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Subject[GIT PULL] tracing: Fix to recursion protection for 5.15


tracing recursion fix:

- While cleaning up some of the tracing recursion protection logic,
I discovered a scenario that the current design would miss, and
would allow an infinite recursion. Removing an optimization trick
that opened the hole, fixes the issue and cleans up the code as well.

Please pull the latest trace-v5.15-rc5 tree, which can be found at:


Tag SHA1: 643b4d78baa695d2c7d27a149c9dfa6fd70ee2df
Head SHA1: ed65df63a39a3f6ed04f7258de8b6789e5021c18

Steven Rostedt (VMware) (1):
tracing: Have all levels of checks prevent recursion

include/linux/trace_recursion.h | 49 ++++++++---------------------------------
kernel/trace/ftrace.c | 4 ++--
2 files changed, 11 insertions(+), 42 deletions(-)
commit ed65df63a39a3f6ed04f7258de8b6789e5021c18
Author: Steven Rostedt (VMware) <>
Date: Mon Oct 18 15:44:12 2021 -0400

tracing: Have all levels of checks prevent recursion

While writing an email explaining the "bit = 0" logic for a discussion on
making ftrace_test_recursion_trylock() disable preemption, I discovered a
path that makes the "not do the logic if bit is zero" unsafe.

The recursion logic is done in hot paths like the function tracer. Thus,
any code executed causes noticeable overhead. Thus, tricks are done to try
to limit the amount of code executed. This included the recursion testing

Having recursion testing is important, as there are many paths that can
end up in an infinite recursion cycle when tracing every function in the
kernel. Thus protection is needed to prevent that from happening.

Because it is OK to recurse due to different running context levels (e.g.
an interrupt preempts a trace, and then a trace occurs in the interrupt
handler), a set of bits are used to know which context one is in (normal,
softirq, irq and NMI). If a recursion occurs in the same level, it is

Then there are infrastructure levels of recursion as well. When more than
one callback is attached to the same function to trace, it calls a loop
function to iterate over all the callbacks. Both the callbacks and the
loop function have recursion protection. The callbacks use the
"ftrace_test_recursion_trylock()" which has a "function" set of context
bits to test, and the loop function calls the internal
trace_test_and_set_recursion() directly, with an "internal" set of bits.

If an architecture does not implement all the features supported by ftrace
then the callbacks are never called directly, and the loop function is
called instead, which will implement the features of ftrace.

Since both the loop function and the callbacks do recursion protection, it
was seemed unnecessary to do it in both locations. Thus, a trick was made
to have the internal set of recursion bits at a more significant bit
location than the function bits. Then, if any of the higher bits were set,
the logic of the function bits could be skipped, as any new recursion
would first have to go through the loop function.

This is true for architectures that do not support all the ftrace
features, because all functions being traced must first go through the
loop function before going to the callbacks. But this is not true for
architectures that support all the ftrace features. That's because the
loop function could be called due to two callbacks attached to the same
function, but then a recursion function inside the callback could be
called that does not share any other callback, and it will be called


traced_function_1: [ more than one callback tracing it ]
call loop_func

trace_recursion set internal bit
call callback

trace_recursion [ skipped because internal bit is set, return 0 ]
call traced_function_2

traced_function_2: [ only traced by above callback ]
call callback

trace_recursion [ skipped because internal bit is set, return 0 ]
call traced_function_2

[ wash, rinse, repeat, BOOM! out of shampoo! ]

Thus, the "bit == 0 skip" trick is not safe, unless the loop function is
call for all functions.

Since we want to encourage architectures to implement all ftrace features,
having them slow down due to this extra logic may encourage the
maintainers to update to the latest ftrace features. And because this
logic is only safe for them, remove it completely.

[*] There is on layer of recursion that is allowed, and that is to allow
for the transition between interrupt context (normal -> softirq ->
irq -> NMI), because a trace may occur before the context update is
visible to the trace recursion logic.


