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Norman Feske avatar

Pine fun - How did we come here?


Some kids from the city once told me about programs called "debuggers". They also use a technology named "green light" to cross the streets. City kids. As we are still far away from urban territory, we are in need of the rural ways of debugging. What are our options?

Remember, at the end of the previous article, we were greeted with the first life sign of the kernel:

 Error: Assertion failed: id < _count && _cpus[id].constructed()
 Error:   File: /.../repos/base-hw/src/core/kernel/cpu.cc:205
 Error:   Function: Kernel::Cpu& Kernel::Cpu_pool::cpu(unsigned int)

This raises the questions: What is the trouble about? How did we get there? What went wrong?

Thankfully, the message gives us a concrete reference to the code cpu.cc at line 205.

 Cpu & Cpu_pool::cpu(unsigned const id)
 {
     assert(id < _count && _cpus[id].constructed());
     return *_cpus[id];
 }

By looking at this code, I'm tempted to draw the connection to the corners we cut regarding the Board::Cpu::wake_up_all_cpus method, which we deliberately left empty. But let us leave this speculation for later.

To get hold of the situation, it is useful to know which part of the condition fails. This can be revealed by adding the following instrumentation at the beginning of the method.

 Genode::log("cpu: id=", id, " _count=", _count, " "
             "constructed=", _cpus[id].constructed());

The Genode::log function is declared in the base/log.h header. Note that it relies on a fair bit of framework infrastructure such as synchronization primitives. In desperate situations during the debugging of lowest-level framework code, the Genode::raw function can become handy as a drop-in replacement. In contrast to log, the raw function relies on less infrastructure. In practice, I use log by default and raw as last resort. After rebuilding the system image and rebooting the board, it turns out that the log function works well at this point.

 ...
 cpu: id=0 _count=4 constructed=0
 Error: Assertion failed: id < _count && _cpus[id].constructed()

The instrumentation tells us that the first element of the _cpus array has not been properly constructed. But how to find out why? We ultimately need to know the call chain that led to execution of the Cpu_pool::cpu method.

Option 1: Walking the source code

The most obvious approach is studying the source code, and determining the immediate callers of the method using grep.

 repos/base-hw$ grep -r "\<cpu("

Unfortunately, "cpu" is a pretty bad pattern to grep for. It is too generic. However, we know that the code in question must reside inside repos/base-hw/ and can thereby restrict the search to only this part of the source tree. Furthermore, by using "\<" (match only the start of a word) and appending "(" to the pattern (as expected at the caller site), we can narrow down the number of matches to a useful level.

 src/test/cpu_quota/main.cc:                  env.cpu()),
 src/test/cpu_quota/main.cc:       Cpu_session::Quota quota = env.cpu().quota();
 src/core/spec/arm_v8/virtualization/kernel/vm.cc: _vcpu_context(cpu_pool().cpu(cpu))
 src/core/spec/arm_v8/virtualization/kernel/vm.cc: affinity(cpu_pool().cpu(cpu));
 src/core/spec/arm_v7/virtualization/kernel/vm.cc: _vcpu_context(cpu_pool().cpu(cpu))
 src/core/spec/arm_v7/virtualization/kernel/vm.cc: affinity(cpu_pool().cpu(cpu));
 src/core/kernel/kernel.cc:    Cpu &cpu = cpu_pool().cpu(Cpu::executing_id());
 src/core/kernel/cpu_mp.cc:    Irq(Board::Pic::IPI, cpu), cpu(cpu)
 src/core/kernel/cpu.h:        Cpu & cpu(unsigned const id);
 src/core/kernel/cpu.h:        Cpu & primary_cpu() { return cpu(Cpu::primary_id()); }
 src/core/kernel/cpu.h:        Cpu & executing_cpu() { return cpu(Cpu::executing_id()); }
 src/core/kernel/cpu.h:            for (unsigned i = 0; i < _count; i++) func(cpu(i));
 src/core/kernel/cpu.cc:Cpu & Cpu_pool::cpu(unsigned const id)
 src/core/kernel/cpu_up.cc:Kernel::Cpu::Ipi::Ipi(Kernel::Cpu & cpu) : Irq(~0U, cpu), cpu(cpu) { }
 src/core/kernel/cpu_context.h:        void cpu(Cpu &cpu) { _cpu = &cpu; }
 src/core/kernel/thread.cc:    Cpu & cpu = cpu_pool().cpu(user_arg_2());

