Re: [PATCH V4 3/8] namespaces: expose ns instance serial numbers in proc

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On Aug 20, 2014 8:12 PM, "Richard Guy Briggs" <rgb@xxxxxxxxxx> wrote:
>
> Expose the namespace instace serial numbers in the proc filesystem at
> /proc/<pid>/ns/<ns>_snum.  The link text gives the serial number in hex.

What's the use case?

I understand the utility of giving unique numbers to the audit code,
but I don't think this part is necessary for that, and I'd like to
understand what else will use this before committing to a duplicative
API like this.

Note that this API is thoroughly incompatible with CRIU.  If we do
this, someone will ask for a namespace number namespace, and that way
lies madness.

--Andy

>
> "snum" was chosen instead of "seq" for consistency with inum and there are a
> number of other uses of "seq" in the namespace code.
>
> Suggested-by: Serge E. Hallyn <serge@xxxxxxxxxx>
> Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
> ---
>  fs/proc/namespaces.c |   33 +++++++++++++++++++++++++--------
>  1 files changed, 25 insertions(+), 8 deletions(-)
>
> diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
> index 8902609..e953e0a 100644
> --- a/fs/proc/namespaces.c
> +++ b/fs/proc/namespaces.c
> @@ -47,12 +47,15 @@ static char *ns_dname(struct dentry *dentry, char *buffer, int buflen)
>         struct inode *inode = dentry->d_inode;
>         const struct proc_ns_operations *ns_ops = PROC_I(inode)->ns.ns_ops;
>
> -       return dynamic_dname(dentry, buffer, buflen, "%s:[%lu]",
> -               ns_ops->name, inode->i_ino);
> +       if (strstr(dentry->d_iname, "_snum"))
> +               return dynamic_dname(dentry, buffer, buflen, "%s_snum:[%llx]",
> +                       ns_ops->name, ns_ops->snum(PROC_I(inode)->ns.ns));
> +       else
> +               return dynamic_dname(dentry, buffer, buflen, "%s:[%lu]",
> +                       ns_ops->name, inode->i_ino);
>  }
>
> -const struct dentry_operations ns_dentry_operations =
> -{
> +const struct dentry_operations ns_dentry_operations = {
>         .d_delete       = always_delete_dentry,
>         .d_dname        = ns_dname,
>  };
> @@ -160,7 +163,10 @@ static int proc_ns_readlink(struct dentry *dentry, char __user *buffer, int bufl
>         if (!ns)
>                 goto out_put_task;
>
> -       snprintf(name, sizeof(name), "%s:[%u]", ns_ops->name, ns_ops->inum(ns));
> +       if (strstr(dentry->d_iname, "_snum"))
> +               snprintf(name, sizeof(name), "%s_snum:[%llx]", ns_ops->name, ns_ops->snum(ns));
> +       else
> +               snprintf(name, sizeof(name), "%s:[%u]", ns_ops->name, ns_ops->inum(ns));
>         res = readlink_copy(buffer, buflen, name);
>         ns_ops->put(ns);
>  out_put_task:
> @@ -210,16 +216,23 @@ static int proc_ns_dir_readdir(struct file *file, struct dir_context *ctx)
>
>         if (!dir_emit_dots(file, ctx))
>                 goto out;
> -       if (ctx->pos >= 2 + ARRAY_SIZE(ns_entries))
> +       if (ctx->pos >= 2 + 2 * ARRAY_SIZE(ns_entries))
>                 goto out;
>         entry = ns_entries + (ctx->pos - 2);
>         last = &ns_entries[ARRAY_SIZE(ns_entries) - 1];
>         while (entry <= last) {
>                 const struct proc_ns_operations *ops = *entry;
> +               char name[50];
> +
>                 if (!proc_fill_cache(file, ctx, ops->name, strlen(ops->name),
>                                      proc_ns_instantiate, task, ops))
>                         break;
>                 ctx->pos++;
> +               snprintf(name, sizeof(name), "%s_snum", ops->name);
> +               if (!proc_fill_cache(file, ctx, name, strlen(name),
> +                                    proc_ns_instantiate, task, ops))
> +                       break;
> +               ctx->pos++;
>                 entry++;
>         }
>  out:
> @@ -247,9 +260,13 @@ static struct dentry *proc_ns_dir_lookup(struct inode *dir,
>
>         last = &ns_entries[ARRAY_SIZE(ns_entries)];
>         for (entry = ns_entries; entry < last; entry++) {
> -               if (strlen((*entry)->name) != len)
> +               char name[50];
> +
> +               snprintf(name, sizeof(name), "%s_snum", (*entry)->name);
> +               if (strlen((*entry)->name) != len && strlen(name) != len)
>                         continue;
> -               if (!memcmp(dentry->d_name.name, (*entry)->name, len))
> +               if (!memcmp(dentry->d_name.name, (*entry)->name, len)
> +                   || !memcmp(dentry->d_name.name, name, len))
>                         break;
>         }
>         if (entry == last)
> --
> 1.7.1
>
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