[gnoi] file: implement Stat in-house using /mnt/host

[hdwhdw]

Why I did it

File.Stat currently delegates to a SONiC host service over D-Bus
(sonic_service_client.GetFileStat). That introduces a runtime
dependency on the host service for what is fundamentally a metadata
read against the host filesystem — metadata that is already reachable
from inside the gNMI container through the existing /mnt/host bind
mount used by File.Put, File.Remove, and File.TransferToRemote.

Bringing Stat in-house removes the D-Bus hop, lets the RPC be served
entirely from the gNMI binary, and aligns Stat with how the rest of
the file RPCs already access the host filesystem. It also unblocks
directory Stat: per the gNOI proto comment ("Stat will list files at
the provided path") a directory input should return one entry per
child, which the D-Bus path did not implement.

How I did it

• Added a pure-Go handler pkg/gnoi/file.HandleStat that reads
  metadata via os.Stat / os.ReadDir and reuses the existing
  translatePathForContainer helper to bridge container-vs-host
  paths.
• The handler now supports both inputs:
  • regular file → single StatInfo for that file
  • directory → one StatInfo per immediate child (non-recursive)
• gnmi_server.FileServer.Stat is now a thin wrapper: authenticate,
  then delegate to HandleStat. The readFileStat helper and the
  D-Bus dependency for Stat are removed.
• Permissions are encoded octal-as-decimal as the proto requires
  (e.g. mode 0644 → Permissions=644).
• umask is reported as a constant 0022 (defaultUmask).
  Rationale: the proto field is "default file creation mask" — a
  process-level attribute, not a per-file one — and it can't be
  derived from os.Stat. syscall.Umask is racy across goroutines
  (it both reads and writes in a single call). The previous D-Bus
  implementation also returned a process umask, so callers that
  treated this as informational see no change.

Corner cases worth flagging

Not blockers, but reviewers may want to confirm these match intent:

1. No path-traversal whitelist on Stat. Cleaned paths like
   /tmp/../etc/hostname resolve to /etc/hostname and Stat returns
   it. This matches the previous D-Bus implementation (which also
   had no whitelist) and is consistent with Stat being a read-only
   metadata RPC. The write/remove RPCs (Put, TransferToRemote,
   Remove) continue to enforce the existing /tmp/, /var/tmp/,
   /host/ whitelist. Easy to add a Stat whitelist later if security
   review wants one.
2. Absolute symlinks point inside the container, not the host.
   os.Stat follows symlinks; if the link target is host-absolute,
   the kernel resolves it against the gNMI container's filesystem
   root, not the host's, and Stat returns NotFound. This is a
   pre-existing limitation of every /mnt/host-based code path
   (internal/diskspace, pkg/gnoi/file.Put, Remove,
   TransferToRemote); it is not introduced by this change.
   Relative symlinks resolve correctly.
3. Asymmetric symlink handling within Stat. Stat on a single
   symlink uses os.Stat (follows). Listing a directory uses
   os.ReadDir + entry.Info(), which is lstat-equivalent (does
   not follow). The proto does not specify either way; ls -l does
   not follow either.
4. PermissionDenied is not exercised by the live test. The
   gNMI container in a vanilla KVM testbed runs as uid=0 with
   CAP_DAC_OVERRIDE, so no filesystem permission can deny it.
   The branch is exercised by the unit tests; it will fire in
   hardened production deployments where the container is
   unprivileged.

How to verify it

Unit tests

• 8 new tests in pkg/gnoi/file/stat_test.go covering nil request,
  empty path, relative path, NotFound, regular-file success,
  directory listing (with a subdir), empty directory, and the
  octal-as-decimal permissions encoding for several modes.
• Existing gnmi_server Stat tests rewritten to drive the new
  handler against real temp files (no D-Bus mocks).
• Pure-test count: 628 → 636 (+8 new). No regressions.

Live end-to-end test on a sonic-mgmt KVM testbed (vlab-01)

Built the deb from the branch, installed it inside the running gNMI
container via the host bind mount, restarted the supervised gNMI
process, and exercised File.Stat over the gNMI container's local
Unix-domain socket.

