Re: [PATCH 1/2] riscv/ftrace: add DYNAMIC_FTRACE_WITH_DIRECT_CALLS support

[Guo Ren]

On Thu, Nov 24, 2022 at 1:27 AM Song Shuai <suagrfillet@xxxxxxxxx> wrote:
>
> Guo Ren <guoren@xxxxxxxxxx> 于2022年11月23日周三 23:02写道：
> >
> > Cool job, thx.
> >
> > On Wed, Nov 23, 2022 at 10:20 PM Song Shuai <suagrfillet@xxxxxxxxx> wrote:
> >>
> >> This patch adds DYNAMIC_FTRACE_WITH_DIRECT_CALLS support for RISC-V.
> >>
> >> select the DYNAMIC_FTRACE_WITH_DIRECT_CALLS to provide the
> >> register_ftrace_direct[_multi] interfaces allowing users to register
> >> the customed trampoline (direct_caller) as the mcount for one or
> >> more target functions. And modify_ftrace_direct[_multi] are also
> >> provided for modifying direct_caller.
> >>
> >> To make the direct_caller and the other ftrace hooks (eg. function/fgraph
> >> tracer, k[ret]probes) co-exist, a temporary register is nominated to
> >> store the address of direct_caller in ftrace_regs_caller. After the
> >> setting of the address direct_caller by direct_ops->func and the
> >> RESTORE_REGS in ftrace_regs_caller, direct_caller will be jumped to
> >> by the `jr` inst.
> >>
> >> Signed-off-by: Song Shuai <suagrfillet@xxxxxxxxx>
> >> ---
> >>  arch/riscv/Kconfig              | 1 +
> >>  arch/riscv/include/asm/ftrace.h | 6 ++++++
> >>  arch/riscv/kernel/mcount-dyn.S  | 4 ++++
> >>  3 files changed, 11 insertions(+)
> >>
> >> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> >> index 39ec8d628cf6..d083ec08d0b6 100644
> >> --- a/arch/riscv/Kconfig
> >> +++ b/arch/riscv/Kconfig
> >> @@ -278,6 +278,7 @@ config ARCH_RV64I
> >>         select ARCH_SUPPORTS_INT128 if CC_HAS_INT128
> >>         select HAVE_DYNAMIC_FTRACE if !XIP_KERNEL && MMU && $(cc-option,-fpatchable-function-entry=8)
> >>         select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
> >> +       select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
> >>         select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
> >>         select HAVE_FUNCTION_GRAPH_TRACER
> >>         select HAVE_FUNCTION_TRACER if !XIP_KERNEL && !PREEMPTION
> >> diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
> >> index 01bebb28eabe..be4d57566139 100644
> >> --- a/arch/riscv/include/asm/ftrace.h
> >> +++ b/arch/riscv/include/asm/ftrace.h
> >> @@ -114,6 +114,12 @@ struct ftrace_regs;
> >>  void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
> >>                        struct ftrace_ops *op, struct ftrace_regs *fregs);
> >>  #define ftrace_graph_func ftrace_graph_func
> >> +
> >> +static inline void arch_ftrace_set_direct_caller(struct pt_regs *regs, unsigned long addr)
> >> +{
> >> +               regs->t1 = addr;
> >
> > How about regs->t0 = addr; ?
> > And delete all mcount-dyn.S modification.
> >
> The direct_caller has the same program layout as the ftrace_caller, which means
> the reg t0 will never be changed when direct_caller returns.
>
> If regs->t0 changes here and ftrace_regs_caller executes `jr t0`,
> direct_caller will enter the dead loop.
?

ftrace_regs_caller->call_direct_funcs->
arch_ftrace_set_direct_caller-> ftrace_regs_caller jr t0.

Only call_direct_funcs call arch_ftrace_set_direct_caller !

static void call_direct_funcs(unsigned long ip, unsigned long pip,
                              struct ftrace_ops *ops, struct ftrace_regs *fregs)
{
        struct pt_regs *regs = ftrace_get_regs(fregs);
        unsigned long addr;

        addr = ftrace_find_rec_direct(ip);
        if (!addr)
                return;

        arch_ftrace_set_direct_caller(regs, addr);
}

>
> Actually the reg t0 always saves the address of function entry with 8B
> offset, it should only
> changed by the IPMODIFY ops instead of the direct_ops.
> >>
> >> +}
> >> +
> >>  #endif /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
> >>
> >>  #endif /* __ASSEMBLY__ */
> >> diff --git a/arch/riscv/kernel/mcount-dyn.S b/arch/riscv/kernel/mcount-dyn.S
> >> index 466c6ef217b1..b89c85a58569 100644
> >> --- a/arch/riscv/kernel/mcount-dyn.S
> >> +++ b/arch/riscv/kernel/mcount-dyn.S
> >> @@ -233,6 +233,7 @@ ENDPROC(ftrace_caller)
> >>  #else /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
> >>  ENTRY(ftrace_regs_caller)
> >>         SAVE_ABI_REGS 1
> >> +       REG_S   x0, PT_T1(sp)
> >>         PREPARE_ARGS
> >>
> >>  ftrace_regs_call:
> >> @@ -241,7 +242,10 @@ ftrace_regs_call:
> >>
> >>
> >>         RESTORE_ABI_REGS 1
> >> +       bnez    t1,.Ldirect
> >>         jr t0
> >> +.Ldirect:
> >> +       jr t1
> >>  ENDPROC(ftrace_regs_caller)
> >>
> >>  ENTRY(ftrace_caller)
> >> --
> >> 2.20.1
> >>
> >
> >
> > --
> > Best Regards
> >  Guo Ren



-- 
Best Regards
 Guo Ren