From a5f7a5bb3b7f28ba7e4fa246775b29a0e5537255 Mon Sep 17 00:00:00 2001 From: Jinke Han Date: Tue, 8 Sep 2026 15:37:42 +0800 Subject: x86/kprobes: Fix crash when probing CS CALL instructions When using eBPF to probe CS CALL instructions within a function, a crash can be triggered. The eBPF tool probes offset 257 of the __hrtimer_run_queues() function: <__hrtimer_run_queues+249>: nopl 0x0(%rax,%rax,1) <__hrtimer_run_queues+254>: mov %r14,%rdi <__hrtimer_run_queues+257>: cs call <__x86_indirect_thunk_r12> <__hrtimer_run_queues+263>: mov %eax,%r12d <__hrtimer_run_queues+266>: xchg %ax,%ax <__hrtimer_run_queues+268>: mov %r13,%rdi Which triggers this crash: BUG: unable to handle page fault for address: 00000000000f41c9 #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page PGD 0 P4D 0 Oops: 0002 [#1] SMP NOPTI CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P RIP: 0010:__hrtimer_run_queues+0x106/0x230 Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is at the 6th byte of the above CS CALL instruction. Since the CS CALL instruction occupies 6 bytes, the exception occurred in the middle of that call instruction. The root cause is that when using eBPF tools to probe in the middle of a function, a kprobe with INT3 is used as the underlying implementation. During single-step emulation of the original CALL instruction, int3_emulate_call() assumes that the probed CALL instruction is 5 bytes long. However, the actual CS-prefixed CALL instruction occupies 6 bytes, so it constructs an incorrect exception return address. When the CPU returns from the kprobe handler, the next instruction to be executed is at the address of the last byte of that CS CALL instruction. Coincidentally, starting from that address, the CPU fetches and decodes a completely different instruction, which ultimately triggers a kernel crash. Fix the issue by using the actual instruction length obtained from the instruction decoder when constructing the exception return address, rather than relying on the hardcoded CALL_INSN_SIZE macro. [ mingo: Refined the changelog ] Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step") Suggested-by: Masami Hiramatsu (Google) Signed-off-by: Jinke Han Signed-off-by: Ingo Molnar Reviewed-by: Masami Hiramatsu (Google) Acked-by: Yafang Shao Acked-by: Borislav Petkov Cc: Peter Zijlstra Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000 --- Documentation/dev-tools/clang-format.rst | 184 +++++++++++++++++++++++++++++++ 1 file changed, 184 insertions(+) create mode 100644 Documentation/dev-tools/clang-format.rst (limited to 'Documentation/dev-tools/clang-format.rst') diff --git a/Documentation/dev-tools/clang-format.rst b/Documentation/dev-tools/clang-format.rst new file mode 100644 index 000000000..6c8a0df5a --- /dev/null +++ b/Documentation/dev-tools/clang-format.rst @@ -0,0 +1,184 @@ +.. _clangformat: + +clang-format +============ + +``clang-format`` is a tool to format C/C++/... code according to +a set of rules and heuristics. Like most tools, it is not perfect +nor covers every single case, but it is good enough to be helpful. + +``clang-format`` can be used for several purposes: + + - Quickly reformat a block of code to the kernel style. Specially useful + when moving code around and aligning/sorting. See clangformatreformat_. + + - Spot style mistakes, typos and possible improvements in files + you maintain, patches you review, diffs, etc. See clangformatreview_. + + - Help you follow the coding style rules, specially useful for those + new to kernel development or working at the same time in several + projects with different coding styles. + +Its configuration file is ``.clang-format`` in the root of the kernel tree. +The rules contained there try to approximate the most common kernel +coding style. They also try to follow :ref:`Documentation/process/coding-style.rst ` +as much as possible. Since not all the kernel follows the same style, +it is possible that you may want to tweak the defaults for a particular +subsystem or folder. To do so, you can override the defaults by writing +another ``.clang-format`` file in a subfolder. + +The tool itself has already been included in the repositories of popular +Linux distributions for a long time. Search for ``clang-format`` in +your repositories. Otherwise, you can either download pre-built +LLVM/clang binaries or build the source code from: + + https://releases.llvm.org/download.html + +See more information about the tool at: + + https://clang.llvm.org/docs/ClangFormat.html + + https://clang.llvm.org/docs/ClangFormatStyleOptions.html + + +.. _clangformatreview: + +Review files and patches for coding style +----------------------------------------- + +By running the tool in its inline mode, you can review full subsystems, +folders or individual files for code style mistakes, typos or improvements. + +To do so, you can run something like:: + + # Make sure your working directory is clean! + clang-format -i kernel/*.