Reproducible example:

# 1. Build the deb (any sonic-gnmi build path works; e.g.
#    dpkg-buildpackage in the sonic-slave-trixie container).
dpkg-buildpackage -rfakeroot -b -us -uc

# 2. Push it to the DUT and install it inside the gNMI container.
scp ../sonic-gnmi_0.1_amd64.deb admin@vlab-01:/tmp/
ssh admin@vlab-01 \
  'docker exec gnmi dpkg -i /mnt/host/tmp/sonic-gnmi_0.1_amd64.deb && \
   docker exec gnmi supervisorctl restart gnmi-native'

# 3. Make a fixture and stat the directory through the gNMI UDS.
ssh admin@vlab-01 'mkdir -p /tmp/stat-smoke && \
  printf "hello stat" > /tmp/stat-smoke/file.txt && \
  touch /tmp/stat-smoke/empty.bin && mkdir /tmp/stat-smoke/sub'

ssh admin@vlab-01 \
  "sudo grpcurl -plaintext -d '{\"path\":\"/tmp/stat-smoke\"}' \
       unix:///var/run/gnmi/gnmi.sock gnoi.file.File/Stat"

Real output:

{
  "stats": [
    {
      "path": "/tmp/stat-smoke/empty.bin",
      "lastModified": "1780514631773086705",
      "permissions": 664,
      "umask": 18
    },
    {
      "path": "/tmp/stat-smoke/file.txt",
      "lastModified": "1780514631769086705",
      "permissions": 664,
      "size": "10",
      "umask": 18
    },
    {
      "path": "/tmp/stat-smoke/sub",
      "lastModified": "1780514631773086705",
      "permissions": 775,
      "umask": 18
    }
  ]
}

Notes on the example:

• unix:///var/run/gnmi/gnmi.sock is the gNMI container's local
  UDS. It bypasses the TLS / mTLS chain that the TCP listeners on
  :50051 / :50052 require, so it is the practical way to
  exercise the server interactively on a test DUT.
• sudo is required because the socket is mode 0660 root:root.
• permissions: 664 is octal 0664 encoded as decimal per the
  gNOI proto. umask: 18 is decimal for 0022 octal — the
  constant defaultUmask discussed above.
• size is omitted (proto default 0) for directories.

Test matrix (all exercised against the deployed binary using the
same grpcurl / UDS flow):

Path	Type	Expected	Got
/tmp/stat-smoke/file.txt	regular file (10 B, 0664)	1 StatInfo with size=10, permissions=664	✅
/tmp/stat-smoke	directory with 3 children	3 StatInfo entries, dirs reported with no size	✅
/tmp/this-does-not-exist-xyz	absent	NotFound	✅
""	—	InvalidArgument: path cannot be empty	✅
relative/foo	—	InvalidArgument: path must be absolute	✅
/etc/hostname	regular file outside /tmp	success — confirms /mnt/host translation is general	✅
/var/log	non-/tmp directory	full child listing	✅
/tmp/stat-extra/../stat-extra/target.txt	path needing filepath.Clean	resolves to canonical path; response carries the cleaned form	✅
/host/reboot-cause	real SONiC dir with mixed entries	lists history/, platform/, reboot-cause.txt, and the previous-reboot-cause.json symlink	✅

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111

Description for the changelog

[gnoi] file: implement Stat in-house using /mnt/host; add directory support

Link to config_db schema for YANG module changes

N/A — no YANG changes.

[mssonicbld]

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[Copilot]

Pull request overview

This PR updates the gNOI File.Stat implementation to remove the D-Bus dependency and instead serve Stat requests directly from the gNMI binary by reading host filesystem metadata via the existing /mnt/host bind-mount translation logic.

Changes:

• Added a pure-Go pkg/gnoi/file.HandleStat that uses os.Stat / os.ReadDir and emits StatInfo (including octal-as-decimal permissions and a constant umask).
• Updated gnmi_server.FileServer.Stat to authenticate and delegate to the new handler; removed the legacy D-Bus-based Stat path.
• Added/updated unit tests to cover Stat behavior for files and directories without D-Bus mocking.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
pkg/gnoi/file/file.go	Adds HandleStat, statInfoFromFileInfo, and defaultUmask; switches Stat to host filesystem reads via translated paths.
pkg/gnoi/file/stat_test.go	Adds new unit tests for HandleStat and permissions encoding.
gnmi_server/gnoi_file.go	Replaces the old D-Bus Stat logic with a thin wrapper delegating to gnoifile.HandleStat.
gnmi_server/gnoi_file_test.go	Reworks Stat tests to exercise the new handler behavior (real temp files/dirs).
gnmi_server/server_test.go	Updates the gNOI Stat integration-style tests to validate the new filesystem-based behavior.

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