Re: [PATCH] riscv: add CALLER_ADDRx support
From: Alexandre Ghiti
Date: Thu Feb 01 2024 - 05:08:06 EST
On 01/02/2024 09:43, Zong Li wrote:
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.
5cb0080f1bfd was introduced in v5.11, so that will make the backport possible up to 5.15, I guess that's ok, nobody will ever use a riscv kernel that old (?).
So I'd add the Fixes tag I proposed above, and let the backport fail for < 5.15. @Conor any opinion?
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,
Alex
Thanks and sorry for the delay!
Alex
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