landlock: Various documentation improvements

* Fix some typos, incomplete or confusing phrases.
* Split paragraphs where appropriate.
* List the same error code multiple times,
  if it has multiple possible causes.
* Bring wording closer to the man page wording,
  which has undergone more thorough review
  (esp. for LANDLOCK_ACCESS_FS_WRITE_FILE).
* Small semantic clarifications
  * Call the ephemeral port range "ephemeral"
  * Clarify reasons for EFAULT in landlock_add_rule()
  * Clarify @rule_type doc for landlock_add_rule()

This is a collection of small fixes which I collected when preparing the
corresponding man pages [1].

Cc: Alejandro Colomar <alx@kernel.org>
Cc: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Link: https://lore.kernel.org/r/20240715155554.2791018-1-gnoack@google.com [1]
Signed-off-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20240715160328.2792835-2-gnoack@google.com
[mic: Add label to link, fix formatting spotted by make htmldocs,
synchronize userspace-api documentation's date]
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Documentation/userspace-api/landlock.rst

@@ -8,7 +8,7 @@ Landlock: unprivileged access control
 =====================================
 
 :Author: Mickaël Salaün
-:Date: April 2024
+:Date: July 2024
 
 The goal of Landlock is to enable to restrict ambient rights (e.g. global
 filesystem or network access) for a set of processes.  Because Landlock

include/uapi/linux/landlock.h

@@ -104,20 +104,21 @@ struct landlock_path_beneath_attr {
  */
 struct landlock_net_port_attr {
 	/**
-	 * @allowed_access: Bitmask of allowed access network for a port
+	 * @allowed_access: Bitmask of allowed network actions for a port
 	 * (cf. `Network flags`_).
 	 */
 	__u64 allowed_access;
 	/**
 	 * @port: Network port in host endianness.
 	 *
-	 * It should be noted that port 0 passed to :manpage:`bind(2)` will
-	 * bind to an available port from a specific port range. This can be
-	 * configured thanks to the ``/proc/sys/net/ipv4/ip_local_port_range``
-	 * sysctl (also used for IPv6). A Landlock rule with port 0 and the
-	 * ``LANDLOCK_ACCESS_NET_BIND_TCP`` right means that requesting to bind
-	 * on port 0 is allowed and it will automatically translate to binding
-	 * on the related port range.
+	 * It should be noted that port 0 passed to :manpage:`bind(2)` will bind
+	 * to an available port from the ephemeral port range.  This can be
+	 * configured with the ``/proc/sys/net/ipv4/ip_local_port_range`` sysctl
+	 * (also used for IPv6).
+	 *
+	 * A Landlock rule with port 0 and the ``LANDLOCK_ACCESS_NET_BIND_TCP``
+	 * right means that requesting to bind on port 0 is allowed and it will
+	 * automatically translate to binding on the related port range.
 	 */
 	__u64 port;
 };
@@ -138,10 +139,10 @@ struct landlock_net_port_attr {
  * The following access rights apply only to files:
  *
  * - %LANDLOCK_ACCESS_FS_EXECUTE: Execute a file.
- * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access. Note that
- *   you might additionally need the %LANDLOCK_ACCESS_FS_TRUNCATE right in order
- *   to overwrite files with :manpage:`open(2)` using ``O_TRUNC`` or
- *   :manpage:`creat(2)`.
+ * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access.  When
+ *   opening files for writing, you will often additionally need the
+ *   %LANDLOCK_ACCESS_FS_TRUNCATE right.  In many cases, these system calls
+ *   truncate existing files when overwriting them (e.g., :manpage:`creat(2)`).
  * - %LANDLOCK_ACCESS_FS_READ_FILE: Open a file with read access.
  * - %LANDLOCK_ACCESS_FS_TRUNCATE: Truncate a file with :manpage:`truncate(2)`,
  *   :manpage:`ftruncate(2)`, :manpage:`creat(2)`, or :manpage:`open(2)` with
@@ -263,7 +264,7 @@ struct landlock_net_port_attr {
  * These flags enable to restrict a sandboxed process to a set of network
  * actions. This is supported since the Landlock ABI version 4.
  *
- * TCP sockets with allowed actions:
+ * The following access rights apply to TCP port numbers:
  *
  * - %LANDLOCK_ACCESS_NET_BIND_TCP: Bind a TCP socket to a local port.
  * - %LANDLOCK_ACCESS_NET_CONNECT_TCP: Connect an active TCP socket to

security/landlock/syscalls.c

@@ -378,8 +378,7 @@ static int add_rule_net_port(struct landlock_ruleset *ruleset,
  *		with the new rule.
  * @rule_type: Identify the structure type pointed to by @rule_attr:
  *             %LANDLOCK_RULE_PATH_BENEATH or %LANDLOCK_RULE_NET_PORT.
- * @rule_attr: Pointer to a rule (only of type &struct
- *             landlock_path_beneath_attr for now).
+ * @rule_attr: Pointer to a rule (matching the @rule_type).
  * @flags: Must be 0.
  *
  * This system call enables to define a new rule and add it to an existing
@@ -390,18 +389,20 @@ static int add_rule_net_port(struct landlock_ruleset *ruleset,
  * - %EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;
  * - %EAFNOSUPPORT: @rule_type is %LANDLOCK_RULE_NET_PORT but TCP/IP is not
  *   supported by the running kernel;
- * - %EINVAL: @flags is not 0, or inconsistent access in the rule (i.e.
+ * - %EINVAL: @flags is not 0;
+ * - %EINVAL: The rule accesses are inconsistent (i.e.
  *   &landlock_path_beneath_attr.allowed_access or
- *   &landlock_net_port_attr.allowed_access is not a subset of the
- *   ruleset handled accesses), or &landlock_net_port_attr.port is
- *   greater than 65535;
- * - %ENOMSG: Empty accesses (e.g. &landlock_path_beneath_attr.allowed_access);
+ *   &landlock_net_port_attr.allowed_access is not a subset of the ruleset
+ *   handled accesses)
+ * - %EINVAL: &landlock_net_port_attr.port is greater than 65535;
+ * - %ENOMSG: Empty accesses (e.g. &landlock_path_beneath_attr.allowed_access is
+ *   0);
  * - %EBADF: @ruleset_fd is not a file descriptor for the current thread, or a
  *   member of @rule_attr is not a file descriptor as expected;
  * - %EBADFD: @ruleset_fd is not a ruleset file descriptor, or a member of
  *   @rule_attr is not the expected file descriptor type;
  * - %EPERM: @ruleset_fd has no write access to the underlying ruleset;
- * - %EFAULT: @rule_attr inconsistency.
+ * - %EFAULT: @rule_attr was not a valid address.
  */
 SYSCALL_DEFINE4(landlock_add_rule, const int, ruleset_fd,
 		const enum landlock_rule_type, rule_type,