RFC-0069: Standard I/O in ELF Runner

RFC-0069: Standard I/O in ELF Runner
  • Component Framework

Mechanism for ELF components to forward stdout and stderr streams to the LogSink service.

Gerrit change
Date submitted (year-month-day)2021-02-02
Date reviewed (year-month-day)2021-02-17


Introduce two new flags, forward_stdout_to and forward_stderr_to, controlling an opt in behavior for components wanting to receive stdout and/or stderr stream upon startup. When enabled, the streams will be backed by the LogSink service.


When component manager is started, its stdout and stderr streams are bound to the debuglog since no alternatives exist at this phase of the boot process. As an example, the Archivist, the component offering the LogSink service is itself started by component manager.

Until recently, component manager redirected all components' stdout and stderr streams into the kernel's debuglog. This was achieved by duplicating component manager's own stdout and stderr handles, themselves bound to the debuglog as detailed above. However, this posed a few issues. First, user-mode components should not write to debuglog as that is reserved for kernel usage and highly privileged user-mode components -- rather, most user-mode components should instead write to the LogSink service. Second, providing two outbound streams without an explicit opt in violates the Principle of Least Privilege.

Today, component manager lacks this feature altogether, and we want to bring it back while addressing the two shortcomings which existed previously. Instead of implicitly granting stdout and/or stderr to all components, we propose adding a new flag to the ELF runner to favor an explicit opt in.

The Component Framework team is in the midst of a long-running migration from appmgr (Components v1) to Component Framework (Components v2). One of the major projects of this effort is migrating all of the components owned by the Netstack team. stdout/stderr support is a prerequisite for migrating all of these components. The reason for this requirement is that the Netstack component is written in Go. Go programs, unlike those written in C++ or Rust, are executed in a runtime environment that emits errors to stderr during setup before developer-written programs begin running. Since errors are logged before a program's entry point, there is no way for authors of Go components to bind the stdout and stderr handles to a logging service. To solve this problem, we can fork and modify Go's runtime to include the necessary logging initialization before it begins execution of user-written Go programs. This will of course add a major technical burden on our end, maintaining a Go fork. Alternatively, component manager can bind the stdout and stderr handles to a logging service before the component (and the Go runtime) is even launched, thus allowing the error messages to be captured by Fuchsia's logging services.

More broadly, we'd like to reduce the technical burden for the v1 -> v2 migration effort mentioned above. Currently, appmgr provides stdout and stderr handles to all v1 components and routes them to debuglog. Thus, it's reasonable to assume that many developers within Fuchsia depend on this feature. As we move into 2021 and start migrating more and more components, we should allow developers to maintain the stdout and stderr support they depend on.


The backing logging service must be LogSink and not debuglog. There are several reasons why we must switch to LogSink. First, debuglog is intended for kernel usage, as mentioned above. Second, debuglog uses a small (128kb) shared ring buffer for all processes and rotates messages on a FIFO basis. Archivist periodically drains these messages and forwards them to LogSink. However, they are liable to get "lost" if they are rolled out of the debuglog buffer before Archivist reads them into LogSink. Using LogSink for the stdout and stderr logging service will not only eliminate any chances of messages being dropped but will also decrease the likelihood that other components and processes that use debuglog will have their messages dropped. Plus, there is no mechanism in place to track how much was lost when the buffer rotates faster than it can be drained. Thirdly, debuglog doesn't support severity levels (e.g. DEBUG, INFO, etc.) This is a critical requirement because we need to distinguish stdout messages from stderr ones. With regards to logging, we could only do this by mapping each output stream to a particular severity level.


The proposal is to introduce two new enum values to the program stanza for ELF components, forward_stdout_to and forward_stderr_to. The enum will be optional for ELF components, and will default to none. When it is none, ELF runner will not bind the stdout and stderr handles to the LogSink service (current behavior) and those streams will continue to be ignored. When these values are set to log, the ELF runner will create a socket that will capture the output of the stdout and stderr streams. It will then forward the bytes read to the LogSink service. For stdout, it'll send INFO messages; for stderr, it'll send WARN messages. Messages will be newline-delimited, and each line will be partitioned as an atomic message to the LogSink service. The maximum message size for the LogSink service is 32KB, so we'll cap the byte stream buffer at 30KB (to allow for some space for message metadata). Bytes beyond that boundary will be discarded and only the partial message will be sent to the LogSink service. This raises an interesting edge case where part of code point can be split at the 30KB boundary. In this case, no special handling will be done and the invalid first half of the code point will be decoded as is. All the input bytes will be decoded as UTF-8 using String::from_utf8_lossy. Per this function's API, all invalid UTF-8 sequences will be replaced with U+FFFD REPLACEMENT CHARACTER.

    program: {
        "runner": "elf",
        "forward_stdout_to": "log",
        "forward_stderr_to": "log",


Since the feature will be restricted to ELF runner, the changes required for implementation are fairly small. We predict that no more than 2 CLs would be needed to implement this change.


