On Fri, 2018-03-23 at 13:05 +0100, Christophe de Dinechin wrote: > From: Christophe de Dinechin <dinec...@redhat.com> > > This change addresses three issues related to plugin version checking: > > 1. It is possible for plugins to bypass version checking or do it wrong > (as a matter of fact, the mjpeg fallback sets a bad example) > > 2. The current plugin version check violates the C++ ODR, i.e. > it relies on undefined behaviors when the header used to compile > the plugin and to compile the agent are not identical, > > 3. A major.minor numbering scheme is not ideal for ABI checks. > In particular, it makes it difficult to react to an incompatibility > that was detected post release. > > [More info] > > 1. Make it impossible to bypass version checking > > The current code depends on the plugin implementing the version check > correctly by calling PluginVersionIsCompatible. To make things worse, > the only publicly available example gets this wrong and uses an > ad-hoc version check, so anybody copy-pasting this code will get it > wrong. > > It is more robust to do the version check in the agent before calling > any method in the plugin. It ensures that version checking cannot be > bypassed, is done consistently and generates consistent error messages. > > As an indication that the aproach is robust, the new check correctly > refuses to load older plugins that use the old version checking method: > > spice-streaming-agent[5167]: > error loading plugin [...].so: no version information > > 2. ODR-related problems > > The C++ One Definition Rule (ODR) states that all translation units > must see the same definitions. In the current code, when we call > Agent::PluginVersionIsCompatible from the plugin, it is an ODR > violation as soon as we have made any change in the Agent class > compared to what the plugin was compiled against. > > The current code therefore relies on implementation dependent knowlege > of how virtual functions are laid out in the vtable, and puts > unnecessary constraints on the changes allowed in the classes > (e.g. it's not allowed to put anything before some member functions) > > 3. Major.minor numbering scheme > > The major.minor numbering scheme initially selected makes it harder > to fixes cases where an incompatibility was detected after release. > > For example, the major.minor version checking assumes that agent 1.21 > is compatible with plugins 1.21, 1.13 or 1.03. If later testing > shows that 1.13 actually introduced an incompatiliy, you have to > special-case 1.13 in the compatibiliy check. > > An approach that does not have this problem is to rely on incremented > version numbers, with a "current" and "oldest compatible" version > number. This is used for example by libtools [1]. > > Since the change required for #1 and #2 introduces an ABI break, > it is as good a time as any to also change the numbering scheme, > since changing it later would introduce another unnecessary ABI break.
Great! AFAIK we haven't made a release yet so we don't need to worry about ABI breakage yet? > [1] > https://www.gnu.org/software/libtool/manual/html_node/Libtool-versioning.html > > Signed-off-by: Christophe de Dinechin <dinec...@redhat.com> > --- > include/spice-streaming-agent/plugin.hpp | 50 > +++++++++++++++++--------------- > src/concrete-agent.cpp | 35 +++++++++++----------- > src/concrete-agent.hpp | 4 --- > src/mjpeg-fallback.cpp | 3 -- > 4 files changed, 45 insertions(+), 47 deletions(-) > > diff --git a/include/spice-streaming-agent/plugin.hpp > b/include/spice-streaming-agent/plugin.hpp > index e08e3a6..0ec5040 100644 > --- a/include/spice-streaming-agent/plugin.hpp > +++ b/include/spice-streaming-agent/plugin.hpp > @@ -23,11 +23,22 @@ namespace streaming_agent { > class FrameCapture; > > /*! > - * Plugin version, only using few bits, schema is 0xMMmm > - * where MM is major and mm is the minor, can be easily expanded > - * using more bits in the future > + * Plugins use a versioning system similar to that implemented by libtool > + * > http://www.gnu.org/software/libtool/manual/html_node/Libtool-versioning.html > + * Update the version information as follows: > + * [ANY CHANGE] If any interfaces have been added, removed, or changed since > the last update, > + * increment PluginInterfaceVersion. > + * [COMPATIBLE CHANGE] If interfaces have only been added since the last > public release, > + * leave PluginInterfaceOldestCompatibleVersion identical. > + * [INCOMPATIBLE CHANGE] If any interfaces have been removed or changed > since the last release, > + * set PluginInterfaceOldestCompatibleVersion to PluginInterfaceVersion. > + * [DETECTED INCOMPATIBILITY]: If an incompatibility is detected after a > release, > + * set PluginInterfaceOldestCompatibleVersion to the last known compatible > version. > */ > -enum Constants : unsigned { PluginVersion = 0x100u }; > +enum Constants : unsigned { > + PluginInterfaceVersion = 1, > + PluginInterfaceOldestCompatibleVersion = 1 > +}; This is still not too pretty, consider at least renaming Constants to something better, or even use something like: struct PluginInterfaceVersion { static constexpr uint16_t current = 1; static constexpr uint16_t oldest_compatible = 1; }; For the encapsulation? Cheers, Lukas > enum Ranks : unsigned { > /// this plugin