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

[Guo Ren <guoren@kernel.org>]

On Fri, Nov 25, 2022 at 2:33 PM Song Shuai <suagrfillet@gmail.com> wrote:
>
> Guo Ren <guoren@kernel.org> 于2022年11月25日周五 03:10写道：
> >
> > On Fri, Nov 25, 2022 at 9:53 AM Song Shuai <suagrfillet@gmail.com> wrote:
> > >
> > > Guo Ren <guoren@kernel.org> 于2022年11月24日周四 15:31写道：
> > > >
> > > > 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)
> > > direct_caller also serves as the first trampoline as ftrace_caller, like this:
> > > ```
> > > func -- direct_caller
> > >         -- ftrace_[regs]_caller
> > > ```
> > > So the t1 in this line has to be t0 to save the PC.
> >
> > direct_caller:
> > add sp,sp,-?
> > sd t1,?(sp)
> > sd t0, ?(so)
> > sd ra,?(sp)
> > mov t0, t1
> This foo is the tracing function along with the direct_caller,
> and it has the same parameters as the target function.
> So the t0 or t1 here means nothing for this foo function.
>
> No offense, but what's the purpose of this mv inst?
Here is my modification: [1]
[1]: https://lore.kernel.org/linux-riscv/20221128101201.4144527-1-guoren@kernel.org/

I've tested like this:
mount -t debugfs none /sys/kernel/debug/
cd /sys/kernel/debug/tracing/
echo handle_mm_fault > set_ftrace_filter
echo 'r:myr handle_mm_fault' > kprobe_events
echo function > current_tracer
echo 1 > events/kprobes/myr/enable
insmod /root/ftrace/ftrace-direct-too.ko
cat trace

             cat-137     [000] .....  3132.231948: handle_mm_fault
<-do_page_fault
             cat-137     [000] .....  3132.231973: my_direct_func:
handle mm fault vma=000000001ba58b17 address=aaaaaaaab689c8 flags=254
             cat-137     [000] .....  3132.232622: myr:
(do_page_fault+0x176/0x424 <- handle_mm_fault)
             cat-137     [000] .....  3132.232724: handle_mm_fault
<-do_page_fault
             cat-137     [000] .....  3132.232750: my_direct_func:
handle mm fault vma=000000001ba58b17 address=aaaaaaaab7df9c flags=254
             cat-137     [000] .....  3132.233385: myr:
(do_page_fault+0x176/0x424 <- handle_mm_fault)
             cat-137     [000] .....  3132.233744: handle_mm_fault
<-do_page_fault
             cat-137     [000] .....  3132.233772: my_direct_func:
handle mm fault vma=000000001ba58b17 address=aaaaaaaab2b2c8 flags=354
             cat-137     [000] .....  3132.234392: myr:
(do_page_fault+0x176/0x424 <- handle_mm_fault)
             cat-137     [000] .....  3132.234480: handle_mm_fault
<-do_page_fault
             cat-137     [000] .....  3132.234505: my_direct_func:
handle mm fault vma=000000001ba58b17 address=aaaaaaaab5afc0 flags=354
             cat-137     [000] .....  3132.235149: myr:
(do_page_fault+0x176/0x424 <- handle_mm_fault)
             cat-137     [000] .....  3132.235263: handle_mm_fault
<-do_page_fault
             cat-137     [000] .....  3132.235289: my_direct_func:
handle mm fault vma=0000000071a096de address=fffffff7fac530 flags=254
             cat-137     [000] .....  3132.235893: myr:
(do_page_fault+0x176/0x424 <- handle_mm_fault)


> > call foo
> > ld t0,?(sp)
> > ld t1,?(sp)
> > ld ra,?(sp)
> > add sp,sp,?
> > jr t1 // <- back to function entry
> When direct_caller works as the first trampoline
> the content of t1 here means nothing for the target function, neither
> PC nor PIP.
> >
> >
> > > > sd ra,?(sp)
> > > > call foo
> > > > ld t1,?(sp)
> > > And this line.
> > > > 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
> > > >
> > > >
> > > >
> > > > --
> > > > Best Regards
> > > >  Guo Ren
> > > Thanks,
> > > Song
> >
> >
> >
> > --
> > Best Regards
> >  Guo Ren



-- 
Best Regards
 Guo Ren