[PATCH 3/3] Documentation/prctl/seccomp_filter.txt: document extensions

[Will Drewry <wad@chromium.org>]

(missed this on the first run)

Add an entry for the PR_SECCOMP_EXT entry point and the
only existing consumer, SECCOMP_EXT_ACT_TSYNC.

Signed-off-by: Will Drewry <wad@chromium.org>
---
 Documentation/prctl/seccomp_filter.txt |   28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt
index 1e469ef..b296701 100644
--- a/Documentation/prctl/seccomp_filter.txt
+++ b/Documentation/prctl/seccomp_filter.txt
@@ -166,10 +166,36 @@ The samples/seccomp/ directory contains both an x86-specific example
 and a more generic example of a higher level macro interface for BPF
 program generation.
 
+Extensions
+----------
+
+SECCOMP_MODE_FILTER supports an additional entry point for accessing
+extended behavior through prctl(PR_SECCOMP_EXT).  Only one extension
+exists today:
+
+SECCOMP_EXT_ACT_TSYNC:
+	If the calling task is running under SECCOMP_MODE_FILTER, it
+	may call prctl() to synchronize the seccomp filter of its
+	threads.  As seccomp behavior is per-task, any thread under
+	SECCOMP_MODE_STRICT will be unaffected, as will any thread
+	under SECCOMP_MODE_FILTER that does not have a filter that is
+	in the filter tree ancestry for the caller.  Any threads that
+	are in SECCOMP_MODE_NONE will be transitioned to
+	SECCOMP_MODE_FILTER if possible.
+
+	Usage:
+		prctl(PR_SECCOMP_EXT, SECCOMP_EXT_ACT_TSYNC, 0, 0);
+
+	If any threads cannot be transitioned, the call will return one
+	of the process ids.  All other threads will have been transitioned.
+	A return value of 0 indicates success. On a negative return value,
+	the errno will be populated appropriately:
+		EINVAL indicates invalid arguments.
+		EACCES indicates invalid seccomp mode.
 
 
 Adding architecture support
------------------------
+---------------------------
 
 See arch/Kconfig for the authoritative requirements.  In general, if an
 architecture supports both ptrace_event and seccomp, it will be able to
-- 
1.7.9.5