Re: [PATCH v3 6/7] agents: add a skill to find other instances of a fixed bug

[Bradley Morgan]

On 16 October 2026 16:28:26 BST, Sasha Levin <sashal@xxxxxxxxxx> wrote:
>A bug fixed in one place often exists elsewhere, in copied code, sibling
>drivers, or other users of the same interface, and reviewers routinely
>ask whether that was checked. Left alone, agents tend to audit the whole
>tree by reading it. Add a skill that states the defect as a rule,
>searches for it through shared interfaces, copied code, the introducing
>series and earlier fixes, and verifies each candidate before reporting
>it.
>
>When Coccinelle is available, the skill searches with a semantic patch:
>it looks for an existing one under scripts/coccinelle/, checks that the
>rule marks the fixed site in the code before the fix, runs it over the
>whole tree with coccicheck, and checks which code spatch could not
>parse. These steps live in references/coccinelle.md, which the agent
>reads only when it uses Coccinelle.
>
>Tested on the parent of commit 989fe6771266 ("drm/nouveau/gsp: fix
>mismatched alloc/free for kvmalloc()"): both the existing
>api/kfree_mismatch.cocci and the example rule mark the two kfree() calls
>that commit fixed, and a whole-tree run found the same bug in four other
>drivers that were later fixed separately.
>
>The sites found are reported, not fixed; each needs its own fix, Fixes
>tag and stable decision. Point the bug-fix procedure in
>coding-assistants.rst at the skill.

