syscalls: normalize _exit(2) declerations

exit(3) is implemented by the runtime and performs a number of shutdown
actions before ultimately calling _exit(2) to terminate the program.  We
historically named the syscall table entry `exit` rather than `_exit`,
but this requires special handling in libc/libsys to cause the `_exit`
symbol to exist while implementing `exit` in libc.

Declare the syscall as `_exit` and flow that through the system.

Because syscall(SYS_exit, code) is fairly widely used, allow a
configured extra line in syscall.h to define SYS_exit to SYS__exit.

I've found no external uses of __sys_exit() so I've not bothered to
create a compatability version of this private symbol.

Reviewed by:	imp, kib, emaste
Differential Revision:	https://reviews.freebsd.org/D51672

lib/libsys/Makefile.sys

@@ -52,7 +52,6 @@ STATICOBJS+=	interposing_table.o
 PSEUDO= \
 	__realpathat \
 	clock_gettime \
-	exit \
 	getlogin \
 	gettimeofday \
 	sched_getcpu

libexec/rtld-elf/rtld-libc/rtld_libc.h

@@ -44,7 +44,7 @@
 #define __libc_interposing error, must not use this variable inside rtld
 
 int	__sys_close(int);
-void	__sys_exit(int) __dead2;
+void	__sys__exit(int) __dead2;
 int	__sys_fcntl(int, int, ...);
 int	__sys_fstat(int fd, struct stat *);
 int	__sys_fstatat(int, const char *, struct stat *, int);
@@ -70,8 +70,8 @@ int __getosreldate(void);
  */
 #define close(fd)	__sys_close(fd)
 #define _close(fd)	__sys_close(fd)
-#define exit(status)	__sys_exit(status)
-#define _exit(status)	__sys_exit(status)
+#define exit(status)	__sys__exit(status)
+#define _exit(status)	__sys__exit(status)
 #define fcntl(fd, cmd, arg)	__sys_fcntl(fd, cmd, arg)
 #define _fcntl(fd, cmd, arg)	__sys_fcntl(fd, cmd, arg)
 #define _fstat(fd, sb)	__sys_fstat(fd, sb)

sys/kern/kern_exit.c

@@ -202,7 +202,7 @@ exit_onexit(struct proc *p)
  * exit -- death of process.
  */
 int
-sys_exit(struct thread *td, struct exit_args *uap)
+sys__exit(struct thread *td, struct _exit_args *uap)
 {
 
 	exit1(td, uap->rval, 0);

sys/kern/syscalls.conf

@@ -1,3 +1,4 @@
 libsysmap="../../lib/libsys/syscalls.map"
 libsys_h="../../lib/libsys/_libsys.h"
 sysmk="../sys/syscall.mk"
+syshdr_extra="#define 	SYS_exit	SYS__exit"

sys/kern/syscalls.master

@@ -125,7 +125,7 @@
 		);
 	}
 1	AUE_EXIT	STD|CAPENABLED {
-		void exit(
+		void _exit(
 		    int rval
 		);
 	}

sys/tools/syscalls/config.lua

@@ -24,6 +24,7 @@ local util = require("tools.util")
 local config = {
 	sysnames = "syscalls.c",
 	syshdr = "../sys/syscall.h",
+	syshdr_extra = nil;
 	sysmk = "/dev/null",
 	syssw = "init_sysent.c",
 	systrace = "systrace_args.c",

sys/tools/syscalls/scripts/syscall_h.lua

@@ -38,6 +38,10 @@ function syscall_h.generate(tbl, config, fh)
 	-- Write the generated preamble.
 	gen:preamble("System call numbers.")
 
+	if config.syshdr_extra then
+		gen:write(string.format("%s\n\n", config.syshdr_extra))
+	end
+
 	for _, v in pairs(s) do
 		local c = v:compatLevel()
 		if v.num > max then

sys/tools/syscalls/scripts/syscalls_map.lua

@@ -39,7 +39,7 @@ function syscalls_map.generate(tbl, config, fh)
 	for _, v in pairs(s) do
 		gen:write(v.prolog)
 		if v:native() and not v.type.NODEF and not v.type.NOLIB then
-			if v.name ~= "exit" and v.name ~= "vfork" then
+			if v.name ~= "_exit" and v.name ~= "vfork" then
 				gen:write(string.format("\t_%s;\n", v.name))
 			end
 			gen:write(string.format("\t__sys_%s;\n", v.name))