sandboxed-api/contrib/pffft

Sandboxing PFFFT library

This library was sandboxed as part of Google's summer 2020 internship program (blog post).

Build System: CMake OS: Linux

How to use from an existing Project

If your project does not include Sandboxed API as a dependency yet, add the following lines to the main CMakeLists.txt:

include(FetchContent)

FetchContent_Declare(sandboxed-api
  GIT_REPOSITORY https://github.com/google/sandboxed-api
  GIT_TAG        main  # Or pin a specific commit/tag
)
FetchContent_MakeAvailable(sandboxed-api)  # CMake 3.14 or higher

add_sapi_subdirectory(contrib/pffft)

The add_sapi_subdirectory() macro sets up the source and binary directories for the sandboxed jsonnet targets.

Afterwards your project's code can link to sapi_contrib::pffft and use the generated header pffft_sapi.sapi.h. An example sandbox policy can be found in main_pffft_sandboxed.cc.

For testing:

cd build, then ./pffft_sandboxed

For debug:

display custom info with ./pffft_sandboxed --logtostderr

About the project

PFFFT library is concerned with 1D Fast-Fourier Transformations finding a compromise between accuracy and speed. It deals with real and complex vectors, both cases being illustrated in the testing part (test_pffft.c for initially and original version, main_pffft_sandboxed.cc for our currently implemented sandboxed version). The original files can be found at: https://bitbucket.org/jpommier/pffft/src.*

The purpose of sandboxing is to limit the permissions and capabilities of library’s methods, in order to secure the usage of them. After obtaining the sandbox, the functions will be called through an Sandbox API (being called api in the current test) and so, the operations, system calls or namspaces access may be controlled. From both pffft.h and fftpack.h headers, useful methods are added to sapi library builded with CMake. There is also a need to link math library as the transformations made require mathematical operators. Regarding the testing of the methods, one main is doing this job by iterating through a set of values, that represents the accuracy of transformations and print the speed for each value and type of transformation. More specifically, the input length is the target for accuracy (named as n) and it stands for the number of data points from the series that calculate the result of transformation. It is also important to mention that the complex variable stands for a boolean value that tells the type of transformation (0 for REAL and 1 for COMPLEX) and it is taken into account while testing. In the end, the performance of PFFFT library it is outlined by the output. There are two output formats available, from which you can choose through --output_format= command-line flag. Without using this type of argument when running, the output format is set by default.*

CMake observations resume:

• linking pffft and fftpack (which contains necessary functions for pffft)
• set math library

Sandboxed main observations resume:

• containing two testing parts (fft / pffft benchmarks)
• showing the performance of the transformations implies testing them through various FFT dimenstions. Variable n, the input length, will take specific values meaning the number of points to which it is set the calculus (more details of mathematical purpose of n - https://en.wikipedia.org/wiki/Cooley%E2%80%93Tukey_FFT_algorithm).
• output shows speed depending on the input length
• use --output_format=0 or --output_format=1 arguments to choose between output formats. 0 is for a detailed output, while 1 is only displaying each transformation process speed.

Bugs history

1. [Solved] pffft benchmark bug: "Sandbox not active"

   n = 64, status OK, pffft_transform generates error n > 64, status not OK Problem on initialising absl::StatusOr<PFFFT_Setup *> s; the memory that stays for s is not the same with the address passed in pffft_transform function. (sapi::v::GenericPtr - to be changed)

   Temporary solution: change the generated files to accept uintptr_t instead of PFFFT_Setup

   Solution: using sapi::v::RemotePtr instead of sapi::v::GenericPtr to access the memory of object s

2. [Unresolved] compiling bug: "No space left on device"

   The building process creates some embed files that use lots of memory, trying to write them on /tmp.

   Temporary solution: clean /tmp directory by sudo rm -rf /tmp/*