Reviewed-by: Bradley Morgan <brads@xxxxxxxxxxxxxx>



>
>Assisted-by: LLM
>Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
>---
> Documentation/process/coding-assistants.rst   |  5 +-
> agents/skills/find-siblings/SKILL.md          | 31 +++++++++
> .../find-siblings/references/coccinelle.md    | 66 +++++++++++++++++++
> 3 files changed, 101 insertions(+), 1 deletion(-)
> create mode 100644 agents/skills/find-siblings/SKILL.md
> create mode 100644 agents/skills/find-siblings/references/coccinelle.md
>
>diff --git a/Documentation/process/coding-assistants.rst b/Documentation/process/coding-assistants.rst
>index fc4e98fd96e11..a89a33980c41f 100644
>--- a/Documentation/process/coding-assistants.rst
>+++ b/Documentation/process/coding-assistants.rst
>@@ -39,6 +39,8 @@ The following skills are available:
>   checks an existing one.
> * agents/skills/stable-tag/SKILL.md decides whether a fix goes to stable
> and
>   returns its stable tag, or checks an existing one.
>+* agents/skills/find-siblings/SKILL.md looks for the same bug elsewhere in the
>+  tree.
> 
> Licensing and Legal Requirements
> ================================
>@@ -101,7 +103,8 @@ these steps:
> 5. Build and verify that the fix works either using the reproducer or by
>    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).
>+   checkpatch.pl checks (see submitting-patches.rst). Then look for the same
>+   bug elsewhere with the find-siblings skill.
> 6. Commit the working fix with a detailed message describing the problem,
> the
>    solution, a Fixes tag determined with the find-fixes skill, and a stable tag
>    if the stable-tag skill calls for one. Do not add a Signed-off-by tag, and
>diff --git a/agents/skills/find-siblings/SKILL.md b/agents/skills/find-siblings/SKILL.md
>new file mode 100644
>index 0000000000000..471ce8d9c0bd6
>--- /dev/null
>+++ b/agents/skills/find-siblings/SKILL.md
>@@ -0,0 +1,31 @@
>+---
>+# SPDX-License-Identifier: GPL-2.0-only
>+name: find-siblings
>+description: >-
>+  Find other places in the Linux kernel tree that have the same defect as a
>+  bug being fixed, and verify each candidate. Use after fixing or reviewing
>+  a bug fix, or when asked whether the same bug exists elsewhere.
>+---
>+
>+# Find other instances of a bug
>+
>+Follow the rules for skills in Documentation/process/coding-assistants.rst.
>+
>+- State the defect as a rule that does not depend on local names, such as
>+  "memory from kvmalloc() is released with kfree()".
>+- Search the tree as it was before the fix, not the working tree. Look at
>+  other users of the same interface, field or lock, code copied from the same
>+  origin, the rest of the series that introduced the bug, and earlier fixes
>+  for the same defect. Limit history searches to the relevant directories.
>+- If Coccinelle is available (`spatch --version` works), search for the
>+  defect with a semantic patch as
>+  [references/coccinelle.md](references/coccinelle.md) describes. Otherwise
>+  use text searches and report that limit.
>+- A match is a lead. Read each candidate with its callers, locking and object
>+  lifetimes, and classify it as confirmed, safe or undetermined, with the
>+  reason. Check whether a confirmed site is already fixed in the current tree
>+  or in a pending tree such as linux-next.
>+
>+Return the searches you ran and their scope, and the classified sites with
>+their maintainers. Do not fix the sites: each confirmed one is a separate bug
>+that needs its own fix, Fixes tag and stable decision.
>diff --git a/agents/skills/find-siblings/references/coccinelle.md b/agents/skills/find-siblings/references/coccinelle.md
>new file mode 100644
>index 0000000000000..6b09ce9d04c8c
>--- /dev/null
>+++ b/agents/skills/find-siblings/references/coccinelle.md
>@@ -0,0 +1,66 @@
>+<!-- SPDX-License-Identifier: GPL-2.0-only -->
>+
>+# Searching for a bug pattern with Coccinelle
>+
>+Documentation/dev-tools/coccinelle.rst describes `make coccicheck` and its
>+options. The semantic patch language is documented at
>+https://coccinelle.gitlabpages.inria.fr/website/docs/.
>+
>+1. Look under `scripts/coccinelle/` for a semantic patch that already covers
>+   the defect. Read its rules rather than its name: `kfree()` of memory from
>+   `kvmalloc()` is covered by `api/kfree_mismatch.cocci`, while
>+   `api/kvmalloc.cocci` only flags an open-coded `is_vmalloc_addr()` test.
>+
>+2. Otherwise, write a semantic patch outside the repository. Match only what
>+   the defect needs, not names or statements specific to the fixed site. Mark
>+   the lines of interest with `*`, and declare `virtual context`: coccicheck
>+   passes `-D context`, and spatch rejects a mode the file does not declare.
>+   A minimal rule for memory from `kvmalloc()` released with `kfree()`, a
>+   case that `api/kfree_mismatch.cocci` covers more completely:
>+
>+       virtual context
>+
>+       @@
>+       expression E;
>+       @@
>+       E = \(kvmalloc\|kvzalloc\|kvcalloc\|kvmalloc_array\)(...);
>+       ...
>+       * kfree(E);
>+
>+3. Check that it finds the site you fixed. Coccinelle reads the files on
>+   disk, where the fix is usually already applied, so copy the file as it was
>+   before the fix to a directory outside the repository and run
>+   `spatch -D <mode> --very-quiet --sp-file <rule> <copy>`, with a mode the
>+   rule declares on a `virtual` line, such as `context` or `report`. Without
>+   `-D`, rules that depend on a mode do not run and nothing is reported. The
>+   rule must mark the fixed site in that copy, and nothing in the fixed
>+   version. If it misses the site, correct the rule before searching further.
>+
>+4. Search the whole tree:
>+
>+       make coccicheck MODE=context COCCI=<rule> DEBUG_FILE=<new log file>
>+
>+   - Use a `MODE` the rule declares on a `virtual` line, as in step 3.
>+   - In context mode the output is a diff, and each line starting with `-`
>+     is a line the rule marked; report mode prints one warning per site.
>+     coccicheck searches the files on disk, so a site you already fixed
>+     there is not marked; step 3 covers it.
>+   - The kernel's `.cocciconfig` makes spatch skip files that do not contain
>+     the rule's identifiers, so a whole-tree search is usually quick when the
>+     rule names distinctive functions. Add `M=<dir>/` only if it is not.
>+   - Give `COCCI` and `DEBUG_FILE` as absolute paths. With `M=`, coccicheck
>+     runs from that directory.
>+   - Read the log. Without `DEBUG_FILE`, spatch errors are discarded and a
>+     broken rule shows only as "coccicheck failed". coccicheck refuses to
>+     reuse an existing log file.
>+   - Header files are searched only with `SPFLAGS=--include-headers`, or with
>+     a `// Options: --include-headers` line at the top of the rule.
>+
>+5. Check what was not searched. Coccinelle skips code it cannot parse without
>+   saying so. Run `spatch --parse-c` on the searched files or directories: a
>+   nonzero `nb bad` count means part of them was skipped. Search those files
>+   another way and report the gap.
>+
>+If the rule marks many sites that turn out to be safe, tighten it, for example
>+with `when !=` constraints, and run it again rather than discarding candidates
>+by hand.
>

--- Thanks!
"I'm not a very positive person" - Linus torvalds