[PATCH v5 1/2] function_graph: Support recording and printing the return value of function

Donglin Peng
2023-03-20T13:18:19Z

When using the function_graph tracer to analyze system call failures,
it can be time-consuming to analyze the trace logs and locate the kernel
function that first returns an error. This change aims to simplify the
process by recording the function return value to the 'retval' member of
'ftrace_graph_ent' and printing it when outputing the trace log.

New trace options are introduced: funcgraph-retval and graph_retval_hex.
The former is used to control whether to display the return value, while
the latter is used to control the display format of the reutrn value.

Note that even if a function's return type is void, a return value will
still be printed, so just ignore it.

Currently, this modification supports the following commonly used
processor architectures: x86_64, x86, arm64, arm, riscv.

Here is an example:

I want to attach the demo process to a cpu cgroup, but it failed:

echo `pidof demo` > /sys/fs/cgroup/cpu/test/tasks
-bash: echo: write error: Invalid argument

The strace logs tells that the write system call returned -EINVAL(-22):
...
write(1, "273\n", 4) = -1 EINVAL (Invalid argument)
...

Use the following commands to capture trace logs when calling the write
system call:

cd /sys/kernel/debug/tracing/
echo 0 > tracing_on
echo > trace
echo *sys_write > set_graph_function
echo *spin* > set_graph_notrace
echo *rcu* >> set_graph_notrace
echo *alloc* >> set_graph_notrace
echo preempt* >> set_graph_notrace
echo kfree* >> set_graph_notrace
echo $$ > set_ftrace_pid
echo function_graph > current_tracer
echo 1 > tracing_on
echo `pidof demo` > /sys/fs/cgroup/cpu/test/tasks
echo 0 > tracing_on
echo 1 > options/funcgraph-retval
cat trace > ~/trace.log

Search the error code -22 directly in the file trace.log and find the
first function that return -22, then read the function code to get the
root cause.

...

1) | cgroup_migrate() {
1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
1) | cgroup_migrate_execute() {
1) | cpu_cgroup_can_attach() {
1) | cgroup_taskset_first() {
1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */
1) 2.335 us | } /* cpu_cgroup_can_attach = -22 */
1) 4.369 us | } /* cgroup_migrate_execute = -22 */
1) 7.143 us | } /* cgroup_migrate = -22 */

...

Signed-off-by: Donglin Peng <pengdonglin@sangfor.com.cn>
---
v5:
- Pass both the return values to ftrace_return_to_handler
- Modify the parameter sequence of ftrace_return_to_handler to
decrease the modification of assembly code, thanks to Russell King
- Wrap __ftrace_return_to_handler with ftrace_return_to_handler
for compatible

v4:
- Modify commit message
- Introduce new option graph_retval_hex to control display format
- Introduce macro CONFIG_FUNCTION_GRAPH_RETVAL and
CONFIG_HAVE_FUNCTION_GRAPH_RETVAL
- add each arch maintainers to review

