[PATCH v2] Documentation: mm/memfd: vm.memfd_noexec

[jeffxu@chromium.org]

From: Jeff Xu <jeffxu@google.com>

Add documentation for sysctl vm.memfd_noexec

Link:https://lore.kernel.org/linux-mm/CABi2SkXUX_QqTQ10Yx9bBUGpN1wByOi_=gZU6WEy5a8MaQY3Jw@mail.gmail.com/T/
Reported-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Jeff Xu <jeffxu@google.com>
---
 Documentation/admin-guide/sysctl/vm.rst | 30 +++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index 45ba1f4dc004..621588041a9e 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -424,6 +424,36 @@ e.g., up to one or two maps per allocation.
 
 The default value is 65530.
 
+memfd_noexec:
+=============
+This pid namespaced sysctl controls memfd_create().
+
+The new MFD_NOEXEC_SEAL and MFD_EXEC flags of memfd_create() allows
+application to set executable bit at creation time.
+
+When MFD_NOEXEC_SEAL is set, memfd is created without executable bit
+(mode:0666), and sealed with F_SEAL_EXEC, so it can't be chmod to
+be executable (mode: 0777) after creation.
+
+when MFD_EXEC flag is set, memfd is created with executable bit
+(mode:0777), this is the same as the old behavior of memfd_create.
+
+The new pid namespaced sysctl vm.memfd_noexec has 3 values:
+0: memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like
+        MFD_EXEC was set.
+1: memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like
+        MFD_NOEXEC_SEAL was set.
+2: memfd_create() without MFD_NOEXEC_SEAL will be rejected.
+
+The default value is 0.
+
+Once set, it can't be downgraded at runtime, i.e. 2=>1, 1=>0
+are denied.
+
+This is pid namespaced sysctl, child processes inherit the parent
+process's memfd_noexec at the time of fork. Changes to the parent
+process after fork are not automatically propagated to the child
+process.
 
 memory_failure_early_kill:
 ==========================
-- 
2.41.0.255.g8b1d071c50-goog