selftests/bpf: Test rhtab kptr cancellation semanticsgrafted

Resizable hash-map updates and deletions must not perform full special-field
destruction in their caller context. In particular, a referenced kptr must
remain attached to the allocation until the memory allocator destructor can
release it safely.

Add separate coverage for both affected paths. The update test stores a task
kptr, replaces the ordinary value bytes with BPF_EXIST, and verifies that the
kptr survived. The delete test removes an element and exchanges its kptr
through the still-valid map-value pointer before the allocation is reclaimed.

Both cases observe a NULL kptr when rhtab uses bpf_obj_free_fields(). They
recover and release the reference after rhtab switches to cancellation
semantics.

Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
[ kkd: Split update and delete coverage and rewrote the commit log ]
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260904104203.345917-5-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 71 insertions, 0 deletions

diff --git a/Documentation/process/debugging/index.rst b/Documentation/process/debugging/index.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+============================================
+Debugging advice for Linux Kernel developers
+============================================
+
+general guides
+--------------
+
+.. toctree::
+   :maxdepth: 1
+
+   driver_development_debugging_guide
+   gdb-kernel-debugging
+   kgdb
+   userspace_debugging_guide
+
+subsystem specific guides
+-------------------------
+
+.. toctree::
+   :maxdepth: 1
+
+   media_specific_debugging_guide
+
+General debugging advice
+========================
+
+Depending on the issue, a different set of tools is available to track down the
+problem or even to realize whether there is one in the first place.
+
+As a first step you have to figure out what kind of issue you want to debug.
+Depending on the answer, your methodology and choice of tools may vary.
+
+Do I need to debug with limited access?
+---------------------------------------
+
+Do you have limited access to the machine or are you unable to stop the running
+execution?
+
+In this case your debugging capability depends on built-in debugging support of
+provided distribution kernel.
+The :doc:`/process/debugging/userspace_debugging_guide` provides a brief
+overview over a range of possible debugging tools in that situation. You can
+check the capability of your kernel, in most cases, by looking into config file
+within the /boot directory.
+
+Do I have root access to the system?
+------------------------------------
+
+Are you easily able to replace the module in question or to install a new
+kernel?
+
+In that case your range of available tools is a lot bigger, you can find the
+tools in the :doc:`/process/debugging/driver_development_debugging_guide`.
+
+Is timing a factor?
+-------------------
+
+It is important to understand if the problem you want to debug manifests itself
+consistently (i.e. given a set of inputs you always get the same, incorrect
+output), or inconsistently. If it manifests itself inconsistently, some timing
+factor might be at play. If inserting delays into the code does change the
+behavior, then quite likely timing is a factor.
+
+When timing does alter the outcome of the code execution using a simple
+printk() for debugging purposes may not work, a similar alternative is to use
+trace_printk() , which logs the debug messages to the trace file instead of the
+kernel log.
+
+**Copyright** ©2024 : Collabora