linux-kernel - Re: [PATCH] tool/perf: Perf build fails on 5.12.0rc2 on s390

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Date:   Tue, 9 Mar 2021 09:43:34 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Thomas Richter <tmricht@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        svens@...ux.ibm.com, gor@...ux.ibm.com, sumanthk@...ux.ibm.com,
        hca@...ux.ibm.com
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@...lp76 perf]# make  util/synthetic-events.o
> ....
>  CC       util/synthetic-events.o
> [root@...lp76 perf]#
> 
> Signed-off-by: Thomas Richter <tmricht@...ux.ibm.com>
> ---
>  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