On Thu, Jan 8, 2015 at 8:27 PM, Stéphane Graber <stgra...@ubuntu.com> wrote:
> This adds a new detect_userns function in virt.c which will check
> whether systemd is running in the host user namespace (single map of all
> available uids and gids) or is using a uid/gid map.
>
> The check makes sure that uid_map and gid_map are both exactly equal to
> the default host map (assuming 32bit uid_t) for a process running in the
> host namespace.
> ---
> src/shared/virt.c | 22 ++++++++++++++++++++++
> src/shared/virt.h | 1 +
> 2 files changed, 23 insertions(+)
>
> diff --git a/src/shared/virt.c b/src/shared/virt.c
> index f10baab..3d94e1f 100644
> --- a/src/shared/virt.c
> +++ b/src/shared/virt.c
> @@ -363,3 +363,25 @@ int detect_virtualization(const char **id) {
>
> return VIRTUALIZATION_NONE;
> }
> +
> +/* Detect whether we run in a uid/gid shifted namespace */
> +int detect_userns(void) {
> + int r;
> + static const char host_id_map[] = " 0 0 4294967295";
> + char *uid_map = NULL;
> + char *gid_map = NULL;
> +
> + /* Check if we are uid-shifted */
> + r = read_one_line_file("/proc/self/uid_map", &uid_map);
> + if (r == 0 && !streq(uid_map, host_id_map))
> + return 1;
> +
> + /* Check if we are gid-shifted */
> + r = read_one_line_file("/proc/self/gid_map", &gid_map);
> + if (r == 0 && !streq(gid_map, host_id_map))
Minor nit: would be nicer to parse these strings into numbers rather
than rely on the whitespace never changing, no? Also, would be a bit
nicer not to use the magic number 4294967295 but some #defined
constant instead.
> + return 1;
> +
> + /* If both uid_map and gid_map don't exist or if they both match
> + * the full uid/gid range, then we're not inside a user namespace */
Hm, this is not necessarily true is it? In my naive test, it works
just fine to set up a usernamespace with the identity mapping.
Moreover, this appears to be functionally different from the initial
user namespaces (somewhat counter-intuitively I might add), so
treating the identity mapping as 'no user namespace' is probably not
the right thing to do.
Is there no better way to test for this?
Cheers,
Tom
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