Performance costs are minimal since lines will just be emitted to LogSink. The most notable overhead that this feature will introduce is the parsing of the byte streams and splitting of newlines. However, this overhead is relatively low-cost since logging is an irregular operation and the byte streams themselves tend to be short. More importantly, this will only affect components that explicitly opt in to this behavior. Therefore if a performance problem does come up, we can quickly address it by simply setting forward_stdout_to and forward_stderr_to to none in those components, temporarily going back to square one until we resolve the underlying performance problem.

Security considerations

Archivist already has the mechanisms in place for attribution and flow control, so a misbehaving component can't deny service by spamming its stdout. So no additional work is needed. However, we should note that this feature provides a mechanism to put arbitrary bytes into another process' address space (from component to Archivist). This could be problematic if the buffer is too large, though we've mentioned mitigation efforts for that above. Also, since this implementation minimally processes the input stream we reduce the risk of escalating privileges. Had this been a complex parser, the likelihood of bugs and vulnerabilities would have increased.

Privacy considerations

The LogSink backend and all LogSink clients are already privacy-compliant, with logs being attributed to their source and with sufficient PII scrubbing mechanisms already being in place. So no additional work is needed.


Alongside unit tests, we'll add integration tests that ensure that logs are written to Archivist. These integrations tests will be written in all supported languages: C/C++, Rust, and Go. Dart will not be tested because Dart component execution is not handled by the ELF runner.


This flag will be documented in the ELF runner section of the Components v2 doc.

Drawbacks, alternatives, and unknowns

Numbered Handles

We've explored raising numbered handles from the process framework to Component Framework as a way of supporting this feature. While we decided against it, the rough design of how the manifest file would have looked like was:

    program: {
        "runner": "elf",
        "handles": ["STDOUT", "STDERR"]

The ELF runner would read the constant strings and map it to its appropriate numbered handle internally. Ultimately, we decided against this because we didn't know of an immediate use case for another numbered handle. Also, if we do find another numbered handle that should be configured in the ELF runner's program stanza, then we can trivially update the manifest file syntax and ELF runner implementation.

Component Manager

We've also explored introducing a new framework-level capability for numbered handles. The rough design of how the manifest file would have looked like was:

    use: [
            "handle": "stdout-to-log",
            "from": "framework",
            "number": "STDOUT",
            "handle": "stderr-to-log",
            "from": "framework",
            "number": "STDERR",

This approach was deemed undesirable for the reasons listed above regarding numbered handles and because introducing this at the component manager-level raises new questions about POSIX-compatibility from component manager. For example, should all runners have to implement this? How would it look like for a runner that doesn't use stdout/stderr, like the "web" runner? Thus, we've decided to pursue the POSIX-compatibility question as a separate workstream, outside of the scope of this RFC.

Introducing New Component

We've also explored implementing the translation layer, the part that parses the stdout/stderr byte streams and forwards them to the LogSink service, in a new component, owned and managed by the Component Framework team. However, after several discussions, it was decided that this approach would be impractical because we'd have to devise a way to retain logging attribution when we log the messages. Archivist uses the event capability to acquire component source info (e.g. moniker) and all that information would be lost if we use a middle-layer component.


We've also explored using fdio, Fuchsia's POSIX-compatibility library, to implement this feature. That is, making a new type that recognizes the stdout/stderr file descriptors and internally (within fdio) redirecting the output to LogSink. However, after several discussions, it was decided to forgo modifying fdio because to do so would make implementation more difficult. We discovered that there are edge cases for POSIX compatibility that couldn't be implemented using a LogSink forwarder in fdio. Also, an fdio-based implementation would yield more uncertainties, and duplicate effort. Alternatively, if we use a socket, as proposed above, it'll be POSIX compliant "out of the box".