refactor all stat functions in terms of fstatat

equivalent logic for fstat+O_PATH fallback and direct use of
stat/lstat syscalls where appropriate is kept, now in the fstatat
function. this change both improves functionality (now, fstatat forms
equivalent to fstat/lstat/stat will work even on kernels too old to
have the at functions) and localizes direct interfacing with the
kernel stat structure to one file.

src/stat/fstat.c

+#define _BSD_SOURCE
 #include <sys/stat.h>
 #include <errno.h>
 #include <fcntl.h>
@@ -5,17 +6,7 @@
 
 int fstat(int fd, struct stat *st)
 {
-	int ret = __syscall(SYS_fstat, fd, st);
-	if (ret != -EBADF || __syscall(SYS_fcntl, fd, F_GETFD) < 0)
-		return __syscall_ret(ret);
-
-	char buf[15+3*sizeof(int)];
-	__procfdname(buf, fd);
-#ifdef SYS_stat
-	return syscall(SYS_stat, buf, st);
-#else
-	return syscall(SYS_fstatat, AT_FDCWD, buf, st, 0);
-#endif
+	return fstatat(fd, "", st, AT_EMPTY_PATH);
 }
 
 weak_alias(fstat, fstat64);

src/stat/fstatat.c

+#define _BSD_SOURCE
 #include <sys/stat.h>
+#include <string.h>
+#include <fcntl.h>
+#include <errno.h>
 #include "syscall.h"
 
-int fstatat(int fd, const char *restrict path, struct stat *restrict buf, int flag)
+int fstatat(int fd, const char *restrict path, struct stat *restrict st, int flag)
 {
-	return syscall(SYS_fstatat, fd, path, buf, flag);
+	int ret;
+
+	if (flag==AT_EMPTY_PATH && fd>=0 && !*path) {
+		ret = __syscall(SYS_fstat, fd, st);
+		if (ret==-EBADF && __syscall(SYS_fcntl, fd, F_GETFD)>=0) {
+			ret = __syscall(SYS_fstatat, fd, path, st, flag);
+			if (ret==-EINVAL) {
+				char buf[15+3*sizeof(int)];
+				__procfdname(buf, fd);
+#ifdef SYS_stat
+				ret = __syscall(SYS_stat, buf, st);
+#else
+				ret = __syscall(SYS_fstatat, AT_FDCWD, buf, st, 0);
+#endif
+			}
+		}
+	}
+#ifdef SYS_lstat
+	else if ((fd == AT_FDCWD || *path=='/') && flag==AT_SYMLINK_NOFOLLOW)
+		ret = __syscall(SYS_lstat, path, st);
+#endif
+#ifdef SYS_stat
+	else if ((fd == AT_FDCWD || *path=='/') && !flag)
+		ret = __syscall(SYS_stat, path, st);
+#endif
+	else ret = __syscall(SYS_fstatat, fd, path, st, flag);
+
+	return __syscall_ret(ret);
 }
 
 weak_alias(fstatat, fstatat64);

src/stat/lstat.c

@@ -4,11 +4,7 @@
 
 int lstat(const char *restrict path, struct stat *restrict buf)
 {
-#ifdef SYS_lstat
-	return syscall(SYS_lstat, path, buf);
-#else
-	return syscall(SYS_fstatat, AT_FDCWD, path, buf, AT_SYMLINK_NOFOLLOW);
-#endif
+	return fstatat(AT_FDCWD, path, buf, AT_SYMLINK_NOFOLLOW);
 }
 
 weak_alias(lstat, lstat64);

src/stat/stat.c

@@ -4,11 +4,7 @@
 
 int stat(const char *restrict path, struct stat *restrict buf)
 {
-#ifdef SYS_stat
-	return syscall(SYS_stat, path, buf);
-#else
-	return syscall(SYS_fstatat, AT_FDCWD, path, buf, 0);
-#endif
+	return fstatat(AT_FDCWD, path, buf, 0);
 }
 
 weak_alias(stat, stat64);