Simplify *chown* syscalls (#16393)

This avoids musl calling the chown/lchown syscalls, and using fchown/fchownat
only. This leaves less work on the syscalls layer, so it will help WasmFS.

Also simplify the logic in fchown. For some reason musl tries chown on a path
of /proc/fd/NNN if fchown NNN fails. It seems like both should fail at equal times?
At least in our implementation of the filesystem they do, unless I'm missing
something.

Also add support for the nofollow flag to fchownat, which is now necessary.

Author: kripken

src/library_syscall.js

@@ -744,20 +744,10 @@ var SyscallsLibrary = {
     var stream = SYSCALLS.getStreamFromFD(fd);
     return SYSCALLS.doStat(FS.stat, stream.path, buf);
   },
-  __syscall_lchown32: function(path, owner, group) {
-    path = SYSCALLS.getStr(path);
-    FS.chown(path, owner, group); // XXX we ignore the 'l' aspect, and do the same as chown
-    return 0;
-  },
   __syscall_fchown32: function(fd, owner, group) {
     FS.fchown(fd, owner, group);
     return 0;
   },
-  __syscall_chown32: function(path, owner, group) {
-    path = SYSCALLS.getStr(path);
-    FS.chown(path, owner, group);
-    return 0;
-  },
   __syscall_getdents64: function(fd, dirp, count) {
     var stream = SYSCALLS.getStreamFromFD(fd)
     if (!stream.getdents) {
@@ -923,11 +913,13 @@ var SyscallsLibrary = {
     err('warning: untested syscall');
 #endif
     path = SYSCALLS.getStr(path);
+    var nofollow = flags & {{{ cDefine('AT_SYMLINK_NOFOLLOW') }}};
+    flags = flags & (~{{{ cDefine('AT_SYMLINK_NOFOLLOW') }}});
 #if ASSERTIONS
     assert(flags === 0);
 #endif
     path = SYSCALLS.calculateAt(dirfd, path);
-    FS.chown(path, owner, group);
+    (nofollow ? FS.lchown : FS.chown)(path, owner, group);
     return 0;
   },
   __syscall_newfstatat: function(dirfd, path, buf, flags) {

system/lib/libc/musl/arch/emscripten/bits/syscall.h

@@ -49,7 +49,6 @@
 #define SYS_stat64		__syscall_stat64
 #define SYS_lstat64		__syscall_lstat64
 #define SYS_fstat64		__syscall_fstat64
-#define SYS_lchown32		__syscall_lchown32
 #define SYS_getuid32		__syscall_getuid32
 #define SYS_getgid32		__syscall_getgid32
 #define SYS_geteuid32		__syscall_geteuid32
@@ -58,7 +57,6 @@
 #define SYS_fchown32		__syscall_fchown32
 #define SYS_getresuid32	__syscall_getresuid32
 #define SYS_getresgid32	__syscall_getresgid32
-#define SYS_chown32		__syscall_chown32
 #define SYS_mincore		__syscall_mincore
 #define SYS_madvise		__syscall_madvise
 #define SYS_getdents64		__syscall_getdents64

system/lib/libc/musl/arch/emscripten/syscall_arch.h

@@ -65,7 +65,6 @@ long __syscall_ftruncate64(long fd, long low, long high);
 long __syscall_stat64(long path, long buf);
 long __syscall_lstat64(long path, long buf);
 long __syscall_fstat64(long fd, long buf);
-long __syscall_lchown32(long path, long owner, long group);
 long __syscall_getuid32(void);
 long __syscall_getgid32(void);
 long __syscall_geteuid32(void);
@@ -78,7 +77,6 @@ long __syscall_setresuid32(long ruid, long euid, long suid);
 long __syscall_getresuid32(long ruid, long euid, long suid);
 long __syscall_setresgid32(long rgid, long egid, long sgid);
 long __syscall_getresgid32(long rgid, long egid, long sgid);
-long __syscall_chown32(long path, long owner, long group);
 long __syscall_setuid32(long uid);
 long __syscall_setgid32(long uid);
 long __syscall_mincore(long addr, long length, long vec);

system/lib/libc/musl/src/internal/syscall.h

@@ -128,7 +128,6 @@ static inline long __alt_socketcall(int sys, int sock, int cp, long a, long b, l
 #undef SYS_setgid
 #undef SYS_setfsuid
 #undef SYS_setfsgid
-#define SYS_lchown SYS_lchown32
 #define SYS_getuid SYS_getuid32
 #define SYS_getgid SYS_getgid32
 #define SYS_geteuid SYS_geteuid32
@@ -142,7 +141,6 @@ static inline long __alt_socketcall(int sys, int sock, int cp, long a, long b, l
 #define SYS_getresuid SYS_getresuid32
 #define SYS_setresgid SYS_setresgid32
 #define SYS_getresgid SYS_getresgid32
-#define SYS_chown SYS_chown32
 #define SYS_setuid SYS_setuid32
 #define SYS_setgid SYS_setgid32
 #define SYS_setfsuid SYS_setfsuid32

system/lib/libc/musl/src/unistd/fchown.c

@@ -10,12 +10,12 @@ int fchown(int fd, uid_t uid, gid_t gid)
 {
 	int ret = __syscall(SYS_fchown, fd, uid, gid);
 #if __EMSCRIPTEN__
-	if (ret != -EBADF || !__wasi_fd_is_valid(fd))
-		return __syscall_ret(ret);
+	// We can't continue onwards to try the /proc/fd/NNN approach that musl does,
+	// as we don't support that much of POSIX.
+	return __syscall_ret(ret);
 #else
 	if (ret != -EBADF || __syscall(SYS_fcntl, fd, F_GETFD) < 0)
 		return __syscall_ret(ret);
-#endif
 
 	char buf[15+3*sizeof(int)];
 	__procfdname(buf, fd);
@@ -24,5 +24,5 @@ int fchown(int fd, uid_t uid, gid_t gid)
 #else
 	return syscall(SYS_fchownat, AT_FDCWD, buf, uid, gid, 0);
 #endif
-
+#endif // EMSCRIPTEN
 }