fix: make zfs_strerror really thread-safe and portable

openzfs#15793 wanted to make zfs_strerror threadsafe, unfortunately, it
turned out that strerror_l() usage was wrong, and also, some libc 
implementations dont have strerror_l().

zfs_strerror() now simply calls original strerror() and copies the 
result to a thread-local buffer, then returns that.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Signed-off-by: Richard Kojedzinszky <[email protected]>
Closes openzfs#15793
Closes openzfs#16640
Closes openzfs#16923

Author: rkojedzinszky

config/user.m4

@@ -33,7 +33,7 @@ AC_DEFUN([ZFS_AC_CONFIG_USER], [
 	ZFS_AC_CONFIG_USER_MAKEDEV_IN_MKDEV
 	ZFS_AC_CONFIG_USER_ZFSEXEC
 
-	AC_CHECK_FUNCS([execvpe issetugid mlockall strerror_l strlcat strlcpy gettid])
+	AC_CHECK_FUNCS([execvpe issetugid mlockall strlcat strlcpy gettid])
 
 	AC_SUBST(RM)
 ])

include/libzutil.h

@@ -27,7 +27,7 @@
 #define	_LIBZUTIL_H extern __attribute__((visibility("default")))
 
 #include <string.h>
-#include <locale.h>
+#include <pthread.h>
 #include <sys/nvpair.h>
 #include <sys/fs/zfs.h>
 
@@ -274,11 +274,14 @@ _LIBZUTIL_H void update_vdev_config_dev_sysfs_path(nvlist_t *nv,
  * Thread-safe strerror() for use in ZFS libraries
  */
 static inline char *zfs_strerror(int errnum) {
-#ifdef HAVE_STRERROR_L
-	return (strerror_l(errnum, uselocale(0)));
-#else
-	return (strerror(errnum));
-#endif
+	static __thread char errbuf[512];
+	static pthread_mutex_t zfs_strerror_lock = PTHREAD_MUTEX_INITIALIZER;
+
+	(void) pthread_mutex_lock(&zfs_strerror_lock);
+	(void) strlcpy(errbuf, strerror(errnum), sizeof (errbuf));
+	(void) pthread_mutex_unlock(&zfs_strerror_lock);
+
+	return (errbuf);
 }
 
 #ifdef	__cplusplus