Re: [PATCH] riscv: add CALLER_ADDRx support

[Zong Li]

On Thu, Feb 1, 2024 at 1:10 AM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> On 05/12/2023 09:59, Zong Li wrote:
> > CALLER_ADDRx returns caller's address at specified level, they are used
> > for several tracers. These macros eventually use
> > __builtin_return_address(n) to get the caller's address if arch doesn't
> > define their own implementation.
> >
> > In RISC-V, __builtin_return_address(n) only works when n == 0, we need
> > to walk the stack frame to get the caller's address at specified level.
>
>
> Is that a bug in gcc or something expected for riscv in general?
>

I think it isn't supported for riscv in general.

>
> >
> > data.level started from 'level + 3' due to the call flow of getting
> > caller's address in RISC-V implementation. If we don't have additional
> > three iteration, the level is corresponding to follows:
> >
> > callsite -> return_address -> arch_stack_walk -> walk_stackframe
> > |           |                 |                  |
> > level 3     level 2           level 1            level 0
> >
> > Signed-off-by: Zong Li <zong.li@xxxxxxxxxx>
> > ---
> >   arch/riscv/include/asm/ftrace.h    |  5 ++++
> >   arch/riscv/kernel/Makefile         |  2 ++
> >   arch/riscv/kernel/return_address.c | 48 ++++++++++++++++++++++++++++++
> >   3 files changed, 55 insertions(+)
> >   create mode 100644 arch/riscv/kernel/return_address.c
> >
> > diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
> > index 2b2f5df7ef2c..42777f91a9c5 100644
> > --- a/arch/riscv/include/asm/ftrace.h
> > +++ b/arch/riscv/include/asm/ftrace.h
> > @@ -25,6 +25,11 @@
> >
> >   #define ARCH_SUPPORTS_FTRACE_OPS 1
> >   #ifndef __ASSEMBLY__
> > +
> > +extern void *return_address(unsigned int level);
> > +
> > +#define ftrace_return_address(n) return_address(n)
> > +
> >   void MCOUNT_NAME(void);
> >   static inline unsigned long ftrace_call_adjust(unsigned long addr)
> >   {
> > diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> > index fee22a3d1b53..40d054939ae2 100644
> > --- a/arch/riscv/kernel/Makefile
> > +++ b/arch/riscv/kernel/Makefile
> > @@ -7,6 +7,7 @@ ifdef CONFIG_FTRACE
> >   CFLAGS_REMOVE_ftrace.o      = $(CC_FLAGS_FTRACE)
> >   CFLAGS_REMOVE_patch.o       = $(CC_FLAGS_FTRACE)
> >   CFLAGS_REMOVE_sbi.o = $(CC_FLAGS_FTRACE)
> > +CFLAGS_REMOVE_return_address.o       = $(CC_FLAGS_FTRACE)
> >   endif
> >   CFLAGS_syscall_table.o      += $(call cc-option,-Wno-override-init,)
> >   CFLAGS_compat_syscall_table.o += $(call cc-option,-Wno-override-init,)
> > @@ -46,6 +47,7 @@ obj-y       += irq.o
> >   obj-y       += process.o
> >   obj-y       += ptrace.o
> >   obj-y       += reset.o
> > +obj-y        += return_address.o
> >   obj-y       += setup.o
> >   obj-y       += signal.o
> >   obj-y       += syscall_table.o
> > diff --git a/arch/riscv/kernel/return_address.c b/arch/riscv/kernel/return_address.c
> > new file mode 100644
> > index 000000000000..c2008d4aa6e5
> > --- /dev/null
> > +++ b/arch/riscv/kernel/return_address.c
> > @@ -0,0 +1,48 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * This code come from arch/arm64/kernel/return_address.c
> > + *
> > + * Copyright (C) 2023 SiFive.
> > + */
> > +
> > +#include <linux/export.h>
> > +#include <linux/kprobes.h>
> > +#include <linux/stacktrace.h>
> > +
> > +struct return_address_data {
> > +     unsigned int level;
> > +     void *addr;
> > +};
> > +
> > +static bool save_return_addr(void *d, unsigned long pc)
> > +{
> > +     struct return_address_data *data = d;
> > +
> > +     if (!data->level) {
> > +             data->addr = (void *)pc;
> > +             return false;
> > +     }
> > +
> > +     --data->level;
> > +
> > +     return true;
> > +}
> > +NOKPROBE_SYMBOL(save_return_addr);
> > +
> > +void *return_address(unsigned int level)
>
>
> Maybe return_address() should be noinline to make sure it's not inlined
> as it would break the "+ 3"? Not sure it's necessary though.

Yes, thanks for pointing it out. We should ensure it's not inlined in
any case. I will send the next version.

>
>
> > +{
> > +     struct return_address_data data;
> > +
> > +     data.level = level + 3;
> > +     data.addr = NULL;
> > +
> > +     arch_stack_walk(save_return_addr, &data, current, NULL);
> > +
> > +     if (!data.level)
> > +             return data.addr;
> > +     else
> > +             return NULL;
> > +
> > +}
> > +EXPORT_SYMBOL_GPL(return_address);
> > +NOKPROBE_SYMBOL(return_address);
>
>
> And I see that with this patch, ftrace_return_address() is now defined
> whether CONFIG_FRAME_POINTER is set or not, is that correct?

Yes, that is what I understand. In this patch, the
ftrace_return_address() is still defined to return_address() when
CONFIG_FRAME_POINTER isn't enabled, and return_address still works
because riscv port can walk stackframe without fp.

>
> This looks like a fix to me so that should go into -fixes with a Fixes
> tag (but we'll have to make sure that the "+ 3" is correct on all
> backports...):
>
> Fixes: 10626c32e382 ("riscv/ftrace: Add basic support")

The return_address() invokes arch_stack_walk(), which enabled by the
'5cb0080f1bfd ("riscv: Enable ARCH_STACKWALK")'

I guess that we are not able to apply it on all backports. Is this
right? "+3" is correct after enabling ARCH_STACKWALK.

>
> And you can finally add for your v2 (or not if you don't respin):
>
> Reviewed-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>

Thanks for the review, Alexandre. I will add it in v2:)

>
> Thanks and sorry for the delay!
>
> Alex
>