SIGSEGV when executing 32 bit binary on 64 bit Linux

Question

You may have heard of StoneKnifeForth, a project by kragen: https://github.com/kragen/stoneknifeforth. It's a Python program that acts as a small Forth interpreter and a Forth program that acts as a Forth compiler. Therefore you can build a Forth compiler binary using those two in unison.

After porting StoneKnifeForth to C++ (https://github.com/tekknolagi/stoneknifecpp), I noticed that all binaries produced by StoneKnifeForth (either variety) segfault on 64 bit Linux. That is, if you clone stoneknifecpp and run:

make
./l01compiler  # produced by the Forth program

You'll get the following:

willow% ./l01compiler                 
[1]    31614 segmentation fault  ./l01compiler

This isn't a very interesting error message, obviously, so I thought I would strace it:

willow% strace ./l01compiler
execve("./l01compiler", ["./l01compiler"], [/* 110 vars */]) = -1 EPERM (Operation not permitted)
--- SIGSEGV {si_signo=SIGSEGV, si_code=SI_KERNEL, si_addr=0} ---
+++ killed by SIGSEGV +++
[1]    31615 segmentation fault (core dumped)  strace ./l01compiler

And got... somewhat more information. It looks like the ELF header is wrong somehow, except for the following two interesting tidbits:

It runs fine under 32 bit qemu
It runs fine if I sudo ./l01compiler

I'm a bit of a loss as to why this is, even after internet searching for possible differences between ELF header formats between 32 bit and 64 bit Linux kernels, etc. If anyone has any information, I would be delighted.

I have attached the header below:

willow% readelf -h l01compiler   
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Intel 80386
  Version:                           0x1
  Entry point address:               0x1e39
  Start of program headers:          52 (bytes into file)
  Start of section headers:          0 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         1
  Size of section headers:           40 (bytes)
  Number of section headers:         0
  Section header string table index: 0

Please show output from readelf -l. It's likely that the problem is with the PT_LOAD segments, not the ELF header. — Employed Russian
– Employed Russian, Commented Aug 18, 2017 at 5:36
@EmployedRussian My friend Tom helped find the issue earlier today. I'll try and post a self answer soon. — tekknolagi
– tekknolagi, Commented Aug 18, 2017 at 7:09
@EmployedRussian I've posted an answer that explains things. — tekknolagi
– tekknolagi, Commented Aug 18, 2017 at 21:41

tekknolagi · Accepted Answer · 2017-08-18 21:53:18Z

2

Big thanks to Tom Hebb (tchebb) for confirming my suspicions and figuring this out. As can be seen in this commit, the problem was that the origin address was simply too low. It's not related to 32 bit or 64 bit, but instead earlier kernel vs newer kernel.

The newer kernels have increased the vm.mmap_min_addr sysctl parameter, meaning that the old origin would prohibit the program from starting at all. That explains why sudo worked. As Tom explained, "Unless you invoke qemu with KVM support, qemu is an emulator not a hypervisor, so it simulates the entire address space and virtual memory subsystem in software and presumably doesn't impose any load address restrictions."

edited Aug 18, 2017 at 21:53

answered Aug 18, 2017 at 21:41

tekknolagi

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2 Comments

Employed Russian Over a year ago

Your answer explains why the program ran ok under QEMU, but not why it ran under sudo. In particular, comment you referenced says "we can't go lower than 0x10000", while your program is linked at 0x1000 (or thereabouts).

tekknolagi Over a year ago

@EmployedRussian you're right. I kind of handwaved over that. I'm not entirely sure, but I am continuing to look it up. wiki.debian.org/mmap_min_addr, for example, says that root solves the problem for dosemu/qemu...

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