CODEWITHSUNDEEP
gem5
Ciro Santilli
Ciro Santilli

Reputation: 4073

How to run a dynamically linked executable syscall emulation mode se.py in gem5?

After How to solve "FATAL: kernel too old" when running gem5 in syscall emulation SE mode? I managed to run a statically linked hello world under certain conditions.

But if I try to run an ARM dynamically linked one against the stdlib with:

./out/common/gem5/build/ARM/gem5.opt ./gem5/gem5/configs/example/se.py -c ./a.out

it fails with:

fatal: Unable to open dynamic executable's interpreter.

How to make it find the interpreter? Hopefully without copying my cross' toolchain's interpreter on my host's root.

For x86_64 it works if I use my native compiler, and as expected strace says that it is using the native interpreter, but it does not work if I use a cross compiler.

The current FAQ says it is not possible to use dynamic executables: http://gem5.org/Frequently_Asked_Questions but I don't trust it, and then these presentations mention it:

but not how to actually use it.

QEMU user mode has the -L option for that.

Tested in gem5 49f96e7b77925837aa5bc84d4c3453ab5f07408e

https://www.mail-archive.com/[email protected]/msg15582.html

Upvotes: 4

Views: 5106

Answers (3)

Ciro Santilli
Ciro Santilli

Reputation: 4073

Support for dynamic linking has been added in November 2019

At: https://gem5-review.googlesource.com/c/public/gem5/+/23066

It was working for sure at that point, but then it broke at some point and needs fixing.....

If you have a root filesystem to use, for example one generated by Buildroot you can do:

./build/ARM/gem5.opt configs/example/se.py \
  --redirects /lib=/path/to/build/target/lib \
  --redirects /lib64=/path/to/build/target/lib64 \
  --redirects /usr/lib=/path/to/build/target/usr/lib \
  --redirects /usr/lib64=/path/to/build/target/usr/lib64 \
  --interp-dir /path/to/build/target \
  --cmd /path/to/build/target/bin/hello

Or if you are using an Ubuntu cross compiler toolchain for example in Ubuntu 18.04:

sudo apt install gcc-aarch64-linux-gnu
aarch64-linux-gnu-gcc -o hello.out hello.c
./build/ARM/gem5.opt configs/example/se.py \
  --interp-dir /usr/aarch64-linux-gnu \
  --redirects /lib=/usr/aarch64-linux-gnu/lib \
  --cmd hello.out

You have to add any paths that might contain dynamic libraries as a separate --redirect as well. Those are enough for C executables.

--interp-dir sets the root directory where the dynamic loader will be searched for, based on ELF metadata which says the path of the loader. For example, buildroot ELF files set that path to /lib/ld-linux-aarch64.so.1, and the loader is a file present at /path/to/build/target/lib/ld-linux-aarch64.so.1. As mentioned by Brandon, this path can be found with:

 readelf -a $bin_name | grep interp

The main difficulty with syscall emulation dynamic linking, is that we want somehow:

  • linker file accesses to go to a magic directory to find libraries there
  • other file accesses from the main application to go to normal paths, e.g. to read an input file in the current working directory

and it is hard to detect if we are in the loader or not, especially because this can happen via dlopen in the middle of a program.

The --redirects option is a simple solution for that.

For example /lib=/path/to/build/target/lib makes it so that if the guest would access the C standard library /lib/libc.so.6, then gem5 sees that this is inside /lib and redirects the path to /path/to/build/target/lib/libc.so.6 instead.

The slight downside is that it becomes impossible to actually access files in the /lib directory of the host, but this is not common, so it works in most cases.

If you miss any --redirect, the dynamic linker will likely complain that the library was not found with a message of type:

hello.out: error while loading shared libraries: libstdc++.so.6: cannot open shared object file: No such file or directory

If that happens, you have to find the libstdc++.so.6 library in the target filesystem / toolchain and add the missing --redirect.

It later broke at https://gem5.atlassian.net/browse/GEM5-430 but was fixed again.

Downsides of dynamic linking

Once I got dynamic linking to work, I noticed that it actually has the following downsides, which might or not be considerable depending on the application:

  • the dynamic linker has to run some instructions, and if you have a very minimal userland test executable, and are running on a low CPU like O3, then this startup can dominate runtime, so watch out for that

  • ExecAll does not show symbol names for stdlib functions, you just get offsets from some random nearest symbol e.g. @__end__+274873692728. Maybe something along these lines would work: Debugging shared libraries with gdbserver but not sure

  • dynamically jumping to a stdlib function for the first time requires going through the dynamic linking machinery, which can create problems if you are trying to control a microbench.

    I actually already hit this once: the dynamic version of a program was doing something extra that and that compounded with a gem5 bug broke my experiment, and cost me a few hours of debugging.

Interpreters like Python and Java

Python and Java are just executables, and the script to execute an argument to the executable.

So in theory, you can run them in syscall emulation mode e.g. with:

build/ARM/gem5.opt configs/example/se.py --cmd /usr/bin/python --options='hello.py arg1 arg2'

In practice however hugely complex executable like interpreters are likely to have syscalls that not yet implemented given the current state of gem5 as of November 2019, see also: When to use full system FS vs syscall emulation SE with userland programs in gem5?

Generally it is not hard to implement / ignore uneeded calls though, so give it a shot. Related threads:

Old answer

I have been told that as of 49f96e7b77925837aa5bc84d4c3453ab5f07408e (May 2018) there is no convenient / well tested way for running dynamically linked cross arch executables in syscall emulation: https://www.mail-archive.com/[email protected]/msg15585.html

I suspect however that it wouldn't be very hard to patch gem5 to support it. QEMU user mode already supports that, you just have to point to the root filesystem with -L.

Upvotes: 6

Brandon Potter
Brandon Potter

Reputation: 51

The cross-compiled binary should have an .interp entry if it's a dynamic executable.

Verify it with readelf:

 readelf -a $bin_name | grep interp

The simulator is setup to find this section in main executable when it loads the executable into the simulated address space. If this sections exists, a c-string is set to point to that text (normally something like /lib64/ld-linux-x86-64.so.2). The simulator then calls glibc's open function with that c-string as the parameter. Effectively, this opens the dynamic linker-loader for the simulator as a normal file. The simulator then maps the file into the simulated address space in phases with mmap and mmap_fixed.

For cross compilation, this code must fail. The error message follows it directly if the simulator cannot open the file. To make this work, you need to debug the opening process and possibly the subsequent pasting of the loader into the address space. There is mechanism to set the program's entry point into the loader rather than directly into the code section of the main binary. (It's done through the auxiliary vector on the stack.) You may need to play around with that as well.

The interesting code is (as of 05/29/19) in src/base/loader/elf_object.cc.

Upvotes: 2

DateBro
DateBro

Reputation: 11

I encountered this problem after I just cross compiled the code. You can try to add "--static" after the command.

Upvotes: 1

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