v3:
- Modify the commit message: add trace logs processed with the btf tool

v2:
- Modify the commit message: use BTF to get the return type of function
---
arch/arm/Kconfig | 1 +
arch/arm/kernel/entry-ftrace.S | 8 +++
arch/arm64/Kconfig | 1 +
arch/arm64/kernel/entry-ftrace.S | 8 +++
arch/riscv/Kconfig | 1 +
arch/riscv/kernel/mcount.S | 12 +++-
arch/x86/Kconfig | 1 +
arch/x86/kernel/ftrace_32.S | 8 +++
arch/x86/kernel/ftrace_64.S | 10 +++
include/linux/ftrace.h | 3 +
kernel/trace/Kconfig | 8 +++
kernel/trace/fgraph.c | 25 +++++++-
kernel/trace/trace.h | 2 +
kernel/trace/trace_entries.h | 26 ++++++++
kernel/trace/trace_functions_graph.c | 95 +++++++++++++++++++++++++---
15 files changed, 197 insertions(+), 12 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index e24a9820e12f..ad03fc868f34 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -99,6 +99,7 @@ config ARM
select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
select HAVE_FUNCTION_ERROR_INJECTION
select HAVE_FUNCTION_GRAPH_TRACER
+ select HAVE_FUNCTION_GRAPH_RETVAL if HAVE_FUNCTION_GRAPH_TRACER
select HAVE_FUNCTION_TRACER if !XIP_KERNEL
select HAVE_GCC_PLUGINS
select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)
diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S
index 3e7bcaca5e07..ba1986e27af8 100644
--- a/arch/arm/kernel/entry-ftrace.S
+++ b/arch/arm/kernel/entry-ftrace.S
@@ -258,7 +258,15 @@ ENDPROC(ftrace_graph_regs_caller)
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
ENTRY(return_to_handler)
stmdb sp!, {r0-r3}
+#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
+ /*
+ * Pass both the function return values in the register r0 and r1
+ * to ftrace_return_to_handler
+ */
+ add r2, sp, #16 @ sp at exit of instrumented routine
+#else
add r0, sp, #16 @ sp at exit of instrumented routine
+#endif
bl ftrace_return_to_handler
mov lr, r0 @ r0 has real ret addr
ldmia sp!, {r0-r3}
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 1023e896d46b..5ad53d565f2b 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -196,6 +196,7 @@ config ARM64
select HAVE_FUNCTION_TRACER
select HAVE_FUNCTION_ERROR_INJECTION
select HAVE_FUNCTION_GRAPH_TRACER
+ select HAVE_FUNCTION_GRAPH_RETVAL if HAVE_FUNCTION_GRAPH_TRACER
select HAVE_GCC_PLUGINS
select HAVE_HW_BREAKPOINT if PERF_EVENTS
select HAVE_IOREMAP_PROT
diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
index 350ed81324ac..d1a5d76e6d72 100644
--- a/arch/arm64/kernel/entry-ftrace.S
+++ b/arch/arm64/kernel/entry-ftrace.S
@@ -276,7 +276,15 @@ SYM_CODE_START(return_to_handler)
stp x4, x5, [sp, #32]
stp x6, x7, [sp, #48]

+#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
+ /*
+ * Pass both the function return values in the register x0 and x1
+ * to ftrace_return_to_handler.
+ */
+ mov x2, x29 // parent's fp
+#else
mov x0, x29 // parent's fp
+#endif
bl ftrace_return_to_handler// addr = ftrace_return_to_hander(fp);
mov x30, x0 // restore the original return address

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index c5e42cc37604..3efa4f645a39 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -139,6 +139,7 @@ config RISCV
select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
select HAVE_FUNCTION_GRAPH_TRACER
+ select HAVE_FUNCTION_GRAPH_RETVAL if HAVE_FUNCTION_GRAPH_TRACER
select HAVE_FUNCTION_TRACER if !XIP_KERNEL && !PREEMPTION

