libc: Fix getentropy POSIX 2024 conformance issues

GETENTROPY_MAX should be defined in limits.h.  EINVAL is the return
value for buflen > GETENTROPY_MAX.

PR:		282783
Reviewed by:	markj, asomers, jhb
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47689

include/limits.h

@@ -137,6 +137,10 @@
 
 #define	MB_LEN_MAX		6	/* 31-bit UTF-8 */
 
+#if __POSIX_VISIBLE >= 202405
+#define GETENTROPY_MAX		256
+#endif
+
 #include <sys/limits.h>
 
 #if __POSIX_VISIBLE

lib/libc/gen/getentropy.3

@@ -15,7 +15,7 @@
 .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd November 20, 2024
+.Dd January 17, 2025
 .Dt GETENTROPY 3
 .Os
 .Sh NAME
@@ -60,16 +60,16 @@ The
 .Fa buf
 parameter points to an
 invalid address.
-.It Bq Er EIO
-Too many bytes requested, or some other fatal error occurred.
+.It Bq Er EINVAL
+Too many bytes requested.
 .El
 .Sh SEE ALSO
 .Xr getrandom 2 ,
 .Xr arc4random 3 ,
 .Xr random 4
 .Sh STANDARDS
 .Fn getentropy
-nearly conforms to
+conforms to
 .St -p1003.1-2024 .
 .Sh HISTORY
 The
@@ -80,9 +80,3 @@ The
 .Fx
 libc compatibility shim first appeared in
 .Fx 12.0 .
-.Sh BUGS
-.In limits.h
-does not define
-.Dv GETENTROPY_MAX .
-Some error values do not match
-.St -p1003.1-2024 .

lib/libc/gen/getentropy.c

@@ -30,6 +30,7 @@
 #include <sys/random.h>
 
 #include <errno.h>
+#include <limits.h>
 #include <signal.h>
 #include <unistd.h>
 #include <ssp/ssp.h>
@@ -47,8 +48,8 @@ __ssp_real(getentropy)(void *buf, size_t buflen)
 {
 	ssize_t rd;
 
-	if (buflen > 256) {
-		errno = EIO;
+	if (buflen > GETENTROPY_MAX) {
+		errno = EINVAL;
 		return (-1);
 	}