Category Archives: Linux

The Hell that MOVAPS Hath Wrought

One of the most difficult bugs we tracked down in SourceMod was a seemingly random crash bug. It occurred quite often in CS:S DM and GunGame:SM, but only on Linux. The crash usually looked like this, although the exact callstack and final function varied:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1209907520 (LWP 5436)]
0xb763ed87 in CPhysicsTrace::SweepBoxIVP () from bin/
(gdb) bt
#0  0xb763ed87 in CPhysicsTrace::SweepBoxIVP () from bin/
#1  0xb7214329 in CEngineTrace::ClipRayToVPhysics () from bin/
#2  0xb7214aad in CEngineTrace::ClipRayToCollideable () from bin/
#3  0xb72156cc in CEngineTrace::TraceRay () from bin/

This crash occurred quite often in normal plugins as well. Finally, one day we were able to reproduce it by calling TraceRay() directly. However, it would only crash from a plugin. The exact same code worked fine if the callstack was C/C++. But as soon as the call emanated from the SourcePawn JIT, it crashed. Something extremely subtle was going wrong in the JIT.

After scratching our heads for a while, we decided to disassemble the function in question — CPhysicsTrace::SweepBoxIVP(). Here is the relevant crash area, with the arrow pointing toward the crashed EIP:

   0xb7667d7c :        mov    DWORD PTR [esp+8],edi
   0xb7667d80 :        lea    edi,[esp+0x260]
-> 0xb7667d87 :        movaps XMMWORD PTR [esp+48],xmm0
   0xb7667d8c :        mov    DWORD PTR [esp+0x244],edx

We generated a quick crash and checked ESP in case the stack was corrupted. It wasn’t, and the memory was both readable and writable. So what does the NASM manual say about MOVAPS?

When the source or destination operand is a memory location, it must be aligned on a 16-byte boundary. To move data in and out of memory locations that are not known to be on 16-byte boundaries, use the MOVUPS instruction.

Aha! GCC decided that it was safe to optimize MOVUPS to MOVAPS because it knows all of its functions will be aligned to 16-byte boundaries. This is a good example of where whole-program optimization doesn’t take external libraries into account. I don’t know how GCC determines when to make this optimization, but for all intents and purposes, it’s reasonable.

The SourcePawn JIT, of course, was making no effort to keep the stack 16-byte aligned for GCC. That’s mostly because the JIT is a 1:1 translation of the compiler’s opcodes, which are processor-independent. As a fix, faluco changed the JIT to align the stack before handing control to external functions.

Suddenly, an entire class of bugs disappeared from SourceMod forever. It was a nice feeling, but at least a week of effort was put into tracking it down. The moral of this story is that source-level debugging for “impossible crashes” is usually in vain.

Linux and C++ ABI, Part 2

Last week I briefly mentioned the nightmares behind Linux and GNU Standard C++ ABI compatibility. This manifested itself at a very bad time — at ESEA, we were getting ready to launch our European servers. Unlike their USA counterparts, two of the physical machines were 32-bit. I made 32-bit builds of our software and uploaded them to both machines. I checked one box and everything was working. Then we launched.

Guess what? The machine I didn’t check? Nothing was working. A critical Metamod plugin was failing to load with the message: cannot handle TLS data

I had seen this before, and quickly panicked; in past cases I’d never managed to fix it. It is virtually undocumented (try a Google search), and the problem is occurring in the ELF loader, not exactly a trivial area of Linux to start dissecting at product launch.

The first thing I did was download the libc source code for the OS – 2.3.4. A grep for “handle TLS data” revealed this in elf/dl-load.c:

Select All Code:
    case PT_TLS:
#ifdef USE_TLS
      /* Uh-oh, the binary expects TLS support but we cannot
         provide it.  */
      errval = 0;
      errstring = N_("cannot handle TLS data");
      goto call_lose;

It looked like, which was required by our binary, had a PT_TLS section in its header. TLS is thread local storage. So, now I needed to find out why the TLS block couldn’t be created.

