This bit will never be supported in the uapi. The purpose of this flag
bit is to allow userspace to distinguish an old kernel that does not
clear unknown sa_flags bits from a kernel that supports every flag bit.

In other words, if userspace finds that this bit remains set in
oldact.sa_flags, it means that the kernel cannot be trusted to have
cleared unknown flag bits from sa_flags, so no assumptions about flag
bit support can be made.

Signed-off-by: Peter Collingbourne <redacted>
---
View this change in Gerrit: https://linux-review.googlesource.com/q/Ic2501ad150a3a79c1cf27fb8c99be342e9dffbcb

 include/uapi/asm-generic/signal-defs.h | 7 +++++++
 kernel/signal.c                        | 6 ++++++
 2 files changed, 13 insertions(+)

diff --git a/include/uapi/asm-generic/signal-defs.h b/include/uapi/asm-generic/signal-defs.h
index 91000b6b97e0..c30a9c1a77b2 100644
--- a/include/uapi/asm-generic/signal-defs.h
+++ b/include/uapi/asm-generic/signal-defs.h

@@ -13,6 +13,12 @@
  * SA_RESETHAND clears the handler when the signal is delivered.
  * SA_NOCLDWAIT flag on SIGCHLD to inhibit zombies.
  * SA_NODEFER prevents the current signal from being masked in the handler.
+ * SA_UNSUPPORTED is a flag bit that will never be supported. Kernels from
+ * before the introduction of SA_UNSUPPORTED did not clear unknown bits from
+ * sa_flags when read using the oldact argument to sigaction and rt_sigaction,
+ * so this bit allows flag bit support to be detected from userspace while
+ * allowing an old kernel to be distinguished from a kernel that supports every
+ * flag bit.
  *
  * SA_ONESHOT and SA_NOMASK are the historical Linux names for the Single
  * Unix names RESETHAND and NODEFER respectively.

@@ -42,6 +48,7 @@
  * The following bits are used in architecture-specific SA_* definitions and
  * should be avoided for new generic flags: 3, 4, 5, 6, 7, 8, 9, 16, 24, 25, 26.
  */
+#define SA_UNSUPPORTED	0x00000400