linux-kernel - Re: [PATCH Linux-next] perf test: Fix test case perf trace BTF general tests

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Message-ID: <aSdNiF2VZMtjQnUQ@google.com>
Date: Wed, 26 Nov 2025 10:57:12 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: acme@...nel.org, Thomas Richter <tmricht@...ux.ibm.com>,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-perf-users@...r.kernel.org, agordeev@...ux.ibm.com,
	gor@...ux.ibm.com, sumanthk@...ux.ibm.com, hca@...ux.ibm.com,
	japo@...ux.ibm.com
Subject: Re: [PATCH Linux-next] perf test: Fix test case perf trace BTF
 general tests

On Wed, Nov 26, 2025 at 12:12:29PM -0500, Steven Rostedt wrote:
> 
> Arnaldo,
> 
> How can I make perf trace not confused by the extra fields in the system
> call trace events?
> 
> Ftrace can now show the contents of the system call user space buffers, but
> it appears that this breaks perf!!!
> 
> system: syscalls
> name: sys_enter_write
> ID: 791
> format:
> 	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
> 	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
> 	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
> 	field:int common_pid;	offset:4;	size:4;	signed:1;
> 
> 	field:int __syscall_nr;	offset:8;	size:4;	signed:1;
> 	field:unsigned int fd;	offset:16;	size:8;	signed:0;
> 	field:const char * buf;	offset:24;	size:8;	signed:0;
> 	field:size_t count;	offset:32;	size:8;	signed:0;
> 	field:__data_loc char[] __buf_val;	offset:40;	size:4;	signed:0;
> 
> That new __buf_val appears to confuse perf, but I'm having a hell of a time
> trying to figure out where it reads it!

I've discussed with Steven and concluded that we should change perf to
ignore fields with "__data_loc char[]" type in syscalls.  Let me take a
look.

Thanks,
Namhyung