[PATCH 2/2] agents: add a skill to determine Fixes tags

[Sasha Levin <sashal@kernel.org>]

Blame and existing Fixes tags can point to code movement instead of the
change that introduced a bug. Add a read-only investigation procedure
that traces historical behavior and checks candidate commits against
their parents before returning a tag with supporting evidence.

Keep the skill entry point and procedure together under
agents/skills/find-fixes/. Connect the procedure to the coding-assistant
guidance so bug fixes get verified attribution before finalization.

If the origin cannot be established, return no trailer and keep the fix
as a draft. Applying the tag and writing the commit message remain outside
the skill.

Validated with controlled histories covering moved code, changed
preconditions, incorrect tags, shallow history, and reintroduction after
a revert, as well as a real kernel fix.

Assisted-by: LLM
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 Documentation/process/coding-assistants.rst |  23 ++-
 agents/skills/find-fixes/SKILL.md           |  23 +++
 agents/skills/find-fixes/guide.rst          | 177 ++++++++++++++++++++
 3 files changed, 220 insertions(+), 3 deletions(-)
 create mode 100644 agents/skills/find-fixes/SKILL.md
 create mode 100644 agents/skills/find-fixes/guide.rst

diff --git a/Documentation/process/coding-assistants.rst b/Documentation/process/coding-assistants.rst
index 3288b32e993f5..8d42d3c10f9ce 100644
--- a/Documentation/process/coding-assistants.rst
+++ b/Documentation/process/coding-assistants.rst
@@ -70,6 +70,21 @@ Example::
 
   Assisted-by: LLM coccinelle sparse
 
+Determining Fixes tags
+======================
+
+Before finalizing a bug-fix patch or its commit message, an assistant must
+follow agents/skills/find-fixes/guide.rst to determine and verify the
+introducing commit. This also applies to fixes for externally reported
+bugs and to checking an existing Fixes tag during review. The skill entry
+point is agents/skills/find-fixes/SKILL.md.
+
+The skill returns an attribution result only. It does not modify files or
+commit messages, and invoking it does not authorize creating or amending a
+commit. If the origin remains unresolved, report the missing evidence and
+keep the fix as a draft. The calling workflow must hold finalization until
+the attribution is resolved.
+
 Procedure for finding and fixing bugs
 =====================================
 