should not be used > @@ -103,20 +114,6 @@ class Agent > { > public: > /*! > - * Get agent version. > - * Plugin should check the version for compatibility before doing > - * everything. > - * \return version specified like PluginVersion > - */ > - virtual unsigned Version() const = 0; > - > - /*! > - * Check if a given plugin version is compatible with this agent > - * \return true is compatible > - */ > - virtual bool PluginVersionIsCompatible(unsigned pluginVersion) const = 0; > - > - /*! > * Register a plugin in the system. > */ > virtual void Register(Plugin& plugin) = 0; > @@ -136,18 +133,25 @@ typedef bool > PluginInitFunc(spice::streaming_agent::Agent* agent); > > #ifndef SPICE_STREAMING_AGENT_PROGRAM > /*! > + * Plugin interface version > + * Each plugin should define this variable and set it to > PluginInterfaceVersion > + * That version will be checked by the agent before executing any plugin code > + */ > +extern "C" unsigned spice_streaming_agent_plugin_interface_version; > + > +/*! > * Plugin main entry point. > - * Plugins should check if the version of the agent is compatible. > - * If is compatible should register itself to the agent and return > - * true. > - * If is not compatible can decide to stay in memory or not returning > - * true (do not unload) or false (safe to unload). This is necessary > + * This entry point is only called if the version check passed. > + * It should return true if it loaded and initialized successfully. > + * If the plugin does not initialize and does not want to be unloaded, > + * it may still return true on failure. This is necessary > * if the plugin uses some library which are not safe to be unloaded. > * This public interface is also designed to avoid exporting data from > * the plugin which could be a problem in some systems. > * \return true if plugin should stay loaded, false otherwise > */ > extern "C" spice::streaming_agent::PluginInitFunc > spice_streaming_agent_plugin_init; > + > #endif > > #endif // SPICE_STREAMING_AGENT_PLUGIN_HPP > diff --git a/src/concrete-agent.cpp b/src/concrete-agent.cpp > index 4cf70e7..eb4f333 100644 > --- a/src/concrete-agent.cpp > +++ b/src/concrete-agent.cpp > @@ -15,28 +15,11 @@ > > using namespace spice::streaming_agent; > > -static inline unsigned MajorVersion(unsigned version) > -{ > - return version >> 8; > -} > - > -static inline unsigned MinorVersion(unsigned version) > -{ > - return version & 0xffu; > -} > - > ConcreteAgent::ConcreteAgent() > { > options.push_back(ConcreteConfigureOption(nullptr, nullptr)); > } > > -bool ConcreteAgent::PluginVersionIsCompatible(unsigned pluginVersion) const > -{ > - unsigned version = Version(); > - return MajorVersion(version) == MajorVersion(pluginVersion) && > - MinorVersion(version) >= MinorVersion(pluginVersion); > -} > - > void ConcreteAgent::Register(Plugin& plugin) > { > plugins.push_back(std::shared_ptr<Plugin>(&plugin)); > @@ -83,6 +66,24 @@ void ConcreteAgent::LoadPlugin(const std::string > &plugin_filename) > return; > } > > + unsigned *version = > + (unsigned *) dlsym(dl, > "spice_streaming_agent_plugin_interface_version"); > + if (!version) { > + syslog(LOG_ERR, "error loading plugin %s: no version information", > + plugin_filename.c_str()); > + return; > + } > + if (*version < PluginInterfaceOldestCompatibleVersion || > + *version > PluginInterfaceVersion) { > + syslog(LOG_ERR, > + "error loading plugin %s: plugin interface version %u, " > + "agent accepts version %u...%u", > + plugin_filename.c_str(), *version, > + PluginInterfaceOldestCompatibleVersion, > + PluginInterfaceVersion); > + return; > + } > + > try { > PluginInitFunc* init_func = > (PluginInitFunc *) dlsym(dl, > "spice_streaming_agent_plugin_init"); > diff --git a/src/concrete-agent.hpp b/src/concrete-agent.hpp > index 5bca23b..c631916 100644 > --- a/src/concrete-agent.hpp > +++ b/src/concrete-agent.hpp > @@ -27,16 +27,12 @@ class ConcreteAgent final : public Agent > { > public: > ConcreteAgent(); > - unsigned Version() const override { > - return PluginVersion; > - } > void Register(Plugin& plugin) override; > const ConfigureOption* Options() const override; > void LoadPlugins(const std::string &directory); > // pointer must remain valid > void AddOption(const char *name, const char *value); > FrameCapture *GetBestFrameCapture(const std::set<SpiceVideoCodecType>& > codecs); > - bool PluginVersionIsCompatible(unsigned pluginVersion) const override; > private: > void LoadPlugin(const std::string &plugin_filename); > std::vector<std::shared_ptr<Plugin>> plugins; > diff --git a/src/mjpeg-fallback.cpp b/src/mjpeg-fallback.cpp > index 68c282f..605a4b3 100644 > --- a/src/mjpeg-fallback.cpp > +++ b/src/mjpeg-fallback.cpp > @@ -180,9 +180,6 @@ SpiceVideoCodecType MjpegPlugin::VideoCodecType() const { > > bool MjpegPlugin::Register(Agent* agent) > { > - if (agent->Version() != PluginVersion) > - return false; > - > std::unique_ptr<MjpegPlugin> plugin(new MjpegPlugin()); > > try { > -- > 2.13.5 (Apple Git-94) > _______________________________________________ > Spice-devel mailing list > Spice-devel@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/spice-devel _______________________________________________ Spice-devel mailing list Spice-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/spice-devel
