[CRIU] [PATCH] mount: detect the newinstance option for devpts (v2)

[Pavel Emelyanov]

On 04/02/2014 11:12 AM, Andrey Vagin wrote:
> The devpts instance was mounted w/o the newinstance option if,
> the device number is equal to the root /dev/pts.
> 
> I think this condition is strong enough to not mount devpts in a
> temporary place.
> 
> v2: move the host.bla-bla-bla in kerndat.c
> Signed-off-by: Andrey Vagin <avagin at openvz.org>
> ---
>  include/fs-magic.h |  4 ++++
>  include/kerndat.h  |  3 +++
>  kerndat.c          | 27 +++++++++++++++++++++++++++
>  mount.c            | 42 ++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 76 insertions(+)
> 
> diff --git a/include/fs-magic.h b/include/fs-magic.h
> index d31a939..9db3ee9 100644
> --- a/include/fs-magic.h
> +++ b/include/fs-magic.h
> @@ -29,4 +29,8 @@
>  # define SOCKFS_MAGIC		0x534f434b
>  #endif
>  
> +#ifndef DEVPTS_SUPER_MAGIC
> +#define DEVPTS_SUPER_MAGIC	0x1cd1
> +#endif
> +
>  #endif /* __CR_FS_MAGIC_H__ */
> diff --git a/include/kerndat.h b/include/kerndat.h
> index 2b3856c..6b9947f 100644
> --- a/include/kerndat.h
> +++ b/include/kerndat.h
> @@ -21,4 +21,7 @@ extern int tcp_max_rshare;
>  extern int kern_last_cap;
>  extern u64 zero_page_pfn;
>  
> +struct stat;
> +extern struct stat *kerndat_get_devpts_stat(void);
> +
>  #endif /* __CR_KERNDAT_H__ */
> diff --git a/kerndat.c b/kerndat.c
> index 00eda2e..2ba1edb 100644
> --- a/kerndat.c
> +++ b/kerndat.c
> @@ -9,6 +9,7 @@
>  #include "log.h"
>  #include "bug.h"
>  #include "kerndat.h"
> +#include "fs-magic.h"
>  #include "mem.h"
>  #include "compiler.h"
>  #include "sysctl.h"
> @@ -52,6 +53,32 @@ static int kerndat_get_shmemdev(void)
>  	return 0;
>  }
>  
> +struct stat *kerndat_get_devpts_stat()
> +{
> +	static struct stat st = {};
> +	struct statfs fst;
> +
> +	if (st.st_dev != 0)
> +		return &st;
> +
> +	if (statfs("/dev/pts", &fst)) {
> +		pr_perror("Unable to statefs /dev/pts");
> +		return NULL;
> +	}
> +	if (fst.f_type != DEVPTS_SUPER_MAGIC) {
> +		pr_err("devpts isn't mount on the host\n");
> +		return NULL;
> +	}
> +
> +	/* The root /dev/pts is mounted w/o newinstance, isn't it? */
> +	if (stat("/dev/pts", &st)) {
> +		pr_perror("Unable to stat /dev/pts");
> +		return NULL;
> +	}
> +
> +	return &st;
> +}
> +
>  /*
>   * Check whether pagemap reports soft dirty bit. Kernel has
>   * this functionality under CONFIG_MEM_SOFT_DIRTY option.
> diff --git a/mount.c b/mount.c
> index f88ac52..f999c39 100644
> --- a/mount.c
> +++ b/mount.c
> @@ -24,6 +24,7 @@
>  #include "image.h"
>  #include "namespaces.h"
>  #include "protobuf.h"
> +#include "kerndat.h"
>  #include "protobuf/mnt.pb-c.h"
>  
>  /*
> @@ -593,6 +594,46 @@ out:
>  	return NULL;
>  }
>  
> +/* Is it mounted w or w/o the newinstance option */
> +static int devpts_dump(struct mount_info *pm)
> +{
> +	static const char newinstance[] = ",newinstance";
> +	struct stat *host_st;
> +	struct stat st;
> +	DIR *fdir = NULL;
> +	char *buf;
> +	int len;
> +
> +	host_st = kerndat_get_devpts_stat();
> +	if (host_st == NULL)
> +		return -1;
> +
> +	fdir = open_mountpoint(pm);
> +	if (fdir == NULL)
> +		return -1;
> +
> +	if (fstat(dirfd(fdir), &st)) {
> +		pr_perror("Unable to statfs %d:%s",
> +				pm->mnt_id, pm->mountpoint);
> +		close_mountpoint(fdir);
> +		return -1;
> +	}
> +	close_mountpoint(fdir);
> +
> +	if (host_st->st_dev == st.st_dev)
> +		return 0;
> +
> +	len = strlen(pm->options);
> +	buf = xrealloc(pm->options, len + sizeof(newinstance));
> +	if (buf == NULL)
> +		return -1;
> +	memcpy(buf, newinstance, sizeof(newinstance));

Are we OK to call mount() with options equals ",newinstance", i.e.
with a comma as the first character?