From these results, we can immediately disregard the lines referring to src/test/. Also the virtualization-related matches are most likely not of interest. When inspecting the remaining lines, the number of potential callers comes down to 5:

 src/core/kernel/kernel.cc:    Cpu &cpu = cpu_pool().cpu(Cpu::executing_id());
 src/core/kernel/cpu.h:        Cpu & primary_cpu() { return cpu(Cpu::primary_id()); }
 src/core/kernel/cpu.h:        Cpu & executing_cpu() { return cpu(Cpu::executing_id()); }
 src/core/kernel/cpu.h:            for (unsigned i = 0; i < _count; i++) func(cpu(i));
 src/core/kernel/thread.cc:    Cpu & cpu = cpu_pool().cpu(user_arg_2());

With such a low amount of callers, we can apply brute force by adding the following line just before each call.

 Genode::log(__FILE__, ":", __LINE__);

The compiler replaces __FILE__ with a string of the file name of the source code and __LINE__ with a string of the line number where __LINE__ appears within the source file. Another useful magic macro is __PRETTY_FUNCTION__, which expands to the name of the surrounding function.

After rebooting the board with the instrumentations in place, we see the origin of the call:

 /.../repos/base-hw/src/core/kernel/kernel.cc:25

A look at the surrounding code reveals that the function call originates from a function called 'kernel':

 extern "C" void kernel()
 {
   ...
   Cpu &cpu = cpu_pool().cpu(Cpu::executing_id());
   ...
 }

You might guess what's next?

 repos/base-hw$ grep -r "\<kernel(" *

There is only one caller, which is at src/core/kernel/init.cc and brings the kernel_init function to our attention.

Granted, this step-wise instrumentation may feel a bit like chopping wood with a nail clipper. But I sometimes enjoy the process anyway. By following call chains in reverse by browsing and instrumenting the code, one develops some kind of peripheral vision for the code around the call path, which fosters the sense of familiarity with the code base.

Of course, using grep manually as described above may be too archaic for your taste. There exist plenty of dedicated tools (like ctags, cscope) and IDEs for aiding source-code discovery after all. Personally, I prefer simple tools. As a small life hack, I have put the following snippet in my Vim configuration:

 nnoremap <leader>g :execute
          \ "grep! -R -I --exclude-dir=.git
                       \ --exclude=*.orig
                       \ --exclude=.*.swp
                       \ --exclude=*.rej
                       \ --exclude=*~ "
                       \ . shellescape("\\<" . expand("<cword>") . "\\>")
                       \ . " ."<cr>:copen<cr><cr>

Similar to how the * and # commands search for the word under the cursor in the current buffer, the <leader>g command above allows me to grep the word under the cursor in the source tree and presents the results in a quickfix window. So I can quickly jump to each occurrence and travel across source files like a poor man's hypertext system.

That all said, once when ending up in a situation with many callers, the approach of manually instrumenting all caller sites becomes a nuisance, which leads us to the second option.

Option 2: One step of ground truth at a time

Instead of instrumenting all potential caller sites, we can let the return addresses as found on the stack guide us by using the following line as instrumentation:

 Genode::log("called from ", __builtin_return_address(0));

When executed, this line prints us the return address of the current function scope. This address corresponds to the caller. By placing this line into the Cpu_pool::cpu method, we get the following output.

 Starting kernel ...

 called from 0xffffffc000058720
 Error: Assertion failed: id < _count && _cpus[id].constructed()

The high number immediately tells us that the executed code resides somewhere high up in virtual memory. That means, we have already passed the bootstrap stage, the MMU is enabled, and core/kernel code is executed. As explained in the previous article, the corresponding ELF binary resides at build/arm_v8a/var/run/log.core and can be inspected via readelf.

 build/arm_v8a$ readelf -l var/run/log.core | grep LOAD
   LOAD           0x0000000000001000 0xffffffc000000000 0xffffffc000000000
   LOAD           0x00000000000c1000 0xffffffc0000c0000 0xffffffc0000c0000
   LOAD           0x00000000000ef5c0 0x0000000000000000 0x0000000000000000

