tools lib traceevent: Separate out tep_strerror() for strerror_r() issues

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, and its in the traceevent
library's own tep_strerror() function, we can copy what was done in perf,
and create yet another external file that undefs _GNU_SOURCE to use the more
portable version of the function. We don't need to worry about the errors
that strerror_r() returns. If the buffer isn't big enough, we simply
truncate it.

Reported-by: Tzvetomir Stoyanov <[e-mail]>
Signed-off-by: Steven Rostedt (VMware) <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Tzvetomir Stoyanov (VMware) <[e-mail]>
Cc: linux trace devel <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/lib/traceevent/Build

···
 libtraceevent-y += parse-filter.o
 libtraceevent-y += parse-utils.o
 libtraceevent-y += kbuffer-parse.o
+libtraceevent-y += tep_strerror.o
 
 plugin_jbd2-y         += plugin_jbd2.o
 plugin_hrtimer-y      += plugin_hrtimer.o

tools/lib/traceevent/event-parse.c

···
 #include <errno.h>
 #include <stdint.h>
 #include <limits.h>
-#include <linux/string.h>
 #include <linux/time64.h>
 
 #include <netinet/in.h>
···
 {
 	struct tep_event_format *event = NULL;
 	return __parse_event(pevent, &event, buf, size, sys);
-}
-
-#undef _PE
-#define _PE(code, str) str
-static const char * const tep_error_str[] = {
-	TEP_ERRORS
-};
-#undef _PE
-
-int tep_strerror(struct tep_handle *pevent __maybe_unused,
-		 enum tep_errno errnum, char *buf, size_t buflen)
-{
-	int idx;
-	const char *msg;
-
-	if (errnum >= 0) {
-		str_error_r(errnum, buf, buflen);
-		return 0;
-	}
-
-	if (errnum <= __TEP_ERRNO__START ||
-	    errnum >= __TEP_ERRNO__END)
-		return -1;
-
-	idx = errnum - __TEP_ERRNO__START - 1;
-	msg = tep_error_str[idx];
-	snprintf(buf, buflen, "%s", msg);
-
-	return 0;
 }
 
 int get_field_val(struct trace_seq *s, struct tep_format_field *field,

tools/lib/traceevent/tep_strerror.c

···
+// SPDX-License-Identifier: LGPL-2.1
+#undef _GNU_SOURCE
+#include <string.h>
+#include <stdio.h>
+
+#include "event-parse.h"
+
+#undef _PE
+#define _PE(code, str) str
+static const char * const tep_error_str[] = {
+	TEP_ERRORS
+};
+#undef _PE
+
+/*
+ * The tools so far have been using the strerror_r() GNU variant, that returns
+ * a string, be it the buffer passed or something else.
+ *
+ * But that, besides being tricky in cases where we expect that the function
+ * using strerror_r() returns the error formatted in a provided buffer (we have
+ * to check if it returned something else and copy that instead), breaks the
+ * build on systems not using glibc, like Alpine Linux, where musl libc is
+ * used.
+ *
+ * So, introduce yet another wrapper, str_error_r(), that has the GNU
+ * interface, but uses the portable XSI variant of strerror_r(), so that users
+ * rest asured that the provided buffer is used and it is what is returned.
+ */
+int tep_strerror(struct tep_handle *tep __maybe_unused,
+		 enum tep_errno errnum, char *buf, size_t buflen)
+{
+	const char *msg;
+	int idx;
+
+	if (!buflen)
+		return 0;
+
+	if (errnum >= 0) {
+		int err = strerror_r(errnum, buf, buflen);
+		buf[buflen - 1] = 0;
+		return err;
+	}
+
+	if (errnum <= __TEP_ERRNO__START ||
+	    errnum >= __TEP_ERRNO__END)
+		return -1;
+
+	idx = errnum - __TEP_ERRNO__START - 1;
+	msg = tep_error_str[idx];
+	snprintf(buf, buflen, "%s", msg);
+
+	return 0;
+}