Cc: Linus Torvalds <>
Cc: Petr Mladek <>
Cc: Ingo Molnar <>
Cc: "James E.J. Bottomley" <>
Cc: Helge Deller <>
Cc: Michael Ellerman <>
Cc: Benjamin Herrenschmidt <>
Cc: Paul Mackerras <>
Cc: Paul Walmsley <>
Cc: Palmer Dabbelt <>
Cc: Albert Ou <>
Cc: Thomas Gleixner <>
Cc: Borislav Petkov <>
Cc: "H. Peter Anvin" <>
Cc: Josh Poimboeuf <>
Cc: Jiri Kosina <>
Cc: Miroslav Benes <>
Cc: Joe Lawrence <>
Cc: Colin Ian King <>
Cc: Masami Hiramatsu <>
Cc: "Peter Zijlstra (Intel)" <>
Cc: Nicholas Piggin <>
Cc: Jisheng Zhang <>
Cc: =?utf-8?b?546L6LSH?= <>
Cc: Guo Ren <>
Fixes: edc15cafcbfa3 ("tracing: Avoid unnecessary multiple recursion checks")
Signed-off-by: Steven Rostedt (VMware) <>

diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
index a9f9c5714e65..fe95f0922526 100644
--- a/include/linux/trace_recursion.h
+++ b/include/linux/trace_recursion.h
@@ -16,23 +16,8 @@
* When function tracing occurs, the following steps are made:
* If arch does not support a ftrace feature:
* call internal function (uses INTERNAL bits) which calls...
- * If callback is registered to the "global" list, the list
- * function is called and recursion checks the GLOBAL bits.
- * then this function calls...
* The function callback, which can use the FTRACE bits to
* check for recursion.
- *
- * Now if the arch does not support a feature, and it calls
- * the global list function which calls the ftrace callback
- * all three of these steps will do a recursion protection.
- * There's no reason to do one if the previous caller already
- * did. The recursion that we are protecting against will
- * go through the same steps again.
- *
- * To prevent the multiple recursion checks, if a recursion
- * bit is set that is higher than the MAX bit of the current
- * check, then we know that the check was made by the previous
- * caller, and we can skip the current check.
enum {
/* Function recursion bits */
@@ -40,12 +25,14 @@ enum {

- /* INTERNAL_BITs must be greater than FTRACE_BITs */
+ /* Internal use recursion bits */

@@ -86,12 +73,6 @@ enum {

- /*
- * When transitioning between context, the preempt_count() may
- * not be correct. Allow for a single recursion to cover this case.
- */
/* Used to prevent recursion recording from recursing. */
@@ -113,12 +94,10 @@ enum {




* Used for setting context
@@ -132,6 +111,7 @@ enum {

static __always_inline int trace_get_context_bit(void)
@@ -160,45 +140,34 @@ extern void ftrace_record_recursion(unsigned long ip, unsigned long parent_ip);

static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsigned long pip,
- int start, int max)
+ int start)
unsigned int val = READ_ONCE(current->trace_recursion);
int bit;

- /* A previous recursion check was made */
- if ((val & TRACE_CONTEXT_MASK) > max)
- return 0;
bit = trace_get_context_bit() + start;
if (unlikely(val & (1 << bit))) {
* It could be that preempt_count has not been updated during
* a switch between contexts. Allow for a single recursion.
+ bit = TRACE_CTX_TRANSITION + start;
if (val & (1 << bit)) {
do_ftrace_record_recursion(ip, pip);
return -1;
- } else {
- /* Normal check passed, clear the transition to allow it again */
- val &= ~(1 << TRACE_TRANSITION_BIT);

val |= 1 << bit;
current->trace_recursion = val;

- return bit + 1;
+ return bit;

static __always_inline void trace_clear_recursion(int bit)
- if (!bit)
- return;
- bit--;

@@ -214,7 +183,7 @@ static __always_inline void trace_clear_recursion(int bit)
static __always_inline int ftrace_test_recursion_trylock(unsigned long ip,
unsigned long parent_ip)
- return trace_test_and_set_recursion(ip, parent_ip, TRACE_FTRACE_START, TRACE_FTRACE_MAX);
+ return trace_test_and_set_recursion(ip, parent_ip, TRACE_FTRACE_START);

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 7efbc8aaf7f6..635fbdc9d589 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -6977,7 +6977,7 @@ __ftrace_ops_list_func(unsigned long ip, unsigned long parent_ip,
struct ftrace_ops *op;
int bit;

- bit = trace_test_and_set_recursion(ip, parent_ip, TRACE_LIST_START, TRACE_LIST_MAX);
+ bit = trace_test_and_set_recursion(ip, parent_ip, TRACE_LIST_START);
if (bit < 0)

@@ -7052,7 +7052,7 @@ static void ftrace_ops_assist_func(unsigned long ip, unsigned long parent_ip,
int bit;

- bit = trace_test_and_set_recursion(ip, parent_ip, TRACE_LIST_START, TRACE_LIST_MAX);
+ bit = trace_test_and_set_recursion(ip, parent_ip, TRACE_LIST_START);
if (bit < 0)

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