linux-kernel - Re: [PATCH v2] perf ftrace: Fix the buffer size in __write_tracing

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Message-ID: <20180212061510.GI31513@sejong>
Date:   Mon, 12 Feb 2018 15:15:10 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     "Du, Changbin" <changbin.du@...el.com>
Cc:     jolsa@...hat.com, peterz@...radead.org, mingo@...hat.com,
        acme@...nel.org, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org, kernel-team@....com,
        rostedt@...dmis.org
Subject: Re: [PATCH v2] perf ftrace: Fix the buffer size in
 __write_tracing_file

On Mon, Feb 12, 2018 at 12:48:15PM +0800, Du, Changbin wrote:
> Hi,
> 
> On Mon, Feb 12, 2018 at 10:55:27AM +0900, Namhyung Kim wrote:
> > Hello,
> > 
> > On Thu, Feb 08, 2018 at 04:13:20PM +0800, changbin.du@...el.com wrote:
> > > From: Changbin Du <changbin.du@...el.com>
> > > 
> > > The terminal character '\0' should take into account into size of the
> > > string buffer. Without this fix, the '--graph-funcs', '--nograph-funcs'
> > > and '--trace-funcs' options didn't work as expected when the <func>
> > > doesn't exist. If usersapce writes a non-terminated string, the kernel
> > > side will always return success but actually no filter applied.
> > > 
> > > As discussed before, the kernel now support '\0' to mark the end of string:
> > > https://lkml.org/lkml/2018/1/16/116
> > > 
> > > After this fix in userspace, the perf will report correct error state. Also
> > > let it print an error if reset_tracing_files() fails.
> > 
> > But what about old kernels?  IIRC there was an error with this change.
> >
> Yes, you're right. I can't find a good compitable change. So what is the compatibilty policy for perf?
> If it must work with recent kernel, I think the only idea is leave as it was.

It should support *both* of kernels.  I suggest adding a space after
function name.  You can simply call write(fd, " ", 1) IMHO.

Thanks,
Namhyung


> > 
> > 
> > > 
> > > The problem:
> > > $ sudo ./perf ftrace -a --graph-depth 1 --graph-funcs abcdefg
> > >  0)   0.140 us    |  rcu_all_qs();
> > >  3)   0.304 us    |  mutex_unlock();
> > >  0)   0.153 us    |  find_vma();
> > >  3)   0.088 us    |  __fsnotify_parent();
> > >  0)   6.145 us    |  handle_mm_fault();
> > >  3)   0.089 us    |  fsnotify();
> > >  3)   0.161 us    |  __sb_end_write();
> > >  3)   0.710 us    |  SyS_close();
> > >  3)   7.848 us    |  exit_to_usermode_loop();
> > > 
> > > On above example, I specified function filter 'abcdefg' but all functions
> > > are enabled. The expected error is hidden.
> > > 
> > > Signed-off-by: Changbin Du <changbin.du@...el.com>
> > > ---
> > >  tools/perf/builtin-ftrace.c | 6 ++++--
> > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
> > > index 25a42ac..a87e9b3 100644
> > > --- a/tools/perf/builtin-ftrace.c
> > > +++ b/tools/perf/builtin-ftrace.c
> > > @@ -69,7 +69,7 @@ static int __write_tracing_file(const char *name, const char *val, bool append)
> > >  {
> > >  	char *file;
> > >  	int fd, ret = -1;
> > > -	ssize_t size = strlen(val);
> > > +	ssize_t size = strlen(val) + 1;
> > >  	int flags = O_WRONLY;
> > >  	char errbuf[512];
> > >  
> > > @@ -280,8 +280,10 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace, int argc, const char **argv)
> > >  	signal(SIGCHLD, sig_handler);
> > >  	signal(SIGPIPE, sig_handler);
> > >  
> > > -	if (reset_tracing_files(ftrace) < 0)
> > > +	if (reset_tracing_files(ftrace) < 0) {
> > > +		pr_err("failed to reset ftrace\n");
> > >  		goto out;
> > > +	}
> > >  
> > >  	/* reset ftrace buffer */
> > >  	if (write_tracing_file("trace", "0") < 0)
> > > -- 
> > > 2.7.4
> > > 
> 
> -- 
> Thanks,
> Changbin Du