config ARCH_MMAP_RND_BITS_MIN
diff --git a/arch/riscv/kernel/mcount.S b/arch/riscv/kernel/mcount.S
index 30102aadc4d7..bba2f3835592 100644
--- a/arch/riscv/kernel/mcount.S
+++ b/arch/riscv/kernel/mcount.S
@@ -69,8 +69,18 @@ ENTRY(return_to_handler)
mv t6, s0
#endif
SAVE_RET_ABI_STATE
-#ifdef HAVE_FUNCTION_GRAPH_FP_TEST
+#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
+ /*
+ * Pass both the function return values in the register a0 and a1
+ * to ftrace_return_to_handler
+ */
+ #ifdef HAVE_FUNCTION_GRAPH_FP_TEST
+ mv a2, t6
+ #endif
+#else
+ #ifdef HAVE_FUNCTION_GRAPH_FP_TEST
mv a0, t6
+ #endif
#endif
call ftrace_return_to_handler
mv a2, a0
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index a825bf031f49..af80f3c75a54 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -215,6 +215,7 @@ config X86
select HAVE_FENTRY if X86_64 || DYNAMIC_FTRACE
select HAVE_FTRACE_MCOUNT_RECORD
select HAVE_FUNCTION_GRAPH_TRACER if X86_32 || (X86_64 && DYNAMIC_FTRACE)
+ select HAVE_FUNCTION_GRAPH_RETVAL if HAVE_FUNCTION_GRAPH_TRACER
select HAVE_FUNCTION_TRACER
select HAVE_GCC_PLUGINS
select HAVE_HW_BREAKPOINT
diff --git a/arch/x86/kernel/ftrace_32.S b/arch/x86/kernel/ftrace_32.S
index a0ed0e4a2c0c..ae8f215d9820 100644
--- a/arch/x86/kernel/ftrace_32.S
+++ b/arch/x86/kernel/ftrace_32.S
@@ -184,7 +184,15 @@ SYM_CODE_END(ftrace_graph_caller)
return_to_handler:
pushl %eax
pushl %edx
+#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
+ /*
+ * Pass both the function return values in the register eax and edx
+ * to ftrace_return_to_handler
+ */
+ movl $0, %ecx
+#else
movl $0, %eax
+#endif
call ftrace_return_to_handler
movl %eax, %ecx
popl %edx
diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
index 1265ad519249..2eefa82814dc 100644
--- a/arch/x86/kernel/ftrace_64.S
+++ b/arch/x86/kernel/ftrace_64.S
@@ -347,7 +347,17 @@ SYM_CODE_START(return_to_handler)
/* Save the return values */
movq %rax, (%rsp)
movq %rdx, 8(%rsp)
+#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
+ /*
+ * Pass both the function return values in the register rax and rdx
+ * to ftrace_return_to_handler
+ */
+ movq %rax, %rdi
+ movq %rdx, %rsi
+ movq %rbp, %rdx
+#else
movq %rbp, %rdi
+#endif

call ftrace_return_to_handler

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 366c730beaa3..be662a11cd13 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -1032,6 +1032,9 @@ struct ftrace_graph_ent {
*/
struct ftrace_graph_ret {
unsigned long func; /* Current function */
+#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
+ unsigned long retval;
+#endif
int depth;
/* Number of functions that overran the depth limit for current task */
unsigned int overrun;
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index a856d4a34c67..319eb6dc2250 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -31,6 +31,9 @@ config HAVE_FUNCTION_GRAPH_TRACER
help
See Documentation/trace/ftrace-design.rst

+config HAVE_FUNCTION_GRAPH_RETVAL
+ bool
+
config HAVE_DYNAMIC_FTRACE
bool
help
@@ -227,6 +230,11 @@ config FUNCTION_GRAPH_TRACER
the return value. This is done by setting the current return
address on the current task structure into a stack of calls.

+config FUNCTION_GRAPH_RETVAL
+ def_bool y
+ depends on HAVE_FUNCTION_GRAPH_RETVAL
+ depends on FUNCTION_GRAPH_TRACER
+
config DYNAMIC_FTRACE
bool "enable/disable function tracing dynamically"
depends on FUNCTION_TRACER
diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index 218cd95bf8e4..f40b10fc6561 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -238,14 +238,24 @@ static struct notifier_block ftrace_suspend_notifier = {

/*
* Send the trace to the ring-buffer.
+ *
+ * For double register return, need to pass both register values.
+ * However only the first one is saved currently.
+ *
+ * @retval_1st: the first return value
+ * @retval_2nd: the second return value
* @return the original return address.
*/
-unsigned long ftrace_return_to_handler(unsigned long frame_pointer)
+static unsigned long __ftrace_return_to_handler(unsigned long retval_1st,
+ unsigned long retval_2nd, unsigned long frame_pointer)
{
struct ftrace_graph_ret trace;
unsigned long ret;

ftrace_pop_return_trace(&trace, &ret, frame_pointer);
+#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
+ trace.retval = retval_1st;
+#endif
trace.rettime = trace_clock_local();
ftrace_graph_return(&trace);
/*
@@ -266,6 +276,19 @@ unsigned long ftrace_return_to_handler(unsigned long frame_pointer)
return ret;
}

