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A trip down into <sys/regset.h>

March 14, 2011

Just the other day I was working with Ryan Dahl on debugging an issue he hit while working on adding support for Crankshaft —  the new JIT for Google’s v8 — for SunOS. This came about from Bryan’s discovery of what can happen when magic collides. Now, this is a rather delicate operation and there is a lot of special stuff that is going on. Since Ryan and I had an interesting little debugging session and both learned something, I thought I’d share a bit of what was going on with an explanation.

As a part of Crankshaft, they are firing a signal to do a bit of the profiling. Some of the code that is in bleeding edge for src/platform-solaris.cc currently looks like:

615 static void ProfilerSignalHandler(int signal, siginfo_t* info, void* context) {
616   USE(info);
617   if (signal != SIGPROF) return;
618   if (active_sampler_ == NULL || !active_sampler_->IsActive()) return;
619   if (vm_tid_ != pthread_self()) return;
621   TickSample sample_obj;
622   TickSample* sample = CpuProfiler::TickSampleEvent();
623   if (sample == NULL) sample = &sample_obj;
625   // Extracting the sample from the context is extremely machine dependent.
626   ucontext_t* ucontext = reinterpret_cast(context);
627   mcontext_t& mcontext = ucontext->uc_mcontext;
628   sample->state = Top::current_vm_state();
630 #if V8_HOST_ARCH_IA32
631   sample->pc = reinterpret_cast(mcontext.gregs[KDIREG_EIP]);
632   sample->sp = reinterpret_cast(mcontext.gregs[KDIREG_ESP]);
633   sample->fp = reinterpret_cast(mcontext.gregs[KDIREG_EBP]);
634 #elif V8_HOST_ARCH_X64
635   sample->pc = reinterpret_cast(mcontext.gregs[KDIREG_RIP]);
636   sample->sp = reinterpret_cast(mcontext.gregs[KDIREG_RSP]);
637   sample->fp = reinterpret_cast(mcontext.gregs[KDIREG_RBP]);
638 #else
640 #endif
641   active_sampler_->SampleStack(sample);
642   active_sampler_->Tick(sample);
643 }

Now for those of you who have spent a long time working with SunOS might notice what’s wrong with this right away. But in some ways it’s not quite so obvious, so let’s talk about what’s happening.

This code is being used as a signal handler, specifically for SIGPROF. If we pull up the manual page for sigaction(2), the Solaris version has the following comment in its notes section:

     The handler routine can be declared:

       void handler (int sig, siginfo_t *sip, ucontext_t *ucp);

     The sig argument is the signal number. The sip argument is a
     pointer (to space on the stack) to  a  siginfo_t  structure,
     which  provides  additional detail about the delivery of the
     signal. The ucp argument is a pointer (again to space on the
     stack)  to  a  ucontext_t  structure  (defined in <sys/ucon-
     text.h>) which contains the context from before the  signal.
     It  is  not  recommended  that ucp be used by the handler to
     restore the context from before the signal delivery.

SunOS 5.11           Last change: 23 Mar 2005                   5

When a signal is delivered on an x86 UNIX system a program stops doing what it is currently doing and if there is a signal handler, executes the code for the signal handler and then returns to what it was previously doing (this is a bit more complicated in a multi-threaded program). We generally describe this as a signal interrupting the thread in question. This third argument to the handler is a context, which is all the information necessary to describe where a user program is executing. If we peek our heads into <sys/ucontext.h> on an x86 based system (the SPARC version is different)) we will find the following declaration for the structure (with a few #ifdefs along for the ride):

 75 #if !defined(_XPG4_2) || defined(__EXTENSIONS__)
 76 struct  ucontext {
 77 #else
 78 struct  __ucontext {
 79 #endif
 80         unsigned long   uc_flags;
 81         ucontext_t      *uc_link;
 82         sigset_t        uc_sigmask;
 83         stack_t         uc_stack;
 84         mcontext_t      uc_mcontext;
 85         long            uc_filler[5];   /* see ABI spec for Intel386 */
 86 };

Specifically here we are interested in the mcontext — what v8 is using. To best understand what the mcontext is, I took a look at what the OpenGroup defines for ucontext.h in SUSv2. They have the following to say about the mcontext:

mcontext_t  uc_mcontext a machine-specific representation of the saved context

More specifically the mcontext_t has two members. From <sys/regset.h> we get:

378 /*
379  * Structure mcontext defines the complete hardware machine state.
380  * (This structure is specified in the i386 ABI suppl.)
381  */
382 typedef struct {
383         gregset_t       gregs;          /* general register set */
384         fpregset_t      fpregs;         /* floating point register set */
385 } mcontext_t;

Well, that’s exactly what v8 is looking for. From the code snippet there, they are saving three registers that describe how the machine works:

  • The Base Pointer – ebp for i386 and rbp on amd64
  • The Instruction Pointer – eip for i386 and rip for amd64
  • The Stack Pointer – esp for i386 and rsp on amd64

Now keeping track of what each of these does can be quite confusing, so let’s do a quick review.

