'llvm-objcopy', 'lli']
     additional_tool_dirs += [config.lldb_tools_dir, config.llvm_tools_dir]
     llvm_config.add_tool_substitutions(support_tools, additional_tool_dirs)

Modified: lldb/trunk/source/Plugins/JITLoader/GDB/JITLoaderGDB.cpp
URL: http://llvm.org/viewvc/llvm-project/lldb/trunk/source/Plugins/JITLoader/GDB/JITLoaderGDB.cpp?rev=360354&r1=360353&r2=360354&view=diff
--- lldb/trunk/source/Plugins/JITLoader/GDB/JITLoaderGDB.cpp (original)
+++ lldb/trunk/source/Plugins/JITLoader/GDB/JITLoaderGDB.cpp Thu May  9 09:40:57 2019
@@ -327,6 +327,10 @@ bool JITLoaderGDB::ReadJITDescriptorImpl
           FileSpec(jit_name), symbolfile_addr, symbolfile_size);
       if (module_sp && module_sp->GetObjectFile()) {
+        // Object formats (like ELF) have no representation for a JIT type.
+        // We will get it wrong, if we deduce it from the header.
+        module_sp->GetObjectFile()->SetType(ObjectFile::eTypeJIT);
         // load the symbol table right away

Modified: lldb/trunk/source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp
URL: http://llvm.org/viewvc/llvm-project/lldb/trunk/source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp?rev=360354&r1=360353&r2=360354&view=diff
--- lldb/trunk/source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp (original)
+++ lldb/trunk/source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp Thu May  9 09:40:57 2019
@@ -1865,7 +1865,7 @@ void ObjectFileELF::CreateSections(Secti
   m_sections_up = llvm::make_unique<SectionList>();
-  VMAddressProvider address_provider(CalculateType());
+  VMAddressProvider address_provider(GetType());
   size_t LoadID = 0;
   for (const auto &EnumPHdr : llvm::enumerate(ProgramHeaders())) {

More information about the lldb-commits mailing list