@@ -93,9 +108,11 @@ these steps:
    re-running a complete analysis; drop any fix that doesn't work and try
    another one. The fix must not add build warnings and must pass the
    checkpatch.pl checks (see submitting-patches.rst).
-6. Commit the working fix with a detailed message describing the problem, the
-   solution and a Fixes tag. Do not add a Signed-off-by tag, and add an
-   Assisted-by tag, as described above.
+6. Determine the Fixes tag as described above. Once attribution is resolved,
+   commit the working fix with a detailed message describing the problem,
+   the solution and the verified Fixes tag. This commit is part of the
+   fix-preparation workflow, not the attribution skill. Do not add a
+   Signed-off-by tag, and add an Assisted-by tag, as described above.
 7. Identify the maintainers and lists using scripts/get_maintainer.pl.
    Documentation/process/security-bugs.rst shows how to do that.
 8. Indicate what could not be done. If the fix could not be built or tested, or
diff --git a/agents/skills/find-fixes/SKILL.md b/agents/skills/find-fixes/SKILL.md
new file mode 100644
index 0000000000000..14978976fdd46
--- /dev/null
+++ b/agents/skills/find-fixes/SKILL.md
@@ -0,0 +1,23 @@
+---
+# SPDX-License-Identifier: GPL-2.0-only
+name: find-fixes
+description: >-
+  Determine the bug-introducing commit and return a verified Fixes tag for
+  a Linux kernel fix. Use when preparing or reviewing bug-fix patches,
+  drafting their commit messages, or checking an existing Fixes tag.
+---
+
+# Determine a Fixes tag
+
+Read and follow [the investigation procedure](guide.rst), relative to this
+skill directory. The canonical path in the kernel repository is
+`agents/skills/find-fixes/guide.rst`. If necessary, locate the
+repository root with `git rev-parse --show-toplevel`.
+
+Perform attribution only. Return the tag and its supporting evidence, or
+an unresolved result explaining the missing evidence. Return no other
+trailers or commit-message draft as part of the skill. Do not change files,
+the index, refs, or commit messages, and do not apply the tag or create a
+commit. An unresolved result must contain no Fixes trailer or placeholder;
+it tells the calling workflow to hold finalization. Applying a determined
+tag belongs to that workflow.
diff --git a/agents/skills/find-fixes/guide.rst b/agents/skills/find-fixes/guide.rst
new file mode 100644
index 0000000000000..7b58c526f3149
--- /dev/null
+++ b/agents/skills/find-fixes/guide.rst
@@ -0,0 +1,177 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+.. _find_fixes:
+
+Finding the commit to name in a Fixes tag
+=========================================
+
+A ``Fixes:`` tag identifies the commit that introduced the bug being fixed.
+Finding that commit requires an explanation of the failure and evidence from
+the history. The last commit touching a line, the oldest matching text, an
+existing tag, and a reproducer's first failure are useful leads; none alone
+establishes the answer.
+
+This procedure applies to bug fixes, including fixes for externally reported
+issues and reviews of proposed Fixes tags. It determines a tag using read-only
+inspection. It does not authorize changing files, applying patches, checking
+out revisions, fetching history, or creating or amending commits. Any additional
+testing or changes require the authority of the surrounding task.
+
+Establish the issue and the history
+-----------------------------------
+
+Record the issue, the proposed fix, and the version being fixed:
+
+* For a commit, resolve its full object ID and inspect its message, diff, and
+  parents. For an ordinary commit, its parent is the pre-fix base. For a merge,
+  identify which parent or combined state the fix addresses.
+* For a patch, identify its base revision. For uncommitted changes, distinguish
+  the staged and unstaged changes and record the base commit. Do not include
+  unrelated work in the analysis. Note relevant untracked files separately.
+* Explain the exact failure: the operation, inputs or interleaving, affected
+  configuration, and invariant that is violated. Identify the functions and
+  callers needed to explain it, not just the lines changed by the fix.
+
+Resolve revisions against the repository being inspected. For example, with
+``fix`` set to the supplied revision::
+
+    git rev-parse --show-toplevel
+    git rev-parse --verify --end-of-options "$fix^{commit}"
+    git show --no-patch --format=fuller "$fix"
+    git rev-list --parents -n 1 "$fix"
+    git show --format= --find-renames "$fix"
+
+Use ``git diff --cached`` and ``git diff`` to inspect staged and unstaged work.
+Read the surrounding code at the pre-fix base with ``git show <base>:<path>``.
+Do not assume the current checkout contains the historical implementation.
+
+If the base or failure mechanism cannot be established, report what is missing
+before selecting an introducer. A broad description such as "a race" is not
+enough to distinguish different bugs in the same function.
+
+Trace candidate introductions
+-----------------------------
+
+Start with the operations responsible for the failure. Inspect their history
+and the history of relevant callers, contracts, guards, and data structures.
+Treat any supplied Fixes tag, blame result, or bisection result as a candidate
+to investigate, rather than as the conclusion.
+
+Useful read-only searches include the following. Here ``base``, ``path``,
+``old_path``, ``new_path``, and ``candidate`` refer to identified revisions or
+paths; replace the example line range and expressions with relevant ones::
+
+    git blame -M -C -L 100,140 "$base" -- "$path"
+    git log --follow -p "$base" -- "$path"
+    git log -p -S 'relevant expression' "$base" -- "$old_path" "$new_path"
+    git log -p -G 'relevant.*pattern' "$base" -- "$old_path" "$new_path"
+    git show --find-renames "$candidate" -- "$old_path" "$new_path"
+    git rev-list --parents -n 1 "$candidate"
+
+Blame and pickaxe narrow the search; they do not prove causality. ``-S`` finds
