A Build ID (or debug info ID) is a value that uniquely identifies a build. It is intended to capture the "meaning" or inputs of the build, and is usually associated with debug info. So for example, programs compiled from different sources would have different build IDs, even if their generated code happened to be the same. A common use case is when a build is created with debug info, and then the debug info is stripped before distribution and archived by the developer. If both the distributed and archived version retain the build ID, then they can be matched, and the distributed version can be debugged or symbolized.
Because the build ID is usually intended to identify debug info, tools that transform Wasm binaries will need to decide whether they will preserve, drop, or recompute the build ID. Generally speaking, if a transformation would invalidate debug info (for example, by rewriting the code section and changing the code offsets), then the tool should drop or recompute the build ID. If a tool updates or regenerates debug info along with its transformation, it should also generally update the build ID, since the debug info no longer matches. Conversely, adding custom sections to a binary would generally not require updating the build ID.
The Build ID section is a custom section
and thus has no semantic effects and can be stripped at any time. It is named
build_id and has no restriction on where in the binary it can appear. It
consists of only 2 fields:
| Field | Type | Description |
|---|---|---|
| length | varuint32 |
number of bytes that follow |
| id | bytes |
sequence of bytes |
Unlike most "string" fields in wasm binaries, the id field is arbitrary binary
data and is not required to be valid UTF-8.
The exact size and method of generating the build ID is unspecified, other than that it should be unique. LLVM lld supports several different methods, including a hash (which includes at least all the code, data, and debug info sections of the output), a random (v4) UUID, and user-specified values. The default is to hash the output and create from it a name-based (v5) UUID, which is deterministic for a given set of inputs.