sandboxed-api/oss-internship-2020/curl

LibCurl Sandbox

This library is a sandboxed version of curl's C API, libcurl, implemented using Sandboxed API.

Setup

The repository can be cloned using: git clone --recursive <URL to this repo> The --recursive flag ensures that submodules are also cloned.

Alternatively, if the repository has already been cloned but the submodules have not, these can be cloned using: git submodule update --init --recursive

The full list of Sandboxed API dependencies can be found on Sandboxed API Getting Started page.

The following commands, used from the current curl/ directory, build the library:

mkdir -p build && cd build
cmake .. -G Ninja -D SAPI_ROOT=<path to sandboxed-api>
cmake --build .

Implementation details

All of libcurl's methods are supported by the library. However, a few of these have different signatures defined in the sandboxed header custom_curl.h, which wraps and extends libcurl.

This is necessary because Sandboxed API sandboxes most of libcurl correctly, but encounters some issues when sandboxing a few methods. The simplest solution is wrapping these methods into wrapper methods that accomplish the same tasks but can also be sandboxed.

The next sections describe the issues encountered and contain some information on the signatures of the wrapper methods solving these issues.

Variadic methods

Variadic methods are currently not supported by Sandboxed API. To solve this, these methods are defined with an additional explicit parameter in custom_curl.h.

The methods are:

• curl_easy_setopt. Use curl_easy_setopt_ptr, curl_easy_setopt_long or curl_easy_setopt_curl_off_t instead.
• curl_easy_getinfo. Use curl_easy_getinfo_ptr instead.
• curl_multi_setopt. Use curl_multi_setopt_ptr, curl_multi_setopt_long or curl_multi_setopt_curl_off_t instead.
• curl_share_setopt. Use curl_share_setopt_ptr or curl_share_setopt_long instead

Methods with incomplete array arguments

Incomplete array arguments are currently not supported by Sandboxed API. To solve this, methods taking an incomplete array argument have a wrapper in custom_curl.h, and take a pointer as the argument.

The methods are:

• curl_multi_poll. Use curl_multi_poll_sapi instead.
• curl_multi_wait. Use curl_multi_wait_sapi instead.

Methods with conflicts on the generated header

Some methods create conflicts on the generated header because of redefined #define directives from files included by the header. To solve this, the conflicting types and methods are redefined in custom_curl.h.

The types are:

• time_t. Use time_t_sapi instead.
• fd_set. Use fd_set_sapi instead.

The methods are:

• curl_getdate. Use curl_getdate_sapi instead.
• curl_multi_fdset. Use curl_multi_fdset_sapi instead.

Function pointers

The functions whose pointers will be passed to the library's methods (callbacks) can't be implemented in the files making use of the library, but must be in other files. These files must be compiled together with the library, and this is done by adding their absolute path to the cmake variable CURL_SAPI_CALLBACKS.

The pointers can then be obtained using an RPCChannel object, as shown in example2.cc.

Examples

The examples directory contains the sandboxed versions of example source codes taken from this page on curl's website. More information about each example can be found in the examples' README.

To build these examples when building the library, the cmake variable CURL_SAPI_ENABLE_EXAMPLES must be set to ON. This enables Sandboxed API examples as well.

Policy

The sandbox.h file contains a policy allowing all is necessary for libcurl to perform simple requests. It is used by all the examples, except by example3. This example needs some additional policies and files in its namespace (since it uses HTTPS), and the file example3.cc shows how to easily extend an existing policy.

Testing

The tests folder contains some test cases created using Google Test. The class CurlTestUtils is used to facilitate some tasks that all test cases need, including the setup of a mock local server on which test requests are performed.

To build these tests when building the library, the cmake variable CURL_SAPI_ENABLE_TESTS must be set to ON. This enables Sandboxed API tests as well.

Callbacks

The callbacks.h and callbacks.cc files implement all the callbacks used by examples and tests.