+changes in occurrence counts, while ``-G`` finds matching changed lines. Follow
+renames, copies, splits, and equivalent older implementations explicitly when
+a search stops at code movement. Inspect complete candidate changes and their
+context, including relevant files outside the fix's diff.
+
+Search all relevant ancestry, not just first-parent history. Choose history
+endpoints from the actual base and repository refs; do not assume a remote
+named ``origin`` or a branch named ``master``. When examining history outside
+the base's ancestry, explain how it relates to the affected tree.
+
+Prove the causal transition
+---------------------------
+
+For each plausible candidate, compare its relevant parent state with the state
+after the commit. Explain the invariant before and after the change, why the
+candidate introduces the defect, and how the proposed fix addresses that same
+defect. Inspect the earlier implementation even if the function or filename
+did not yet exist under its current name.
+
+Distinguish three events:
+
+* Introduction of the code or pattern. This is provenance, and may predate any
+  defect in it.
+* Introduction of the semantic defect. This is the change that makes the
+  implementation violate the applicable contract or invariant.
+* A later trigger, newly reachable path, or change that makes the failure easier
+  to observe. This may expose an existing defect, or may itself introduce the
+  defect by changing the contract or execution context.
+
+Do not select a rename or refactor if it merely carries an existing defect.
+Conversely, do not blame old code that was correct under the earlier contract.
+When a later change exposes a latent defect, identify both commits and explain
+why the older implementation was already defective. The location of the fix
+does not prove that the oldest version of the modified code was wrong. If the
+distinction cannot be justified, keep the result unresolved.
+
+A reproducer that fails after the candidate and succeeds before it provides
+useful evidence when both tests exercise equivalent conditions. Build failures,
+missing features, unsupported configurations, and changes to the test itself
+are not evidence of a good parent. A bisection may locate an activation change
+rather than the semantic origin. Historical source analysis is acceptable when
+it establishes the transition; describe its reasoning and limits without
+claiming tests were run.
+
+Handle discontinuities in history
+---------------------------------
+
+* **Merges:** inspect each relevant parent. A bad merge resolution or the
+  interaction of two branches can introduce a defect absent from either parent;
+  do not assume that the first parent alone explains the transition.
+* **Reverts and reintroductions:** trace whether the defect was removed and
+  subsequently restored. Establish the introduction relevant to the affected
+  lineage instead of automatically choosing the earliest occurrence.
+* **Backports:** distinguish the upstream commit from its downstream copy and
+  inspect any adaptation. For an upstream defect, identify the upstream origin
+  and document its relationship to the affected branch. If the defect exists
+  only because of a backport adaptation, identify that downstream introduction.
+  An upstream SHA need not be an ancestor of a downstream base; verify the
+  correspondence from the changes rather than ancestry or subjects alone.
+* **Multiple causes:** keep independent bugs separate. If a single fix repairs
+  distinct introductions, justify each separately before proposing multiple
+  tags. Do not list competing guesses as multiple Fixes tags.
+* **Incomplete history:** check ``git rev-parse --is-shallow-repository`` and
+  whether the required commits and historical blobs are available. A shallow
+  boundary, missing object, or initial import is not proof of introduction.
+  If the defect predates available history, including pre-Git history, report
+  that limit rather than assigning the first available commit by default.
+
+Existing tags and external reports can suggest candidates, but inspect the
+actual historical changes. Reject alternatives with a concrete reason, such
+as unchanged behavior across a file split or a caller that could not supply
+the problematic input under the earlier contract.
+
+Return a conclusion supported by evidence
+-----------------------------------------
+
+Return the attribution result to the caller. Drafting a commit message or
+adding other trailers is outside this procedure.
+
+For a determined result, provide:
+
+* The issue and fix/base revisions analyzed.
+* The full introducing commit ID and an explanation of the parent-to-candidate
+  transition, citing the relevant historical code or test results.
+* Any distinct activation commit, material rejected candidates, and limitations
+  such as tests not run or history not available.
+* The proposed Fixes tag, rendered from the verified commit's Git metadata.
+
+For example, after resolving ``candidate`` to the selected full commit ID::
+
+    git rev-parse --verify --end-of-options "$candidate^{commit}"
+    git show --no-patch --abbrev=12 --format='Fixes: %h ("%s")' "$candidate"
+
+Git extends the abbreviation if necessary to make it unique in the available
+repository. Preserve the exact subject and keep the tag on one line, as
+described in Documentation/process/submitting-patches.rst. Formatting checks
+such as ``scripts/checkpatch.pl`` do not validate the causal conclusion.
+
+For an unresolved result, provide the candidates, evidence, missing information,
+and the next investigation that could distinguish them. Do not emit a Fixes
+trailer or placeholder. The calling bug-fix workflow must hold finalization
+and surface the unresolved origin instead of fabricating a tag or silently
+treating the investigation as complete.
+
+Determining a Fixes tag does not establish stable eligibility or replace the
+requirements in Documentation/process/stable-kernel-rules.rst.