linux-kernel - Re: [PATCH v3] tools/lib/traceevent: Replace str_error

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Date:   Fri, 5 Oct 2018 11:30:56 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Jiri Olsa <jolsa@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Tzvetomir Stoyanov <tz.stoyanov@...il.com>,
        Linux Trace Devel <linux-trace-devel@...r.kernel.org>,
        Colin Patrick McCabe <cmccabe@...mni.cmu.edu>
Subject: Re: [PATCH v3] tools/lib/traceevent: Replace str_error_r() with an
 open coded implementation

On Tue, 2 Oct 2018 17:55:39 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> While working on having PowerTop use libtracevent as a shared object
> library, Tzvetomir hit "str_error_r not defined". This was added by commit
> c3cec9e68f12d ("tools lib traceevent: Use str_error_r()") because
> strerror_r() has two definitions, where one is GNU specific, and the other
> is XSI complient. The strerror_r() is in a wrapper str_error_r() to keep the
> code from having to worry about which compiler is being used.
> 
> The problem is that str_error_r() is external to libtraceevent, and not part
> of the library. If it is used as a shared object then the tools using it
> will need to define that function. I do not want that function defined in
> libtraceevent itself, as it is out of scope for that library.
> 
> As there's only a single instance of this call, I replaced it with an open
> coded algorithm that uses sys_nerr and sys_errlist error array with
> strncpy() to place the error message in the given buffer. We don't need to
> worry about the errors that strerror_r() returns. If the buffer isn't big
> enough, we simply truncate it.
> 
> The sys_nerr and sys_errlist idea was found here:
> 
>   http://www.club.cc.cmu.edu/~cmccabe/blog_strerror.html
> 
> Cc: Colin Patrick McCabe <cmccabe@...mni.cmu.edu>
> Reported-by: Tzvetomir Stoyanov <tstoyanov@...are.com>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
> Changes since v2:
> 
>   Use sys_nerr and sys_errlist idea.
> 
>  tools/lib/traceevent/event-parse.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/lib/traceevent/event-parse.c b/tools/lib/traceevent/event-parse.c
> index 7980fc6c3bac..d23d10bc5314 100644
> --- a/tools/lib/traceevent/event-parse.c
> +++ b/tools/lib/traceevent/event-parse.c
> @@ -18,7 +18,6 @@
>  #include <errno.h>
>  #include <stdint.h>
>  #include <limits.h>
> -#include <linux/string.h>
>  #include <linux/time64.h>
>  
>  #include <netinet/in.h>
> @@ -6215,7 +6214,13 @@ int tep_strerror(struct tep_handle *pevent __maybe_unused,
>  	const char *msg;
>  
>  	if (errnum >= 0) {
> -		str_error_r(errnum, buf, buflen);
> +		if (buflen > 0) {
> +			if (errnum < sys_nerr)
> +				strncpy(buf, sys_errlist[errnum], buflen);
> +			else
> +				snprintf(buf, buflen, "Unknown error %d", errnum);
> +			buf[buflen - 1] = 0;
> +		}

Bah, I now get warnings that sys_nerr and sys_errlist are deprecated.

OK, so going back to just using the racy strerror() should be good
enough, as this incompatibility with strerror_r() is a disaster!

-- Steve


>  		return 0;
>  	}
>