[ch] + +And then take a look at the git diff. + +Counting the lines of such a diff is also useful for improving/tweaking +the style options in the configuration file; as well as testing new +``clang-format`` features/versions. + +``clang-format`` also supports reading unified diffs, so you can review +patches and git diffs easily. See the documentation at: + + https://clang.llvm.org/docs/ClangFormat.html#script-for-patch-reformatting + +To avoid ``clang-format`` formatting some portion of a file, you can do:: + + int formatted_code; + // clang-format off + void unformatted_code ; + // clang-format on + void formatted_code_again; + +While it might be tempting to use this to keep a file always in sync with +``clang-format``, specially if you are writing new files or if you are +a maintainer, please note that people might be running different +``clang-format`` versions or not have it available at all. Therefore, +you should probably refrain yourself from using this in kernel sources; +at least until we see if ``clang-format`` becomes commonplace. + + +.. _clangformatreformat: + +Reformatting blocks of code +--------------------------- + +By using an integration with your text editor, you can reformat arbitrary +blocks (selections) of code with a single keystroke. This is specially +useful when moving code around, for complex code that is deeply indented, +for multi-line macros (and aligning their backslashes), etc. + +Remember that you can always tweak the changes afterwards in those cases +where the tool did not do an optimal job. But as a first approximation, +it can be very useful. + +There are integrations for many popular text editors. For some of them, +like vim, emacs, BBEdit and Visual Studio you can find support built-in. +For instructions, read the appropriate section at: + + https://clang.llvm.org/docs/ClangFormat.html + +For Atom, Eclipse, Sublime Text, Visual Studio Code, XCode and other +editors and IDEs you should be able to find ready-to-use plugins. + +For this use case, consider using a secondary ``.clang-format`` +so that you can tweak a few options. See clangformatextra_. + + +.. _clangformatmissing: + +Missing support +--------------- + +``clang-format`` is missing support for some things that are common +in kernel code. They are easy to remember, so if you use the tool +regularly, you will quickly learn to avoid/ignore those. + +In particular, some very common ones you will notice are: + + - Aligned blocks of one-line ``#defines``, e.g.:: + + #define TRACING_MAP_BITS_DEFAULT 11 + #define TRACING_MAP_BITS_MAX 17 + #define TRACING_MAP_BITS_MIN 7 + + vs.:: + + #define TRACING_MAP_BITS_DEFAULT 11 + #define TRACING_MAP_BITS_MAX 17 + #define TRACING_MAP_BITS_MIN 7 + + - Aligned designated initializers, e.g.:: + + static const struct file_operations uprobe_events_ops = { + .owner = THIS_MODULE, + .open = probes_open, + .read = seq_read, + .llseek = seq_lseek, + .release = seq_release, + .write = probes_write, + }; + + vs.:: + + static const struct file_operations uprobe_events_ops = { + .owner = THIS_MODULE, + .open = probes_open, + .read = seq_read, + .llseek = seq_lseek, + .release = seq_release, + .write = probes_write, + }; + + +.. _clangformatextra: + +Extra features/options +---------------------- + +Some features/style options are not enabled by default in the configuration +file in order to minimize the differences between the output and the current +code. In other words, to make the difference as small as possible, +which makes reviewing full-file style, as well diffs and patches as easy +as possible. + +In other cases (e.g. particular subsystems/folders/files), the kernel style +might be different and enabling some of these options may approximate +better the style there. + +For instance: + + - Aligning assignments (``AlignConsecutiveAssignments``). + + - Aligning declarations (``AlignConsecutiveDeclarations``). + + - Reflowing text in comments (``ReflowComments``). + + - Sorting ``#includes`` (``SortIncludes``). + +They are typically useful for block re-formatting, rather than full-file. +You might want to create another ``.clang-format`` file and use that one +from your editor/IDE instead. -- cgit v1.3.1