execveat.2: Fix prototype

It's been 6 years since execveat(2) was added to the kernel,
and there's still no glibc wrapper.  Let's document the kernel
syscall prototype.

I reported a bug against glibc requesting a wrapper;
when glibc adds the wrapper, this commit should probably be
reverted.

......

.../linux$ grep_syscall execveat
fs/exec.c:2062:
SYSCALL_DEFINE5(execveat,
		int, fd, const char __user *, filename,
		const char __user *const __user *, argv,
		const char __user *const __user *, envp,
		int, flags)
fs/exec.c:2083:
COMPAT_SYSCALL_DEFINE5(execveat, int, fd,
		       const char __user *, filename,
		       const compat_uptr_t __user *, argv,
		       const compat_uptr_t __user *, envp,
		       int,  flags)
include/linux/compat.h:815:
asmlinkage long compat_sys_execveat(int dfd, const char __user *filename,
		     const compat_uptr_t __user *argv,
		     const compat_uptr_t __user *envp, int flags);
include/linux/syscalls.h:980:
asmlinkage long sys_execveat(int dfd, const char __user *filename,
			const char __user *const __user *argv,
			const char __user *const __user *envp, int flags);
.../linux$

function grep_syscall()
{
	if ! [ -v 1 ]; then
		>&2 echo "Usage: ${FUNCNAME[0]} <syscall>";
		return ${EX_USAGE};
	fi

	find * -type f \
	|grep '\.c$' \
	|sort -V \
	|xargs pcregrep -Mn "(?s)^\w*SYSCALL_DEFINE.\(${1},.*?\)" \
	|sed -E 's/^[^:]+:[0-9]+:/&\n/';

	find * -type f \
	|grep '\.[ch]$' \
	|sort -V \
	|xargs pcregrep -Mn "(?s)^asmlinkage\s+[\w\s]+\**sys_${1}\s*\(.*?\)" \
	|sed -E 's/^[^:]+:[0-9]+:/&\n/';
}

Glibc bug: <https://sourceware.org/bugzilla/show_bug.cgi?id=27364>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/execveat.2

@@ -31,7 +31,7 @@ execveat \- execute program relative to a directory file descriptor
 .B #include <unistd.h>
 .PP
 .BI "int execveat(int " dirfd ", const char *" pathname ,
-.BI "             char *const " argv "[], char *const " envp [],
+.BI "             const char *const " argv "[], const char *const " envp [],
 .BI "             int " flags );
 .fi
 .PP