2021-03-09 11:07:52

by Thomas Richter

Subject: [PATCH] tool/perf: Perf build fails on 5.12.0rc2 on s390

perf build fails on 5.12.0rc2 on s390 with this error message:

util/synthetic-events.c: In function
‘__event__synthesize_thread.part.0.isra’:
util/synthetic-events.c:787:19: error: ‘kernel_thread’ may be
used uninitialized in this function [-Werror=maybe-uninitialized]
787 | if (_pid == pid && !kernel_thread) {
| ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~

The build succeeds using command 'make DEBUG=y'.

The variable kernel_thread is set by this function sequence:

__event__synthesize_thread()
| defines bool kernel_thread; as local variable and calls
+--> perf_event__prepare_comm(..., &kernel_thread)
+--> perf_event__get_comm_ids(..., bool *kernel);
On return of this function variable kernel is always
set to true of false.

To prevent this compile error, assign variable kernel_thread
a value when it is defined.

Output after:
[root@m35lp76 perf]# make util/synthetic-events.o
....
CC util/synthetic-events.o
[root@m35lp76 perf]#

Signed-off-by: Thomas Richter <[email protected]>
---
tools/perf/util/synthetic-events.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c
index b698046ec2db..5dd451695f33 100644
--- a/tools/perf/util/synthetic-events.c
+++ b/tools/perf/util/synthetic-events.c
@@ -758,7 +758,7 @@ static int __event__synthesize_thread(union perf_event *comm_event,
for (i = 0; i < n; i++) {
char *end;
pid_t _pid;
- bool kernel_thread;
+ bool kernel_thread = false;

_pid = strtol(dirent[i]->d_name, &end, 10);
if (*end)
--
2.29.2

2021-03-09 12:45:24

by Arnaldo Carvalho de Melo

Subject: Re: [PATCH] tool/perf: Perf build fails on 5.12.0rc2 on s390

Em Tue, Mar 09, 2021 at 12:04:47PM +0100, Thomas Richter escreveu:
> perf build fails on 5.12.0rc2 on s390 with this error message:
>
> util/synthetic-events.c: In function
> ‘__event__synthesize_thread.part.0.isra’:
> util/synthetic-events.c:787:19: error: ‘kernel_thread’ may be
> used uninitialized in this function [-Werror=maybe-uninitialized]
> 787 | if (_pid == pid && !kernel_thread) {
> | ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
>
> The build succeeds using command 'make DEBUG=y'.
>
> The variable kernel_thread is set by this function sequence:
>
> __event__synthesize_thread()
> | defines bool kernel_thread; as local variable and calls
> +--> perf_event__prepare_comm(..., &kernel_thread)
> +--> perf_event__get_comm_ids(..., bool *kernel);
> On return of this function variable kernel is always
> set to true of false.

s/of/or/

But it is only called for the host 'struct machine', if that is not the
case, then the value of 'kernel_thread' is left undefined/uninitialized,
right?

>
> To prevent this compile error, assign variable kernel_thread
> a value when it is defined.

Applied, and added:

Fixes: c1b907953b2cd9ff ("perf tools: Skip PERF_RECORD_MMAP event synthesis for kernel threads")

Changed the subject to:

perf synthetic-events: Fix uninitialized 'kernel_thread' variable

As this doesn't affect just s/390, it is entirely possible that that
variable gets used with an undefined value.

- Arnaldo

> Output after:
> [root@m35lp76 perf]# make util/synthetic-events.o
> ....
> CC util/synthetic-events.o
> [root@m35lp76 perf]#
>
> Signed-off-by: Thomas Richter <[email protected]>
> ---
> tools/perf/util/synthetic-events.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c
> index b698046ec2db..5dd451695f33 100644
> --- a/tools/perf/util/synthetic-events.c
> +++ b/tools/perf/util/synthetic-events.c
> @@ -758,7 +758,7 @@ static int __event__synthesize_thread(union perf_event *comm_event,
> for (i = 0; i < n; i++) {
> char *end;
> pid_t _pid;
> - bool kernel_thread;
> + bool kernel_thread = false;
>
> _pid = strtol(dirent[i]->d_name, &end, 10);
> if (*end)
> --
> 2.29.2
>

--

- Arnaldo