linux-kernel - Re: [PATCH] riscv: add CALLER

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Message-ID: <139cdbd6-73d9-4452-94ce-825689b7c0c8@ghiti.fr>
Date: Wed, 31 Jan 2024 18:10:48 +0100
From: Alexandre Ghiti <alex@...ti.fr>
To: Zong Li <zong.li@...ive.com>, palmer@...belt.com,
 paul.walmsley@...ive.com, aou@...s.berkeley.edu,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: add CALLER_ADDRx support

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?


>
> 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@...ive.com>
> ---
>   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.


> +{
> +	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?

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")

And you can finally add for your v2 (or not if you don't respin):

Reviewed-by: Alexandre Ghiti <alexghiti@...osinc.com>

Thanks and sorry for the delay!

Alex