[CRIU] [PATCH 2/3] file: add a general approach for external files

[Andrew Vagin]

On Wed, Jan 27, 2016 at 04:40:44PM +0300, Pavel Emelyanov wrote:
> On 01/21/2016 05:07 AM, Andrey Vagin wrote:
> > From: Andrew Vagin <avagin at virtuozzo.com>
> > 
> > The idea is to call open_cb for /proc/self/fd/X,
> > where X is an externa file descriptor.

where X is an inherit file descriptor

> > 
> > This approach works for files and fifo.
> 
> Plz, describe how this is supposed to be used and how this correlates
> with the --inherit-fd one.

This patch is about inherit-fd.

The idea is that we use an inherit file descriptor to get a path to a
file. We open /proc/self/fd/X instead of using the inherit file
descriptor directly. In this case we don't need to worry about access
mode flags, what is significant in a case when we have a few users for
one file.

I know that this method works for files and fifos. You can find a test
in the third patch.

Thanks,
Andrew

> 
> > Signed-off-by: Andrew Vagin <avagin at virtuozzo.com>
> > ---
> >  crtools.c   |  2 ++
> >  files-reg.c | 29 +++++++++++++++++++++++++----
> >  2 files changed, 27 insertions(+), 4 deletions(-)
> > 
> > diff --git a/crtools.c b/crtools.c
> > index 039b7a0..fa27622 100644
> > --- a/crtools.c
> > +++ b/crtools.c
> > @@ -781,6 +781,7 @@ usage:
> >  "  --external RES        dump objects from this list as external resources:\n"
> >  "                        Formats of RES:\n"
> >  "                            tty[rdev:dev]\n"
> > +"                            files[mnt_id:inode]\n"
> >  "  --inherit-fd fd[<num>]:<existing>\n"
> >  "                        Inherit file descriptors. This allows to treat file descriptor\n"
> >  "                        <num> as being already opened via <existing> one and instead of\n"
> > @@ -788,6 +789,7 @@ usage:
> >  "                            tty[rdev:dev]\n"
> >  "                            pipe[inode]\n"
> >  "                            socket[inode]\n"
> > +"                            files[mnt_id:inode]\n"
> >  "\n"
> >  "* Logging:\n"
> >  "  -o|--log-file FILE    log file name\n"
> > diff --git a/files-reg.c b/files-reg.c
> > index ea66af0..ec74272 100644
> > --- a/files-reg.c
> > +++ b/files-reg.c
> > @@ -1021,6 +1021,7 @@ int dump_one_reg_file(int lfd, u32 id, const struct fd_parms *p)
> >  	struct fd_link _link, *link;
> >  	struct ns_id *nsid;
> >  	struct cr_img *rimg;
> > +	char ext_id[64];
> >  
> >  	RegFileEntry rfe = REG_FILE_ENTRY__INIT;
> >  
> > @@ -1031,6 +1032,14 @@ int dump_one_reg_file(int lfd, u32 id, const struct fd_parms *p)
> >  	} else
> >  		link = p->link;
> >  
> > +
> > +	snprintf(ext_id, sizeof(ext_id), "@file[%x:%"PRIx64"]", p->mnt_id, p->stat.st_ino);
> > +	if (external_lookup_id(ext_id + 1)) {
> > +		/* the first symbol will be cut on restore to get an relative path*/
> > +		rfe.name = xstrdup(ext_id);
> > +		goto ext;
> > +	}
> > +
> >  	nsid = lookup_nsid_by_mnt_id(p->mnt_id);
> >  	if (nsid == NULL) {
> >  		pr_err("Can't lookup mount=%d for fd=%d path=%s\n",
> > @@ -1056,12 +1065,12 @@ int dump_one_reg_file(int lfd, u32 id, const struct fd_parms *p)
> >  
> >  	if (check_path_remap(link, p, lfd, id, nsid))
> >  		return -1;
> > -
> > +	rfe.name	= &link->name[1];
> > +ext:
> >  	rfe.id		= id;
> >  	rfe.flags	= p->flags;
> >  	rfe.pos		= p->pos;
> >  	rfe.fown	= (FownEntry *)&p->fown;
> > -	rfe.name	= &link->name[1];
> >  
> >  	if (S_ISREG(p->stat.st_mode) && should_check_size(rfe.flags)) {
> >  		rfe.has_size = true;
> > @@ -1216,11 +1225,22 @@ int open_path(struct file_desc *d,
> >  	struct reg_file_info *rfi;
> >  	int tmp, mntns_root;
> >  	char *orig_path = NULL;
> > +	char path[PATH_MAX];
> >  
> >  	if (inherited_fd(d, &tmp))
> >  		return tmp;
> >  
> >  	rfi = container_of(d, struct reg_file_info, d);
> > +
> > +	tmp = inherit_fd_lookup_id(rfi->path);
> > +	if (tmp >= 0) {
> > +		mntns_root = open_pid_proc(PROC_SELF);
> > +		snprintf(path, sizeof(path), "fd/%d", tmp);
> > +		orig_path = rfi->path;
> > +		rfi->path = path;
> > +		goto ext;
> > +	}
> > +
> >  	if (rfi->remap) {
> >  		mutex_lock(ghost_file_mutex);
> >  		if (rfi->remap->is_dir) {
> > @@ -1262,6 +1282,7 @@ int open_path(struct file_desc *d,
> >  	}
> >  
> >  	mntns_root = mntns_get_root_by_mnt_id(rfi->rfe->mnt_id);
> > +ext:
> >  	tmp = open_cb(mntns_root, rfi, arg);
> >  	if (tmp < 0) {
> >  		pr_perror("Can't open file %s", rfi->path);
> > @@ -1303,10 +1324,10 @@ int open_path(struct file_desc *d,
> >  			unlinkat(mntns_root, rfi->remap->rpath, rfi->remap->is_dir ? AT_REMOVEDIR : 0);
> >  		}
> >  
> > -		if (orig_path)
> > -			rfi->path = orig_path;
> >  		mutex_unlock(ghost_file_mutex);
> >  	}
> > +	if (orig_path)
> > +		rfi->path = orig_path;
> >  
> >  	if (restore_fown(tmp, rfi->rfe->fown))
> >  		return -1;
> > 
>