First, I opened up in a diassembler. The function name in question was _dl_map_object_from_fd. I then had to find where the PT_TLS case was handled (constant value is 7). From the source code, a _dl_next_tls_modid() is called shortly after. I found that call and traced back the jumps using IDA’s cross-reference feature. I found this:

.text:4AA03A8C                 cmp     eax, 7
.text:4AA03A8F                 nop
.text:4AA03A90                 jnz     short loc_4AA03A50
.text:4AA03A92                 mov     eax, [esi+14h]
.text:4AA03A95                 test    eax, eax
.text:4AA03A97                 jz      short loc_4AA03A50

Bingo! Clearly, USE_TLS is defined, otherwise it wouldn’t bother with a jump. So the problem is definitely in the logic somewhere, rather than a lack of TLS support. It was time for some debugging with gdb:

(gdb) set disas intel
(gdb) display/i $pc
(gdb) br _dl_map_object_from_fd
Breakpoint 1 at 0x4aa03866

I got lucky with the matching address and put a direct breakpoint:

(gdb) del 1
(gdb) br *0x4AA03A8C
Breakpoint 2 at 0x4aa03a8c

I stepped through the assembly, following along in my source code. I narrowed the failing condition down to this code:

         /* If GL(dl_tls_dtv_slotinfo_list) == NULL, then rtld.c did
         not set up TLS data structures, so don't use them now.  */
          || __builtin_expect (GL(dl_tls_dtv_slotinfo_list) != NULL, 1))

I opened up rtld.c and searched for dl_tls_dtv_slotinfo_list — and the answer was immediately apparent:

  /* We do not initialize any of the TLS functionality unless any of the
     initial modules uses TLS.  This makes dynamic loading of modules with
     TLS impossible, but to support it requires either eagerly doing setup
     now or lazily doing it later.  Doing it now makes us incompatible with
     an old kernel that can't perform TLS_INIT_TP, even if no TLS is ever
     used.  Trying to do it lazily is too hairy to try when there could be
     multiple threads (from a non-TLS-using libpthread).  */
  if (!TLS_INIT_TP_EXPENSIVE || GL(dl_tls_max_dtv_idx) > 0)

And there it was. That version of glibc refused to late-load dynamic libraries that had TLS requirements. When I checked the working server, it had a much later libc (2.5 from Centos 5, versus 2.3.4 from Centos 4.4).

I am hardly worthy of nit-picking the likes of glibc maintainers, but I find it lame that the error message was completely undocumented, as was the (lack of) functionality therein. While researching this, I also looked through the glibc CVS – the bug was first fixed here. The big comment explaining the bug remains, even though it appears that as of this revision, it is no longer applicable. Whether that’s true or not, I don’t know. The actual revision comments are effectively useless for determining what the changes mean. I may never really know.

How did I end up solving this? Rather than do an entire system upgrade, I removed our libstdc++ dependency. It just so happened it was there by accident. Oops. Note that earlier versions of libstdc++ had no PT_TLS references — which is why this is a subtle ABI issue.

In the end, the moral of the story is: binary compatibility on Linux is a nightmare. It’s no fault of the just the kernel, or GNU — it’s the fault of everyone picking and enforcing their own standards.

As a final tirade, Glibc needs to get dlerror() message documentation. “Use the source, Luke,” is not an acceptable API reference.

Linux and C++ ABI, Part 1

One particularly annoying gem about developing on Linux is the general disregard for cross-distribution binary compatibility. There are two big reasons for this, one philosophical and the other is semi-technical:

  • Philosophical: There is a tendency to believe that almost everything on Linux is (or should be) open source; instead of vendors distributing binaries, they should distribute source code for users to compile.
  • Technical: Linux Distribution vendors can theoretically call or place libraries whatever or wherever they want, or compile them with any options they feel necessary, or leave options out which they feel are unnecessary.

Projects which distribute cross-distro binaries (whether open source or not) have to fight an uphill battle on both of these counts. But it only gets worse when dealing with the GNU Standard C++ Library. The huge problem with libstdc++ is that the compiler offers no easy way to either exclude it or to link it statically.

Excluding libstdc++ is possible if you invoke the compiler with gcc instead of g++ — but you need to overload new, new[], delete, and delete[] (simple malloc() wrappers will do). If you use try/catch/throw or dynamic_cast, forget it. You have to link to libstdc++ no matter what.

Static linking is out of the question for DSOs. You can run into all sorts of problems when loading other dynamic libraries or another libstdc++ in the same process. Not only that, but static linking can be nearly impossible.

It gets worse when you realize that the ABI version to libstdc++ tends to get bumped quite often. AMX Mod X experienced compatibility bumps from GCC versions 2.95, 3.0-3.3, and 3.4. As each distribution decides to package different libstdc++ versions/builds, and some distributions are too old to even support the compiler we choose to use, it becomes increasingly difficult to distribute Linux binaries. Asking average users to compile from source is out of the question — many falter on even a graphical installer.

The continual problems with Linux distributions and libstdc++ are well-known and documented by now. This was just your introduction. Next week I will share an extremely frustrating and subtle ABI problem I ran into with libstdc++ and glibc.