The instruction pointer holds the address of the next assembly instruction that the CPU should execute for this program. The Base Pointer and Stack Pointer are unfortunately, not quite as intuitive. Memory is laid out in the stack from high addresses towards low addresses. The stack pointer tells us where the bottom of the stack is, i.e. if we decrement the address we can store a new value. When we use the stack, we break it up into what are called stack frames. A stack frame contains everything necessary to run a function: arguments to the function, copies of registers that are expected to be saved, the instruction to return to after the function completes (the eip) and a pointer to the previous stack frame. The ebp points into the current stack frame.

After this brief interlude, we now return to the code that we were working on v8 src/platform_solaris.cc. Now, every so often that code would segfault. With a brief bit of debugging work and comparing the registers before the interrupt was taken with those in the mcontext, we found that we were using the wrong value! Now, if you look back, you’ll see that we’re using macros with prefix KDIREG. These are generally gotten from <sys/kdi_regs.h>. Specifically the definitions used are architecture dependent and for x86 will be found in <ia32/sys/kdi_regs.h> and in <amd64/sys/kdi_regs.h> for amd64. This is the interface that kmdb uses for operating.

In this context, kdi stands for the Kernel/Debugger Interface. So these definitions are meant for structures that are using that interface. When we specified KDIREGS_ESP the value it ended up actually getting out of the register actually was giving us the register ECX. ECX can be used as a general purpose and historically CX was used for loop counters, so the chances that we’re getting an invalid address are pretty high.

However, it turned out it was not too hard to use the correct registers. Looking at <sys/regset.h> had the answer right in front of us:

 91 /*
 92  * The names and offsets defined here are specified by i386 ABI suppl.
 93  */
 95 #define SS              18      /* only stored on a privilege transition */
 96 #define UESP            17      /* only stored on a privilege transition */
 97 #define EFL             16
 98 #define CS              15
 99 #define EIP             14
100 #define ERR             13
101 #define TRAPNO          12
102 #define EAX             11
103 #define ECX             10
104 #define EDX             9
105 #define EBX             8
106 #define ESP             7
107 #define EBP             6
108 #define ESI             5
109 #define EDI             4
110 #define DS              3
111 #define ES              2
112 #define FS              1
113 #define GS              0

This led us to making the obvious substitutions:

630 #if V8_HOST_ARCH_IA32
631   sample->pc = reinterpret_cast(mcontext.gregs[EIP]);
632   sample->sp = reinterpret_cast(mcontext.gregs[ESP]);
633   sample->fp = reinterpret_cast(mcontext.gregs[EBP]);

Well, actually it was almost too obvious, because it segfaulted as well in the same location. However, instead of using address 0xf (a reasonable value for ECX), it actually had 0x0 in the ESP register! Now wait a minute, this is what tells us where the bottom of the stack is, that’s not right, if the bottom of the stack is at 0 we’re in a lot of trouble.

Now, on Solaris x86/amd64 we take interrupts on the stack. These days, most systems use a 1:1 threading model (for reasons why, ask Bryan or read his paper) so for each userland thread there is a kernel thread that corresponds to it which means that each thread has a stack in both userland and the kernel. So here ESP really could be called KESP — referring to the ESP of the kernel thread. So really what we are interested in here is the ESP for userland or the register UESP.

Now that we know that we need to be using UESP, I took another look at the header file and found the following snippet:

115 /* aliases for portability */
117 #if defined(__amd64)
119 #define REG_PC  REG_RIP
120 #define REG_FP  REG_RBP
121 #define REG_SP  REG_RSP
122 #define REG_PS  REG_RFL
123 #define REG_R0  REG_RAX
124 #define REG_R1  REG_RDX
126 #else   /* __i386 */
128 #define REG_PC  EIP
129 #define REG_FP  EBP
130 #define REG_SP  UESP
131 #define REG_PS  EFL
132 #define REG_R0  EAX
133 #define REG_R1  EDX

One of the nice things about this here is that it makes it easier to write code that works across both the x86 and amd64 architectures. Of course, this doesn’t really work when looking at SPARC platforms because the ABI and calling conventions are different due to the differences in CPU architecture. This is one of the things that I personally enjoy about SunOS. The act of defining these more portable aliases is really helpful and if we ever get a 128 bit processor for some reason, those macros will be extended to make sense for it as well. Those portable definitions allowed us to take those architecture ifdefs and just replace it with the following three lines:

631   sample->pc = reinterpret_cast(mcontext.gregs[REG_PC]);
632   sample->sp = reinterpret_cast(mcontext.gregs[REG_SP]);
633   sample->fp = reinterpret_cast(mcontext.gregs[REG_FP]);

That’s about it for our little trip down to sys/regset.h. The fix should hopefully land in v8 (it may even have by the time I get around to posting this) shortly. It should be fun to play around with node and a proper Crankshaft on v8.

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