Fix fchownat(2) system call ABI on x86.

This system call has five arguments, of which one is a 64-bit uid_t, and
another is a 64-bit gid_t, which means that 7 registers are needed. However,
x86 only has 5 registers available for system calls. Wrap the system call
with a structure like with mmap(2).

kernel/include/sortix/kernel/syscall.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011, 2012, 2013, 2014, 2015 Jonas 'Sortie' Termansen.
+ * Copyright (c) 2011, 2012, 2013, 2014, 2015, 2016 Jonas 'Sortie' Termansen.
  *
  * Permission to use, copy, modify, and distribute this software for any
  * purpose with or without fee is hereby granted, provided that the above
@@ -46,6 +46,9 @@
 
 namespace Sortix {
 
+#if defined(__i386__)
+struct fchownat_request;
+#endif
 struct mmap_request;
 
 int sys_accept4(int, void*, size_t*, int);
@@ -71,6 +74,9 @@ int sys_fchmod(int, mode_t);
 int sys_fchmodat(int, const char*, mode_t, int);
 int sys_fchown(int, uid_t, gid_t);
 int sys_fchownat(int, const char*, uid_t, gid_t, int);
+#if defined(__i386__)
+int sys_fchownat_wrapper(const struct fchownat_request*);
+#endif
 int sys_fchroot(int);
 int sys_fchrootat(int, const char*);
 int sys_fcntl(int, int, uintptr_t);

kernel/io.cpp

@@ -472,6 +472,26 @@ int sys_fchownat(int dirfd, const char* path, uid_t owner, gid_t group, int flag
 	return desc->chown(&ctx, owner, group);
 }
 
+#if defined(__i386__)
+struct fchownat_request /* duplicated in libc/unistd/fchownat.c */
+{
+	int dirfd;
+	const char* path;
+	uid_t owner;
+	gid_t group;
+	int flags;
+};
+
+int sys_fchownat_wrapper(const struct fchownat_request* user_request)
+{
+	struct fchownat_request request;
+	if ( !CopyFromUser(&request, user_request, sizeof(request)) )
+		return -1;
+	return sys_fchownat(request.dirfd, request.path, request.owner,
+	                    request.group, request.flags);
+}
+#endif
+
 int sys_fchmod(int fd, mode_t mode)
 {
 	Ref<Descriptor> desc = CurrentProcess()->GetDescriptor(fd);

kernel/syscall.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011, 2012, 2013, 2014, 2015 Jonas 'Sortie' Termansen.
+ * Copyright (c) 2011, 2012, 2013, 2014, 2015, 2016 Jonas 'Sortie' Termansen.
  *
  * Permission to use, copy, modify, and distribute this software for any
  * purpose with or without fee is hereby granted, provided that the above
@@ -96,7 +96,11 @@ void* syscall_list[SYSCALL_MAX_NUM + 1] =
 	[SYSCALL_MKDIRAT] = (void*) sys_mkdirat,
 	[SYSCALL_FCHDIR] = (void*) sys_fchdir,
 	[SYSCALL_TRUNCATEAT] = (void*) sys_truncateat,
+#if !defined(__i386__)
 	[SYSCALL_FCHOWNAT] = (void*) sys_fchownat,
+#else
+	[SYSCALL_FCHOWNAT] = (void*) sys_fchownat_wrapper,
+#endif
 	[SYSCALL_FCHOWN] = (void*) sys_fchown,
 	[SYSCALL_FCHMOD] = (void*) sys_fchmod,
 	[SYSCALL_FCHMODAT] = (void*) sys_fchmodat,

libc/unistd/fchownat.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012 Jonas 'Sortie' Termansen.
+ * Copyright (c) 2012, 2016 Jonas 'Sortie' Termansen.
  *
  * Permission to use, copy, modify, and distribute this software for any
  * purpose with or without fee is hereby granted, provided that the above
@@ -23,9 +23,35 @@
 #include <fcntl.h>
 #include <unistd.h>
 
+#if !defined(__i386__)
+
 DEFN_SYSCALL5(int, sys_fchownat, SYSCALL_FCHOWNAT, int, const char*, uid_t, gid_t, int);
 
 int fchownat(int dirfd, const char* path, uid_t owner, gid_t group, int flags)
 {
 	return sys_fchownat(dirfd, path, owner, group, flags);
 }
+
+#else
+
+// TODO: The i386 system call ABI doesn't have enough registers to do this with
+//       64-bit uid_t and gid_t.
+
+struct fchownat_request /* duplicated in kernel/io.cpp */
+{
+	int dirfd;
+	const char* path;
+	uid_t owner;
+	gid_t group;
+	int flags;
+};
+
+DEFN_SYSCALL1(int, sys_fchownat_wrapper, SYSCALL_FCHOWNAT, const struct fchownat_request*);
+
+int fchownat(int dirfd, const char* path, uid_t owner, gid_t group, int flags)
+{
+	struct fchownat_request request = { dirfd, path, owner, group, flags };
+	return sys_fchownat_wrapper(&request);
+}
+
+#endif