files: Support opened-and-unlinked files with non zero link count

These are not ghost, as they are still on fs, so we cannot take
them with us in the image. Neither we can easily find the other name
of that file. Sad :(

To make it work we linkat() the new name to that file using the
AT_EMPTY_PATH flag to link directly to the opened fd. If we could
openat() the fd's parent we would better do it, but we can't and
thus have to create the link name by explicit absolute path :(

This modifies the fs we're dumping, so I'll introduce one more cmd
line option for that soon.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

files-reg.c

 #include <stdlib.h>
 #include <unistd.h>
 #include <errno.h>
-
+#include <string.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 
 #include "crtools.h"
-
+#include "file-ids.h"
 #include "mount.h"
 #include "files.h"
 #include "image.h"
@@ -135,6 +135,32 @@ err:
 	return -1;
 }
 
+static int open_remap_linked(struct reg_file_info *rfi,
+		RemapFilePathEntry *rfe)
+{
+	struct file_remap *rm;
+	struct file_desc *rdesc;
+	struct reg_file_info *rrfi;
+
+	rdesc = find_file_desc_raw(FD_TYPES__REG, rfe->remap_id);
+	if (!rdesc) {
+		pr_err("Can't find target file %x\n", rfe->remap_id);
+		return -1;
+	}
+
+	rm = xmalloc(sizeof(*rm));
+	if (!rm)
+		return -1;
+
+	rrfi = container_of(rdesc, struct reg_file_info, d);
+	pr_info("Remapped %s -> %s\n", rfi->path, rrfi->path);
+
+	rm->path = rrfi->path;
+	rm->users = 1;
+	rfi->remap = rm;
+	return 0;
+}
+
 static int collect_remaps(void)
 {
 	int fd, ret = 0;
@@ -153,13 +179,6 @@ static int collect_remaps(void)
 			break;
 
 		ret = -1;
-
-		if (!(rfe->remap_id & REMAP_GHOST)) {
-			pr_err("Non ghost remap not supported @%#x\n",
-					rfe->orig_id);
-			goto tail;
-		}
-
 		fdesc = find_file_desc_raw(FD_TYPES__REG, rfe->orig_id);
 		if (fdesc == NULL) {
 			pr_err("Remap for non existing file %#x\n",
@@ -169,7 +188,11 @@ static int collect_remaps(void)
 
 		rfi = container_of(fdesc, struct reg_file_info, d);
 		pr_info("Configuring remap %#x -> %#x\n", rfi->rfe->id, rfe->remap_id);
-		ret = open_remap_ghost(rfi, rfe);
+
+		if (rfe->remap_id & REMAP_GHOST)
+			ret = open_remap_ghost(rfi, rfe);
+		else
+			ret = open_remap_linked(rfi, rfe);
 tail:
 		remap_file_path_entry__free_unpacked(rfe, NULL);
 		if (ret)
@@ -256,6 +279,60 @@ dump_entry:
 			&rpe, PB_REMAP_FPATH);
 }
 
+static int create_link_remap(char *path, int len, int lfd, u32 *idp)
+{
+	char link_name[PATH_MAX], *tmp;
+	RegFileEntry rfe = REG_FILE_ENTRY__INIT;
+	FownEntry fwn = FOWN_ENTRY__INIT;
+
+	/*
+	 * Linked remapping -- we create a hard link on a removed file
+	 * in the directory original file used to sit.
+	 *
+	 * Bad news is than we can't easily open lfd's parent dir. Thus
+	 * we have to just generate an absolute path and use it. The linkat
+	 * will fail if we chose the bad one.
+	 */
+
+	memcpy(link_name, path, len);
+	tmp = link_name + len;
+	while (*tmp != '/') {
+		BUG_ON(tmp == link_name);
+		tmp--;
+	}
+
+	rfe.id = *idp 	= fd_id_generate_special();
+	rfe.flags	= 0;
+	rfe.pos		= 0;
+	rfe.fown	= &fwn;
+	rfe.name	= link_name;
+
+	/* Any 'unique' name works here actually. Remap works by reg-file ids. */
+	sprintf(tmp + 1, "link_remap.%d", rfe.id);
+
+	if (linkat(lfd, "", 0, link_name, AT_EMPTY_PATH) < 0) {
+		pr_perror("Can't link remap to %s", path);
+		return -1;
+	}
+
+	return pb_write_one(fdset_fd(glob_fdset, CR_FD_REG_FILES), &rfe, PB_REG_FILES);
+}
+
+static int dump_linked_remap(char *path, int len, const struct stat *ost, int lfd, u32 id)
+{
+	u32 lid;
+	RemapFilePathEntry rpe = REMAP_FILE_PATH_ENTRY__INIT;
+
+	if (create_link_remap(path, len, lfd, &lid))
+		return -1;
+
+	rpe.orig_id = id;
+	rpe.remap_id = lid;
+
+	return pb_write_one(fdset_fd(glob_fdset, CR_FD_REMAP_FPATH),
+			&rpe, PB_REMAP_FPATH);
+}
+
 static int check_path_remap(char *rpath, int plen, const struct stat *ost, int lfd, u32 id)
 {
 	int ret;
@@ -278,6 +355,10 @@ static int check_path_remap(char *rpath, int plen, const struct stat *ost, int l
 		 * uning linkat with AT_EMPTY_PATH flag and remap it to this
 		 * name.
 		 */
+
+		if (errno == ENOENT)
+			return dump_linked_remap(rpath + 1, plen - 1, ost, lfd, id);
+
 		pr_perror("Can't stat path");
 		return -1;
 	}

include/files.h

@@ -91,4 +91,8 @@ extern int get_filemap_fd(int pid, VmaEntry *vma_entry);
 extern int prepare_fs(int pid);
 extern int set_fd_flags(int fd, int flags);
 
+#ifndef AT_EMPTY_PATH
+#define AT_EMPTY_PATH 0x1000
+#endif
+
 #endif /* FILES_H_ */