The addresses of the ELF segments correlate nicely with the value printed by our instrumentation. To determine the exact source-code location for the given return address, the objdump tool becomes handy. It allows one to disassemble an ELF binary while displaying the source-code intermixed. The tool is specific to the used CPU architecture. That is, for 64-bit ARM, it is called genode-aarch64-objdump. To use it interactively from the shell, the tool chain's bin/ directory should be added to the shell's PATH variable:

 $ export PATH=/usr/local/genode/tool/current/bin/:$PATH

With the PATH variable set, we can disassemble the log.core ELF binary and pipe the result to less for inspection:

 build/arm_v8a$ genode-aarch64-objdump -lSd var/run/log.core | less

Note that the amount of output generated by objdump can be huge. By replacing less by wc -l one can see that the output comprises more than 300,000 lines! Still, this amount of data leaves less unimpressed, which leaves me impressed. We can simply search for our address ffffffc000058720 (with the 0x prefix stripped away) via the slash (/) command and end up at the following section of output:

 kernel():
 /.../base-hw/src/core/kernel/kernel.cc:25
 ffffffc000058718:       12001c21        and     w1, w1, #0xff
 ffffffc00005871c:       97fff8e3        bl      ffffffc000056aa8 <_ZN6Kernel8Cpu
 _pool3cpuEj>
 ffffffc000058720:       aa0003f4        mov     x20, x0
 /.../base-hw/src/core/kernel/kernel.cc:29
         Cpu_job * new_job;

The source location kernel.cc line 25 looks familiar.

Alternatively to going though the disassembled output of objdump, the addr2line utility can be used to streamline the lookup of a source-code location by a given instruction address.

 $ genode-aarch64-addr2line -e var/run/log.core 0xffffffc000058720
 /.../base-hw/src/core/kernel/kernel.cc:25

This is fast and convenient. But sometimes, in particular when code is excessively inlined, the contextual information given by the objdump output can be valuable. Most often, I scroll upwards until hitting the next occurrence of a .cc file and watch silently the lines - header-file names and fragments of source code - that scroll by. Again, peripheral vision at play.

Option 3: Backtraces

The __builtin_return_address feature of the compiler allows us to follow the call chain one step at a time. Each step involves a manual instrumentation, a compile-test cycle, and the invocation of the addr2line utility.

To avoid such repetitive work, Genode provides the utility function Genode::backtrace() to walk the stack and print the return addresses along the way. This function is declared in the os/backtrace.h header. An instrumentation of the Cpu_pool::cpu method would look as follows.

 #include <os/backtrace.h>

 Cpu & Cpu_pool::cpu(unsigned const id)
 {
   Genode::backtrace();
   ...
 }

To assist the backtrace() function to parse stack frames correctly, the Genode build system must be instructed to preserve frame-pointer information. This can be achieved by placing the following line to the build directory's etc/tools.conf file. Note that by default there is no such file. So you will have to create one containing this line.

 CC_OPT += -fno-omit-frame-pointer

After rebuilding and running the system image the next time, we are greeted with quite a lot of output:

 Starting kernel ...

 0xffffffc000058738
 0xffffffc00000085c
 0xffffffc0000568e0
 0xffffffc000056a60
 0xffffffc000057e44
 0x400273d8
 0x40026754
 0x40010068
 0xffffffc000058738

Each of the values starting with 0xfff... is a valid return address and can be used with objdump or addr2line as described above. To make matters more convenient, the addr2line utility can be used in an "interactive" fashion be running the following command in a separate terminal.

 build/arm_v8a$ genode-aarch64-addr2line -e var/run/log.core

With no address specified at the command line, the tool simply waits for standard input. So we can paste multiple lines of the Genode::backtrace() output directly into it and get the following result:

 0xffffffc000058738
 0xffffffc00000085c
 0xffffffc0000568e0
 0xffffffc000056a60
 0xffffffc000057e44
 /.../base-hw/src/core/kernel/kernel.cc:25
 :?
 /.../base-hw/src/core/spec/arm/virtualization/gicv2.h:22
 /.../base/include/util/reconstructible.h:56
 /.../base-hw/src/core/kernel/init.cc:64 (discriminator 1)

We can spot both of the familiar locations kernel.cc line 25 and init.cc line 64.

As shown above, the standard GNU binutils and compiler features can bring us quite far without using a debugger. We have gathered a lot of input for investigating the error. Our next job will be using this information to discharge it. Read on ...