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

Guo Ren
2022-11-24T15:31:40Z

On Thu, Nov 24, 2022 at 1:27 AM Song Shuai <suagrfillet@gmail.com> wrote:
>
> Guo Ren <guoren@kernel.org> 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@gmail.com> 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@gmail.com>
> >> ---
> >> 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.
>
> 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.
How about:
static inline void arch_ftrace_set_direct_caller(struct pt_regs *regs,
unsigned long addr)
{
regs->t1 = regs->t0;
regs->t0 = addr;

direct_caller:
add sp,sp,-?
sd t1,?(sp)
sd ra,?(sp)
call foo
ld t1,?(sp)
ld ra,?(sp)
add sp,sp,?
jr t1 // <- back to function entry

And delete all mcount-dyn.S modification.

> >>
> >> +}
> >> +
> >> #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