Hi Günther,
On 4/14/23 17:59, Günther Noack wrote:
> Signed-off-by: Günther Noack <gnoack3000@xxxxxxxxx>
> ---
> man7/landlock.7 | 70 ++++++++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 66 insertions(+), 4 deletions(-)
>
> diff --git a/man7/landlock.7 b/man7/landlock.7
> index 24488465e..64bfa0752 100644
> --- a/man7/landlock.7
> +++ b/man7/landlock.7
> @@ -394,11 +394,14 @@ accessible through these system call families:
> Future Landlock evolutions will enable to restrict them.
> .SH EXAMPLES
> We first need to create the ruleset that will contain our rules.
> +.PP
> For this example,
> the ruleset will contain rules that only allow read actions,
> but write actions will be denied.
> The ruleset then needs to handle both of these kinds of actions.
> -See below for the description of filesystem actions.
> +See the
> +.B DESCRIPTION
> +section for the description of filesystem actions.
> .PP
> .in +4n
> .EX
> @@ -421,7 +424,62 @@ attr.handled_access_fs =
> LANDLOCK_ACCESS_FS_MAKE_SYM |
> LANDLOCK_ACCESS_FS_REFER |
> LANDLOCK_ACCESS_FS_TRUNCATE;
> +.EE
> +.in
> +.PP
> +To be compatible with older Linux versions,
> +we detect the available Landlock ABI version,
> +and only use the available subset of access rights:
> +.PP
> +.in +4n
> +.EX
> +/*
> + * Table of available file system access rights by ABI version,
> + * numbers hardcoded to keep the example short.
> + */
> +__u64 landlock_fs_access_rights[] = {
> + (1ULL << 13) \- 1, /* ABI v1 */
> + (1ULL << 14) \- 1, /* ABI v2: add "refer" */
> + (1ULL << 15) \- 1, /* ABI v3: add "truncate" */
> +};
> +
> +int abi = landlock_create_ruleset(NULL, 0,
> + LANDLOCK_CREATE_RULESET_VERSION);
> +if (abi <= 0) {
> + perror("Giving up \- No Landlock support");
Using perror(3) will already print "Operation not supported", since
errno is ENOTSUP. Maybe this string is redundant? How about the
following?
perror("landlock_create_ruleset"); // EOPNOTSUPP
BTW, now I checked that while in Linux ENOTSUP and EOPNOTSUPP are
equivalent, in POSIX the latter has a connotation that it's about
sockets. Should we document ENOTSUP in landlock_create_ruleset(2)
instead of EOPNOTSUPP?
> + exit(EXIT_FAILURE);
> +}
> +if (abi > 3)
> + abi = 3;
This makes the example a line shorter (see MIN(3)):
abi = MIN(abi, 3);
Cheers,
Alex
>
> +/* Only use the available rights in the ruleset. */
> +attr.handled_access_fs &= landlock_fs_access_rights[abi \- 1];
> +.EE
> +.in
> +.PP
> +The available access rights for each ABI version are listed in the
> +.B VERSIONS
> +section.
> +.PP
> +If our program needed to create hard links
> +or rename files between different directories
> +.RB ( LANDLOCK_ACCESS_FS_REFER ),
> +we would require the following change to the backwards compatibility logic:
> +Directory reparenting is not possible
> +in a process restricted with Landlock ABI version 1.
> +Therefore,
> +if the program needed to do file reparenting,
> +and if only Landlock ABI version 1 was available,
> +we could not restrict the process.
> +.PP
> +Now that the ruleset attributes are determined,
> +we create the Landlock ruleset
> +and acquire a file descriptor as a handle to it,
> +using
> +.BR landlock_create_ruleset (2):
> +.PP
> +.in +4n
> +.EX
> ruleset_fd = landlock_create_ruleset(&attr, sizeof(attr), 0);
> if (ruleset_fd == \-1) {
> perror("Failed to create a ruleset");
> @@ -430,9 +488,13 @@ if (ruleset_fd == \-1) {
> .EE
> .in
> .PP
> -We can now add a new rule to this ruleset thanks to the returned file
> -descriptor referring to this ruleset.
> -The rule will only allow reading the file hierarchy
> +We can now add a new rule to the ruleset through the ruleset's file descriptor.
> +The requested access rights must be a subset of the access rights
> +which were specified in
> +.I attr.handled_access_fs
> +at ruleset creation time.
> +.PP
> +In this example, the rule will only allow reading the file hierarchy
> .IR /usr .
> Without another rule, write actions would then be denied by the ruleset.
> To add
--
<http://www.alejandro-colomar.es/>
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