+#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
+unsigned long ftrace_return_to_handler(unsigned long retval_1st,
+ unsigned long retval_2nd, unsigned long frame_pointer)
+{
+ return __ftrace_return_to_handler(retval_1st, retval_2nd, frame_pointer);
+}
+#else
+unsigned long ftrace_return_to_handler(unsigned long frame_pointer)
+{
+ return __ftrace_return_to_handler(0, 0, frame_pointer);
+}
+#endif
+
/**
* ftrace_graph_get_ret_stack - return the entry of the shadow stack
* @task: The task to read the shadow stack from
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 616e1aa1c4da..0a3c4582df10 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -831,6 +831,8 @@ static __always_inline bool ftrace_hash_empty(struct ftrace_hash *hash)
#define TRACE_GRAPH_PRINT_TAIL 0x100
#define TRACE_GRAPH_SLEEP_TIME 0x200
#define TRACE_GRAPH_GRAPH_TIME 0x400
+#define TRACE_GRAPH_PRINT_RETVAL 0x800
+#define TRACE_GRAPH_PRINT_RETVAL_HEX 0x1000
#define TRACE_GRAPH_PRINT_FILL_SHIFT 28
#define TRACE_GRAPH_PRINT_FILL_MASK (0x3 << TRACE_GRAPH_PRINT_FILL_SHIFT)

diff --git a/kernel/trace/trace_entries.h b/kernel/trace/trace_entries.h
index cd41e863b51c..340b2fa98218 100644
--- a/kernel/trace/trace_entries.h
+++ b/kernel/trace/trace_entries.h
@@ -86,6 +86,30 @@ FTRACE_ENTRY_PACKED(funcgraph_entry, ftrace_graph_ent_entry,
);

/* Function return entry */
+#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
+
+FTRACE_ENTRY_PACKED(funcgraph_exit, ftrace_graph_ret_entry,
+
+ TRACE_GRAPH_RET,
+
+ F_STRUCT(
+ __field_struct( struct ftrace_graph_ret, ret )
+ __field_packed( unsigned long, ret, func )
+ __field_packed( unsigned long, ret, retval )
+ __field_packed( int, ret, depth )
+ __field_packed( unsigned int, ret, overrun )
+ __field_packed( unsigned long long, ret, calltime)
+ __field_packed( unsigned long long, ret, rettime )
+ ),
+
+ F_printk("<-- %ps (%d) (start: %llx end: %llx) over: %d retval: %lx",
+ (void *)__entry->func, __entry->depth,
+ __entry->calltime, __entry->rettime,
+ __entry->depth, __entry->retval)
+);
+
+#else
+
FTRACE_ENTRY_PACKED(funcgraph_exit, ftrace_graph_ret_entry,

TRACE_GRAPH_RET,
@@ -105,6 +129,8 @@ FTRACE_ENTRY_PACKED(funcgraph_exit, ftrace_graph_ret_entry,
__entry->depth)
);

+#endif
+
/*
* Context switch trace entry - which task (and prio) we switched from/to:
*
diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index 203204cadf92..e26f548a4d75 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -58,6 +58,12 @@ static struct tracer_opt trace_opts[] = {
{ TRACER_OPT(funcgraph-irqs, TRACE_GRAPH_PRINT_IRQS) },
/* Display function name after trailing } */
{ TRACER_OPT(funcgraph-tail, TRACE_GRAPH_PRINT_TAIL) },
+#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
+ /* Display function return value ? */
+ { TRACER_OPT(funcgraph-retval, TRACE_GRAPH_PRINT_RETVAL) },
+ /* Display function return value in hexadecimal format ? */
+ { TRACER_OPT(graph_retval_hex, TRACE_GRAPH_PRINT_RETVAL_HEX) },
+#endif
/* Include sleep time (scheduled out) between entry and return */
{ TRACER_OPT(sleep-time, TRACE_GRAPH_SLEEP_TIME) },

@@ -619,6 +625,58 @@ print_graph_duration(struct trace_array *tr, unsigned long long duration,
trace_seq_puts(s, "| ");
}

