signal: Verify the alignment and size of siginfo_t

Update the static assertions about siginfo_t to also describe
it's alignment and size.

While investigating if it was possible to add a 64bit field into
siginfo_t[1] it became apparent that the alignment of siginfo_t
is as much a part of the ABI as the size of the structure.

If the alignment changes siginfo_t when embedded in another structure
can move to a different offset.  Which is not acceptable from an ABI
structure.

So document that fact and add static assertions to notify developers
if they change change the alignment by accident.

[1] https://lkml.kernel.org/r/YJEZdhe6JGFNYlum@elver.google.com
Acked-by: Marco Elver <elver@google.com>
v1: https://lkml.kernel.org/r/20210505141101.11519-4-ebiederm@xmission.co
Link: https://lkml.kernel.org/r/875yxaxmyl.fsf_-_@disp2133
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

arch/arm/kernel/signal.c

@@ -737,6 +737,8 @@ static_assert(NSIGBUS	== 5);
 static_assert(NSIGTRAP	== 6);
 static_assert(NSIGCHLD	== 6);
 static_assert(NSIGSYS	== 2);
+static_assert(sizeof(siginfo_t) == 128);
+static_assert(__alignof__(siginfo_t) == 4);
 static_assert(offsetof(siginfo_t, si_signo)	== 0x00);
 static_assert(offsetof(siginfo_t, si_errno)	== 0x04);
 static_assert(offsetof(siginfo_t, si_code)	== 0x08);

arch/arm64/kernel/signal.c

@@ -1011,6 +1011,8 @@ static_assert(NSIGBUS	== 5);
 static_assert(NSIGTRAP	== 6);
 static_assert(NSIGCHLD	== 6);
 static_assert(NSIGSYS	== 2);
+static_assert(sizeof(siginfo_t) == 128);
+static_assert(__alignof__(siginfo_t) == 8);
 static_assert(offsetof(siginfo_t, si_signo)	== 0x00);
 static_assert(offsetof(siginfo_t, si_errno)	== 0x04);
 static_assert(offsetof(siginfo_t, si_code)	== 0x08);

arch/arm64/kernel/signal32.c

@@ -469,6 +469,8 @@ static_assert(NSIGBUS	== 5);
 static_assert(NSIGTRAP	== 6);
 static_assert(NSIGCHLD	== 6);
 static_assert(NSIGSYS	== 2);
+static_assert(sizeof(compat_siginfo_t) == 128);
+static_assert(__alignof__(compat_siginfo_t) == 4);
 static_assert(offsetof(compat_siginfo_t, si_signo)	== 0x00);
 static_assert(offsetof(compat_siginfo_t, si_errno)	== 0x04);
 static_assert(offsetof(compat_siginfo_t, si_code)	== 0x08);

arch/sparc/kernel/signal32.c

@@ -757,6 +757,8 @@ static_assert(NSIGBUS	== 5);
 static_assert(NSIGTRAP	== 6);
 static_assert(NSIGCHLD	== 6);
 static_assert(NSIGSYS	== 2);
+static_assert(sizeof(compat_siginfo_t) == 128);
+static_assert(__alignof__(compat_siginfo_t) == 4);
 static_assert(offsetof(compat_siginfo_t, si_signo)	== 0x00);
 static_assert(offsetof(compat_siginfo_t, si_errno)	== 0x04);
 static_assert(offsetof(compat_siginfo_t, si_code)	== 0x08);

arch/sparc/kernel/signal_64.c

@@ -567,6 +567,8 @@ static_assert(NSIGBUS	== 5);
 static_assert(NSIGTRAP	== 6);
 static_assert(NSIGCHLD	== 6);
 static_assert(NSIGSYS	== 2);
+static_assert(sizeof(siginfo_t) == 128);
+static_assert(__alignof__(siginfo_t) == 8);
 static_assert(offsetof(siginfo_t, si_signo)	== 0x00);
 static_assert(offsetof(siginfo_t, si_errno)	== 0x04);
 static_assert(offsetof(siginfo_t, si_code)	== 0x08);

arch/x86/kernel/signal_compat.c

@@ -34,7 +34,13 @@ static inline void signal_compat_build_tests(void)
 	BUILD_BUG_ON(NSIGSYS  != 2);
 
 	/* This is part of the ABI and can never change in size: */
+	BUILD_BUG_ON(sizeof(siginfo_t) != 128);
 	BUILD_BUG_ON(sizeof(compat_siginfo_t) != 128);
+
+	/* This is a part of the ABI and can never change in alignment */
+	BUILD_BUG_ON(__alignof__(siginfo_t) != 8);
+	BUILD_BUG_ON(__alignof__(compat_siginfo_t) != 4);
+
 	/*
 	 * The offsets of all the (unioned) si_fields are fixed
 	 * in the ABI, of course.  Make sure none of them ever

include/uapi/asm-generic/siginfo.h

@@ -29,6 +29,11 @@ typedef union sigval {
 #define __ARCH_SI_ATTRIBUTES
 #endif
 
+/*
+ * Be careful when extending this union.  On 32bit siginfo_t is 32bit
+ * aligned.  Which means that a 64bit field or any other field that
+ * would increase the alignment of siginfo_t will break the ABI.
+ */
 union __sifields {
 	/* kill() */
 	struct {