Re: [PATCH v2 3/4] LoongArch: BPF: Enhance trampoline support for kernel and module tracing

[Chenghao Duan <duanchenghao@kylinos.cn>]

On Sun, Dec 14, 2025 at 08:36:16PM +0800, Tiezhu Yang wrote:
> On 12/12/25 17:11, Chenghao Duan wrote:
> > This patch addresses two main issues in the LoongArch BPF trampoline
> > implementation:
> > 
> > 1. BPF-to-BPF call handling:
> >   - Modify the build_prologue function to ensure that the value of the
> >   return address register ra is saved to t0 before entering the
> >   trampoline operation.
> >   - This ensures that the return address handling logic is accurate and
> >   error-free when a BPF program calls another BPF program.
> > 
> > 2. Enable Module Function Tracing Support:
> >   - Remove the previous restrictions that blocked the tracing of kernel
> >   module functions.
> >   - Fix the issue that previously caused kernel lockups when attempting
> >   to trace module functions
> > 
> > 3. Related Function Optimizations:
> >   - Adjust the jump offset of tail calls to ensure correct instruction
> >     alignment.
> >   - Enhance the bpf_arch_text_poke() function to enable accurate location
> >   of BPF program entry points.
> >   - Refine the trampoline return logic to ensure that the register data
> >   is correct when returning to both the traced function and the parent
> >   function.
> > 
> > Signed-off-by: Chenghao Duan <duanchenghao@kylinos.cn>
> 
> As described in the commit message, your changes include many kinds
> of contents, thanks for the fixes and optimizations.
> 
> In order to avoid introducing bugs in the middle, please separate each
> logical change into a separate patch, each patch should make an easily
> understood change that can be verified by reviewers, each patch should
> be justifiable on its own merits.
> 
> The current patch #4 can be put after the current patch #2 as a
> preparation for the bpf patches.
> 

Got it. I will incorporate your suggestions in the next version.

> Furthermore, it would be better to put the related test cases in the
> commit message of each patch rather than in the cover letter, so that
> it can be verified easily to know what this patch affected and can be
> recorded in the git log.

I fully agree with your suggestions. In fact, the current three patches
(excluding 0002-ftrace-samples-xxx.patch) are all fixes for the failed
test cases of module_attach. The test items included in the cover letter
of 0000-xxx.patch are intended to verify that the trampoline-related
test cases can pass after the current changes. I will follow your advice
and place the relevant test cases in the commit message of the
corresponding patches in the next version.

Chenghao

> 
> And also please add Fixes tag for each patch if possible.
> 
> Thanks,
> Tiezhu