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-rw-r--r--include/uapi/asm-generic/signal-defs.h7
-rw-r--r--kernel/signal.c6
2 files changed, 13 insertions, 0 deletions
diff --git a/include/uapi/asm-generic/signal-defs.h b/include/uapi/asm-generic/signal-defs.h
index 44f070982752..c790f67304ba 100644
--- a/include/uapi/asm-generic/signal-defs.h
+++ b/include/uapi/asm-generic/signal-defs.h
@@ -14,6 +14,12 @@
* SA_RESTART flag to get restarting signals (which were the default long ago)
* SA_NODEFER prevents the current signal from being masked in the handler.
* SA_RESETHAND clears the handler when the signal is delivered.
+ * 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.
@@ -34,6 +40,7 @@
/* 0x00000080 used on parisc */
/* 0x00000100 used on sparc */
/* 0x00000200 used on sparc */
+#define SA_UNSUPPORTED 0x00000400
/* 0x00010000 used on mips */
/* 0x01000000 used on x86 */
/* 0x02000000 used on x86 */
diff --git a/kernel/signal.c b/kernel/signal.c
index 8f5bd12ee41b..8f34819e80de 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3986,6 +3986,12 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
*oact = *k;
/*
+ * Make sure that we never accidentally claim to support SA_UNSUPPORTED,
+ * e.g. by having an architecture use the bit in their uapi.
+ */
+ BUILD_BUG_ON(UAPI_SA_FLAGS & SA_UNSUPPORTED);
+
+ /*
* Clear unknown flag bits in order to allow userspace to detect missing
* support for flag bits and to allow the kernel to use non-uapi bits
* internally.