fsgofer: add extension interface for custom backends#13216
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Building a custom gofer (e.g. for network-backed storage, encrypted filesystems, or tiered caches) currently requires forking the runsc binary and copying/maintaining unexported setup and seccomp code. This adds an `Extension` interface that lets custom filesystem backends register with the stock gofer and serve LISAFS connections for specific mounts without forking. This builds on #13180, which moved LISAFS implementation selection and connection options to the connection. With that in place, custom backends can plug into the stock gofer without creating separate `lisafs.Server` instances. All mounts continue to share one `lisafs.Server`, preserving the server-side filesystem tree and synchronization across stock and extension-backed mounts. Registered extensions are queried in order for each mount. `NewConnection` returns a nil `lisafs.ConnectionImpl` to decline a mount, and the first extension that returns a non-nil implementation handles it. `NewConnection` receives the sandbox's OCI runtime spec, the specific `*specs.Mount` being served, the resolved mount path, and readonly state, so extensions can read sandbox-wide configuration from `spec.Annotations` and per-mount configuration from the mount itself without a side channel. Stock `fsgofer` remains the default when no extension claims a mount. Extensions only choose the per-connection `lisafs.ConnectionImpl` and `lisafs.ConnectionOpts` now supported by `lisafs.Server.CreateConnection`. `SeccompRules` lets extensions declare additional syscalls, merged with the stock gofer allowlist before installation. There are no behavior changes when no extensions are registered. Also adds documentation in `g3doc/user_guide/filesystem.md` and `pkg/lisafs/README.md` describing how to use the extension interface. FUTURE_COPYBARA_INTEGRATE_REVIEW=#12950 from shayonj:s/gofer-backend-v2 7139233 PiperOrigin-RevId: 918225708
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fsgofer: add extension interface for custom backends
Building a custom gofer (e.g. for network-backed storage, encrypted filesystems, or tiered caches) currently requires forking the runsc binary and copying/maintaining unexported setup and seccomp code. This adds an
Extensioninterface that lets custom filesystem backends register with the stock gofer and serve LISAFS connections for specific mounts without forking.This builds on #13180, which moved LISAFS implementation selection and connection options to the connection. With that in place, custom backends can plug into the stock gofer without creating separate
lisafs.Serverinstances. All mounts continue to share onelisafs.Server, preserving the server-side filesystem tree and synchronization across stock and extension-backed mounts.Registered extensions are queried in order for each mount.
NewConnectionreturns a nillisafs.ConnectionImplto decline a mount, and the first extension that returns a non-nil implementation handles it.NewConnectionreceives the sandbox's OCI runtime spec, the specific*specs.Mountbeing served, the resolved mount path, and readonly state, so extensions can read sandbox-wide configuration fromspec.Annotationsand per-mount configuration from the mount itself without a side channel.Stock
fsgoferremains the default when no extension claims a mount. Extensions only choose the per-connectionlisafs.ConnectionImplandlisafs.ConnectionOptsnow supported bylisafs.Server.CreateConnection.SeccompRuleslets extensions declare additional syscalls, merged with the stock gofer allowlist before installation.There are no behavior changes when no extensions are registered.
Also adds documentation in
g3doc/user_guide/filesystem.mdandpkg/lisafs/README.mddescribing how to use the extension interface.FUTURE_COPYBARA_INTEGRATE_REVIEW=#12950 from shayonj:s/gofer-backend-v2 7139233