Unikraft C++ Large Global Data Issue

Defining a large string vector as a global data results in page fault. This is a program that highlights the issue.

The main.cpp program defines the CXX_VALUES vector of strings. For 2725 of items (each using the "EXAMPLE" string) it doesn't fail. With 2726 items it fails.

Trigger the issue by assigning the USE_CXX_VALUES macro to 1 and the SHOW_ISSUE macro to 1 at the beginning of the main.cpp program. This is the default assignment.

Building

Build the program either using kraft:

kraft configure -p kvm -m x86_64
kraft build

Build it for linuxu using:

kraft configure -p linuxu -m x86_64
kraft build

Or by using the Makefile-based approach:

make menuconfig
make

When running make menuconfig simply save the configuration. This step is required to load the configuration from Config.uk.

In make menuconfig you can select both the kvm and linuxu platforms to build an image for each platform.

Running

The images are part of the build/ folder. Run them either using kraft:

kraft run -p kvm

or

kraft run -p linuxu

Or directly:

./build/app-cxx-data-issue_linuxu-x86_64

or

./qemu-guest -k build/app-cxx-data-issue_kvm-x86_64.dbg

Summary of Issue

Running for linuxu results in a Segmentation fault message. Running for kvm results in a page fault message.

A stack trace of the issue points to:

(gdb) bt
#0  std::__1::ios_base::Init::Init (this=<optimized out>) at /home/razvan/projects/unicore/unikraft-my/apps/t/build/libcxx/origin/libcxx-7.0.0.src/src/iostream.cpp:85
#1  0x0000000000204fae in __static_initialization_and_destruction_0 (__priority=65535, __initialize_p=1) at /home/razvan/projects/unicore/unikraft-my/apps/t/build/libcxx/origin/libcxx-7
.0.0.src/src/iostream.cpp:80
#2  _GLOBAL__sub_I__ZNSt3__13cinE () at /home/razvan/projects/unicore/unikraft-my/apps/t/build/libcxx/origin/libcxx-7.0.0.src/src/iostream.cpp:124
#3  0x000000000010c0b2 in main_thread_func (arg=0x3fdff50) at /home/razvan/projects/unicore/unikraft-my/unikraft.git/lib/ukboot/boot.c:135
#4  0x0000000000105757 in asm_thread_starter () at /home/razvan/projects/unicore/unikraft-my/unikraft.git/plat/common/x86/thread_start.S:39
#5  0x0000000000000000 in ?? ()

The highlighted instruction (=>) is the one where the issue occurs.

(gdb) disass
Dump of assembler code for function std::__1::ios_base::Init::Init():
   0x0000000000141e10 <+0>:     push   %rbp
   0x0000000000141e11 <+1>:     mov    %rsp,%rbp
   0x0000000000141e14 <+4>:     push   %rbx
   0x0000000000141e15 <+5>:     sub    $0x18,%rsp
   0x0000000000141e19 <+9>:     callq  0x110ec0 <__getreent>
   0x0000000000141e1e <+14>:    mov    $0x29a3a0,%edi
=> 0x0000000000141e23 <+19>:    mov    0x8(%rax),%rbx
   0x0000000000141e27 <+23>:    callq  0x213640 <std::__1::basic_streambuf<char, std::__1::char_traits<char> >::basic_streambuf()>
   0x0000000000141e2c <+28>:    lea    -0x18(%rbp),%rdi

It seems to point to a C++ initialization issue in Unikraft, before the calling of the main() function. It's possible to be related to memory alocation / initialization in the Unikraft startup sequence.