linux: Use waitid on wait4 if __NR_wait4 is not defined

If the wait4 syscall is not available (such as y2038 safe 32-bit
systems) waitid should be used instead.  However prior Linux 5.4
waitid is not a full superset of other wait syscalls, since it
does not include support for waiting for the current process group.

It is possible to emulate wait4 by issuing an extra syscall to get
the current process group, but it is inherent racy: after the current
process group is received and before it is passed to waitid a signal
could arrive causing the current process group to change.

So waitid is used if wait4 is not defined iff the build is
enabled with a minimum kernel if 5.4+.  The new assume
__ASSUME_WAITID_PID0_P_PGID is added and an error is issued if waitid
can not be implemented by either __NR_wait4 or
__NR_waitid && __ASSUME_WAITID_PID0_P_PGID.

Checked on x86_64-linux-gnu and i686-linux-gnu.

Co-authored-by: Alistair Francis <alistair.francis@wdc.com>

sysdeps/unix/sysv/linux/kernel-features.h

@@ -208,4 +208,10 @@
 # define __ASSUME_TIME64_SYSCALLS 1
 #endif
 
+/* Linux waitid prior kernel 5.4 does not support waiting for the current
+   process group.  */
+#if __LINUX_KERNEL_VERSION >= 0x050400
+# define __ASSUME_WAITID_PID0_P_PGID
+#endif
+
 #endif /* kernel-features.h */

sysdeps/unix/sysv/linux/syscalls.list

@@ -67,7 +67,6 @@ swapoff		-	swapoff		i:s	__swapoff	swapoff
 unshare		EXTRA	unshare		i:i	unshare
 uselib		EXTRA	uselib		i:s	__compat_uselib	uselib@GLIBC_2.0:GLIBC_2.23
 utime		-	utime		i:sP	utime
-wait4		-	wait4		i:iWiP	__wait4		wait4
 
 chown		-	chown		i:sii	__libc_chown	__chown chown
 

sysdeps/unix/sysv/linux/wait4.c

@@ -0,0 +1,84 @@
+/* Wait for process to change state.  Linux version.
+   Copyright (C) 2019 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sys/wait.h>
+#include <sys/resource.h>
+#include <sysdep-cancel.h>
+
+pid_t
+__wait4 (pid_t pid, int *stat_loc, int options, struct rusage *usage)
+{
+#ifdef __NR_wait4
+  return SYSCALL_CANCEL (wait4, pid, stat_loc, options, usage);
+#elif defined (__ASSUME_WAITID_PID0_P_PGID)
+  idtype_t idtype = P_PID;
+
+  if (pid < -1)
+    {
+      idtype = P_PGID;
+      pid *= -1;
+    }
+  else if (pid == -1)
+    idtype = P_ALL;
+  else if (pid == 0)
+    idtype = P_PGID;
+
+  options |= WEXITED;
+
+  siginfo_t infop;
+  if (SYSCALL_CANCEL (waitid, idtype, pid, &infop, options, usage) < 0)
+    return -1;
+
+  if (stat_loc)
+    {
+      switch (infop.si_code)
+        {
+        case CLD_EXITED:
+          *stat_loc = W_EXITCODE (infop.si_status, 0);
+          break;
+        case CLD_DUMPED:
+          *stat_loc = WCOREFLAG | infop.si_status;
+	  break;
+        case CLD_KILLED:
+          *stat_loc = infop.si_status;
+          break;
+        case CLD_TRAPPED:
+        case CLD_STOPPED:
+          *stat_loc = W_STOPCODE (infop.si_status);
+          break;
+        case CLD_CONTINUED:
+          *stat_loc = __W_CONTINUED;
+          break;
+	default:
+	  *stat_loc = 0;
+	  break;
+        }
+    }
+
+  return infop.si_pid;
+# else
+/* Linux waitid prior kernel 5.4 does not support waiting for the current
+   process.  It is possible to emulate wait4 it by calling getpgid for
+   PID 0, however, it would require an additional syscall and it is inherent
+   racy: after the current process group is received and before it is passed
+   to waitid a signal could arrive causing the current process group to
+   change.  */
+# error "The kernel ABI does not provide a way to implement wait4"
+#endif
+}
+weak_alias (__wait4, wait4)