implement fstatat with SYS_statx, conditional on undersized kstat time

this commit adds a new backend for fstatat (and thereby the whole stat
family) using the SYS_statx syscall, but conditions the new code on
the kernel stat structure's time fields being smaller than time_t. in
principle that should make it all dead code at present, but mips64 has
a broken stat structure with 32-bit time fields despite having 64-bit
time_t elsewhere, so on mips64 it is a functional change that makes
post-Y2038 filesystem timestamps accessible.

whenever the 32-bit archs end up getting 64-bit time_t, regardless of
how that happens, the changes in this commit will automatically take
effect for them too.

src/stat/fstatat.c

@@ -3,10 +3,65 @@
 #include <string.h>
 #include <fcntl.h>
 #include <errno.h>
+#include <stdint.h>
+#include <sys/sysmacros.h>
 #include "syscall.h"
 #include "kstat.h"
 
-int fstatat(int fd, const char *restrict path, struct stat *restrict st, int flag)
+struct statx {
+	uint32_t stx_mask;
+	uint32_t stx_blksize;
+	uint64_t stx_attributes;
+	uint32_t stx_nlink;
+	uint32_t stx_uid;
+	uint32_t stx_gid;
+	uint16_t stx_mode;
+	uint16_t pad1;
+	uint64_t stx_ino;
+	uint64_t stx_size;
+	uint64_t stx_blocks;
+	uint64_t stx_attributes_mask;
+	struct {
+		int64_t tv_sec;
+		uint32_t tv_nsec;
+		int32_t pad;
+	} stx_atime, stx_btime, stx_ctime, stx_mtime;
+	uint32_t stx_rdev_major;
+	uint32_t stx_rdev_minor;
+	uint32_t stx_dev_major;
+	uint32_t stx_dev_minor;
+	uint64_t spare[14];
+} stx;
+
+static int fstatat_statx(int fd, const char *restrict path, struct stat *restrict st, int flag)
+{
+	struct statx stx;
+
+	int ret = __syscall(SYS_statx, fd, path, flag, 0x7ff, &stx);
+	if (ret) return ret;
+
+	*st = (struct stat){
+		.st_dev = makedev(stx.stx_dev_major, stx.stx_dev_minor),
+		.st_ino = stx.stx_ino,
+		.st_mode = stx.stx_mode,
+		.st_nlink = stx.stx_nlink,
+		.st_uid = stx.stx_uid,
+		.st_gid = stx.stx_gid,
+		.st_rdev = makedev(stx.stx_rdev_major, stx.stx_rdev_minor),
+		.st_size = stx.stx_size,
+		.st_blksize = stx.stx_blksize,
+		.st_blocks = stx.stx_blocks,
+		.st_atim.tv_sec = stx.stx_atime.tv_sec,
+		.st_atim.tv_nsec = stx.stx_atime.tv_nsec,
+		.st_mtim.tv_sec = stx.stx_mtime.tv_sec,
+		.st_mtim.tv_nsec = stx.stx_mtime.tv_nsec,
+		.st_ctim.tv_sec = stx.stx_ctime.tv_sec,
+		.st_ctim.tv_nsec = stx.stx_ctime.tv_nsec,
+	};
+	return 0;
+}
+
+static int fstatat_kstat(int fd, const char *restrict path, struct stat *restrict st, int flag)
 {
 	int ret;
 	struct kstat kst;
@@ -36,7 +91,7 @@ int fstatat(int fd, const char *restrict path, struct stat *restrict st, int fla
 #endif
 	else ret = __syscall(SYS_fstatat, fd, path, &kst, flag);
 
-	if (ret) return __syscall_ret(ret);
+	if (ret) return ret;
 
 	*st = (struct stat){
 		.st_dev = kst.st_dev,
@@ -60,4 +115,15 @@ int fstatat(int fd, const char *restrict path, struct stat *restrict st, int fla
 	return 0;
 }
 
+int fstatat(int fd, const char *restrict path, struct stat *restrict st, int flag)
+{
+	int ret;
+	if (sizeof((struct kstat){0}.st_atime_sec) < sizeof(time_t)) {
+		ret = fstatat_statx(fd, path, st, flag);
+		if (ret!=-ENOSYS) return __syscall_ret(ret);
+	}
+	ret = fstatat_kstat(fd, path, st, flag);
+	return __syscall_ret(ret);
+}
+
 weak_alias(fstatat, fstatat64);