files: Add ability to use read link in dump_one_reg_file

To be able to dump procfs namespace entries we will need to
analyze the link name first and then figure out if the file
referred indeed a procfs entry or it's plain regular file.

This means we read link early, and to escape double reading
of same link, once we read it we remember it in fd_parms
structure which we pass to dump_one_reg_file.

Still the dump_one_reg_file is not solely used from inside
of dump_one_file, but from a number of other places (fifo,
special files) where fd_parms is filled only partially
and fill_fd_params is not even called. Thus dump_one_reg_file
must be ready for such scenario and read the link name by own
if the caller has not provided one.

To achieve all this we do

 - extend fd_parms structure to have a reference on a new
   fd_link structure, thus if caller already read the link
   name it might assign the reference and call for dump_one_reg_file

 - tune up dump_one_reg_file to fill own copy of fd_link
   structure if the caller has not provied one

  [ xemul: Added const to fill_fdlink arg ]
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

files-reg.c

@@ -432,36 +432,37 @@ static int check_path_remap(char *rpath, int plen, const struct stat *ost, int l
 
 int dump_one_reg_file(int lfd, u32 id, const struct fd_parms *p)
 {
-	char rpath[PATH_MAX + 1] = ".", *path = rpath + 1;
-	int len, rfd;
+	struct fd_link _link, *link;
+	int rfd;
 
 	RegFileEntry rfe = REG_FILE_ENTRY__INIT;
 
-	len = read_fd_link(lfd, path, sizeof(rpath) - 1);
-	if (len < 0) {
-		pr_err("Can't read link\n");
-		return len;
-	}
+	if (!p->link) {
+		if (fill_fdlink(lfd, p, &_link))
+			return -1;
+		link = &_link;
+	} else
+		link = p->link;
 
 	pr_info("Dumping path for %d fd via self %d [%s]\n",
-			p->fd, lfd, path);
+			p->fd, lfd, &link->name[1]);
 
 	/*
 	 * The regular path we can handle should start with slash.
 	 */
-	if (path[0] != '/') {
-		pr_err("The path [%s] is not supported\n", path);
+	if (link->name[1] != '/') {
+		pr_err("The path [%s] is not supported\n", &link->name[1]);
 		return -1;
 	}
 
-	if (check_path_remap(rpath, len, &p->stat, lfd, id))
+	if (check_path_remap(link->name, link->len, &p->stat, lfd, id))
 		return -1;
 
 	rfe.id		= id;
 	rfe.flags	= p->flags;
 	rfe.pos		= p->pos;
 	rfe.fown	= (FownEntry *)&p->fown;
-	rfe.name	= path;
+	rfe.name	= &link->name[1];
 
 	rfd = fdset_fd(glob_fdset, CR_FD_REG_FILES);
 

files.c

@@ -135,6 +135,22 @@ int do_dump_gen_file(struct fd_parms *p, int lfd,
 	return pb_write_one(fdinfo, &e, PB_FDINFO);
 }
 
+int fill_fdlink(int lfd, const struct fd_parms *p, struct fd_link *link)
+{
+	size_t len;
+
+	link->name[0] = '.';
+
+	len = read_fd_link(lfd, &link->name[1], sizeof(link->name) - 1);
+	if (len < 0) {
+		pr_err("Can't read link for pid %d fd %d\n", p->pid, p->fd);
+		return -1;
+	}
+
+	link->len = len + 1;
+	return 0;
+}
+
 static int fill_fd_params(struct parasite_ctl *ctl, int fd, int lfd,
 				struct fd_opts *opts, struct fd_parms *p)
 {
@@ -244,8 +260,15 @@ static int dump_one_file(struct parasite_ctl *ctl, int fd, int lfd, struct fd_op
 			return dump_unsupp_fd(&p);
 	}
 
-	if (S_ISREG(p.stat.st_mode) || S_ISDIR(p.stat.st_mode))
+	if (S_ISREG(p.stat.st_mode) || S_ISDIR(p.stat.st_mode)) {
+		struct fd_link link;
+
+		if (fill_fdlink(lfd, &p, &link))
+			return -1;
+
+		p.link = &link;
 		return dump_reg_file(&p, lfd, fdinfo);
+	}
 
 	if (S_ISFIFO(p.stat.st_mode)) {
 		if (statfs.f_type == PIPEFS_MAGIC)

include/files.h

@@ -17,6 +17,11 @@ struct cr_fdset;
 struct rst_info;
 struct parasite_ctl;
 
+struct fd_link {
+	char		name[PATH_MAX + 1];
+	size_t		len;
+};
+
 struct fd_parms {
 	int		fd;
 	unsigned long	pos;
@@ -25,6 +30,7 @@ struct fd_parms {
 	struct stat	stat;
 	pid_t		pid;
 	FownEntry	fown;
+	struct fd_link	*link;
 
 	struct parasite_ctl *ctl;
 };
@@ -33,8 +39,11 @@ struct fd_parms {
 (struct fd_parms) {			\
 	.fd	= FD_DESC_INVALID,	\
 	.fown	= FOWN_ENTRY__INIT,	\
+	.link	= NULL,			\
 }
 
+extern int fill_fdlink(int lfd, const struct fd_parms *p, struct fd_link *link);
+
 struct file_desc;
 
 struct fdinfo_list_entry {