+#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
+
+#define __TRACE_GRAPH_PRINT_RETVAL TRACE_GRAPH_PRINT_RETVAL
+
+static void print_graph_retval(struct trace_seq *s, unsigned long retval,
+ bool leaf, void *func, bool hex_format)
+{
+ unsigned long err_code = 0;
+
+ if (retval == 0 || hex_format)
+ goto done;
+
+ /* Guess whether the retval looks like an error code */
+ if ((retval & BIT(7)) && (retval >> 8) == 0)
+ err_code = (unsigned long)(s8)retval;
+ else if ((retval & BIT(15)) && (retval >> 16) == 0)
+ err_code = (unsigned long)(s16)retval;
+ else if ((retval & BIT(31)) && (((u64)retval) >> 32) == 0)
+ err_code = (unsigned long)(s32)retval;
+ else
+ err_code = retval;
+
+ if (!IS_ERR_VALUE(err_code))
+ err_code = 0;
+
+done:
+ if (leaf) {
+ if (hex_format || (err_code == 0))
+ trace_seq_printf(s, "%ps(); /* = 0x%lx */\n",
+ func, retval);
+ else
+ trace_seq_printf(s, "%ps(); /* = %ld */\n",
+ func, err_code);
+ } else {
+ if (hex_format || (err_code == 0))
+ trace_seq_printf(s, "} /* %ps = 0x%lx */\n",
+ func, retval);
+ else
+ trace_seq_printf(s, "} /* %ps = %ld */\n",
+ func, err_code);
+ }
+}
+
+#else
+
+#define __TRACE_GRAPH_PRINT_RETVAL 0
+
+#define print_graph_retval(_seq, _retval, _leaf, _func, _format) \
+ do {} while (0)
+
+#endif
+
/* Case of a leaf function on its call entry */
static enum print_line_t
print_graph_entry_leaf(struct trace_iterator *iter,
@@ -663,7 +721,15 @@ print_graph_entry_leaf(struct trace_iterator *iter,
for (i = 0; i < call->depth * TRACE_GRAPH_INDENT; i++)
trace_seq_putc(s, ' ');

- trace_seq_printf(s, "%ps();\n", (void *)call->func);
+ /*
+ * Write out the function return value if the option function-retval is
+ * enabled.
+ */
+ if (flags & __TRACE_GRAPH_PRINT_RETVAL)
+ print_graph_retval(s, graph_ret->retval, true, (void *)call->func,
+ !!(flags & TRACE_GRAPH_PRINT_RETVAL_HEX));
+ else
+ trace_seq_printf(s, "%ps();\n", (void *)call->func);

print_graph_irq(iter, graph_ret->func, TRACE_GRAPH_RET,
cpu, iter->ent->pid, flags);
@@ -942,16 +1008,25 @@ print_graph_return(struct ftrace_graph_ret *trace, struct trace_seq *s,
trace_seq_putc(s, ' ');

/*
- * If the return function does not have a matching entry,
- * then the entry was lost. Instead of just printing
- * the '}' and letting the user guess what function this
- * belongs to, write out the function name. Always do
- * that if the funcgraph-tail option is enabled.
+ * Always write out the function name and its return value if the
+ * function-retval option is enabled.
*/
- if (func_match && !(flags & TRACE_GRAPH_PRINT_TAIL))
- trace_seq_puts(s, "}\n");
- else
- trace_seq_printf(s, "} /* %ps */\n", (void *)trace->func);
+ if (flags & __TRACE_GRAPH_PRINT_RETVAL) {
+ print_graph_retval(s, trace->retval, false, (void *)trace->func,
+ !!(flags & TRACE_GRAPH_PRINT_RETVAL_HEX));
+ } else {
+ /*
+ * If the return function does not have a matching entry,
+ * then the entry was lost. Instead of just printing
+ * the '}' and letting the user guess what function this
+ * belongs to, write out the function name. Always do
+ * that if the funcgraph-tail option is enabled.
+ */
+ if (func_match && !(flags & TRACE_GRAPH_PRINT_TAIL))
+ trace_seq_puts(s, "}\n");
+ else
+ trace_seq_printf(s, "} /* %ps */\n", (void *)trace->func);
+ }

/* Overrun */
if (flags & TRACE_GRAPH_PRINT_OVERRUN)
--
2.25.1