On Fri, Jan 09, 2026 at 10:34:53AM +0100, Jiri Olsa wrote: > Mahe reported issue with bpf_override_return helper not working when > executed from kprobe.multi bpf program on arm. > > The problem is that on arm we use alternate storage for pt_regs object > that is passed to bpf_prog_run and if any register is changed (which > is the case of bpf_override_return) it's not propagated back to actual > pt_regs object. > > Fixing this by introducing and calling ftrace_partial_regs_update function > to propagate the values of changed registers (ip and stack). > > Reported-by: Mahe Tardy <[email protected]> > Signed-off-by: Jiri Olsa <[email protected]> > --- > v2 changes: > - moved ftrace_partial_regs_update to generic code [Will] > > include/linux/ftrace_regs.h | 25 +++++++++++++++++++++++++ > kernel/trace/bpf_trace.c | 1 + > 2 files changed, 26 insertions(+) > > diff --git a/include/linux/ftrace_regs.h b/include/linux/ftrace_regs.h > index 15627ceea9bc..f9a7c009cdae 100644 > --- a/include/linux/ftrace_regs.h > +++ b/include/linux/ftrace_regs.h > @@ -33,6 +33,31 @@ struct ftrace_regs; > #define ftrace_regs_get_frame_pointer(fregs) \ > frame_pointer(&arch_ftrace_regs(fregs)->regs) > > +static __always_inline void > +ftrace_partial_regs_update(const struct ftrace_regs *fregs, struct pt_regs > *regs) { } > + > +#else > + > +/* > + * ftrace_partial_regs_update - update the original ftrace_regs from regs > + * @fregs: The ftrace_regs to update from @regs > + * @regs: The partial regs from ftrace_partial_regs() that was updated > + * > + * Some architectures have the partial regs living in the ftrace_regs > + * structure, whereas other architectures need to make a different copy > + * of the @regs. If a partial @regs is retrieved by ftrace_partial_regs() and > + * if the code using @regs updates a field (like the instruction pointer or > + * stack pointer) it may need to propagate that change to the original @fregs > + * it retrieved the partial @regs from. Use this function to guarantee that > + * update happens. > + */ > +static __always_inline void > +ftrace_partial_regs_update(const struct ftrace_regs *fregs, struct pt_regs > *regs) > +{ > + ftrace_regs_set_instruction_pointer(fregs, instruction_pointer(regs)); > + ftrace_regs_set_return_value(fregs, regs_return_value(regs)); > +}
I think the AI thingy is right about dropping the const qualifier here but overall I prefer this to the previous revisions. Thanks for sticking with it! Acked-by: Will Deacon <[email protected]> Will
