don't call file_pos_write() if vfs_{read,write}{,v}() fails

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/read_write.c

@@ -477,7 +477,8 @@ SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)
 	if (f.file) {
 		loff_t pos = file_pos_read(f.file);
 		ret = vfs_read(f.file, buf, count, &pos);
-		file_pos_write(f.file, pos);
+		if (ret >= 0)
+			file_pos_write(f.file, pos);
 		fdput(f);
 	}
 	return ret;
@@ -492,7 +493,8 @@ SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,
 	if (f.file) {
 		loff_t pos = file_pos_read(f.file);
 		ret = vfs_write(f.file, buf, count, &pos);
-		file_pos_write(f.file, pos);
+		if (ret >= 0)
+			file_pos_write(f.file, pos);
 		fdput(f);
 	}
 
@@ -780,7 +782,8 @@ SYSCALL_DEFINE3(readv, unsigned long, fd, const struct iovec __user *, vec,
 	if (f.file) {
 		loff_t pos = file_pos_read(f.file);
 		ret = vfs_readv(f.file, vec, vlen, &pos);
-		file_pos_write(f.file, pos);
+		if (ret >= 0)
+			file_pos_write(f.file, pos);
 		fdput(f);
 	}
 
@@ -799,7 +802,8 @@ SYSCALL_DEFINE3(writev, unsigned long, fd, const struct iovec __user *, vec,
 	if (f.file) {
 		loff_t pos = file_pos_read(f.file);
 		ret = vfs_writev(f.file, vec, vlen, &pos);
-		file_pos_write(f.file, pos);
+		if (ret >= 0)
+			file_pos_write(f.file, pos);
 		fdput(f);
 	}
 
@@ -959,7 +963,8 @@ COMPAT_SYSCALL_DEFINE3(readv, unsigned long, fd,
 		return -EBADF;
 	pos = f.file->f_pos;
 	ret = compat_readv(f.file, vec, vlen, &pos);
-	f.file->f_pos = pos;
+	if (ret >= 0)
+		f.file->f_pos = pos;
 	fdput(f);
 	return ret;
 }
@@ -1025,7 +1030,8 @@ COMPAT_SYSCALL_DEFINE3(writev, unsigned long, fd,
 		return -EBADF;
 	pos = f.file->f_pos;
 	ret = compat_writev(f.file, vec, vlen, &pos);
-	f.file->f_pos = pos;
+	if (ret >= 0)
+		f.file->f_pos = pos;
 	fdput(f);
 	return ret;
 }