Adjust to glibc __rseq_size semantic change

In commit 2e456ccf0c34a056e3ccafac4a0c7effef14d918 ("Linux: Make
__rseq_size useful for feature detection (bug 31965)") glibc 2.40
changed the meaning of __rseq_size slightly: it is now the size
of the active/feature area (20 bytes initially), and not the size
of the entire initially defined struct (32 bytes including padding).
The reason for the change is that the size including padding does not
allow detection of newly added features while previously unused
padding is consumed.

The prep_libc_rseq_info change in criu/cr-restore.c is not necessary
on kernels which have full ptrace support for obtaining rseq
information because the code is not used.  On older kernels, it is
a correctness fix because with size 20 (the new value), rseq
registeration would fail.

The two other changes are required to make rseq unregistration work
in tests.

criu/cr-restore.c

@@ -2618,7 +2618,15 @@ static void prep_libc_rseq_info(struct rst_rseq_param *rseq)
 	if (!kdat.has_ptrace_get_rseq_conf) {
 #if defined(__GLIBC__) && defined(RSEQ_SIG)
 		rseq->rseq_abi_pointer = encode_pointer(__criu_thread_pointer() + __rseq_offset);
+		/*
+		 * Current glibc reports the feature/active size in
+		 * __rseq_size, not the size passed to the kernel.
+		 * This could be 20, but older kernels expect 32 for
+		 * the size argument even if only 20 bytes are used.
+		 */
 		rseq->rseq_abi_size = __rseq_size;
+		if (rseq->rseq_abi_size < 32)
+			rseq->rseq_abi_size = 32;
 		rseq->signature = RSEQ_SIG;
 #else
 		rseq->rseq_abi_pointer = 0;

test/zdtm/static/rseq00.c

@@ -51,7 +51,10 @@ static inline void unregister_glibc_rseq(void)
 	rseq->cpu_id = RSEQ_CPU_ID_REGISTRATION_FAILED;
 
 	/* unregister rseq */
-	syscall(__NR_rseq, (void *)rseq, __rseq_size, 1, RSEQ_SIG);
+	unsigned int size = __rseq_size;
+	if (__rseq_size < 32)
+		size = 32;
+	syscall(__NR_rseq, (void *)rseq, size, 1, RSEQ_SIG);
 }
 #else
 static inline void unregister_glibc_rseq(void)
@@ -140,4 +143,4 @@ int main(int argc, char *argv[])
 	return 0;
 }
 
-#endif /* #if defined(__x86_64__) */
+#endif /* #if defined(__x86_64__) */

test/zdtm/transition/rseq01.c

@@ -33,7 +33,10 @@ static inline void *thread_pointer(void)
 static inline void unregister_old_rseq(void)
 {
 	/* unregister rseq */
-	syscall(__NR_rseq, (void *)((char *)thread_pointer() + __rseq_offset), __rseq_size, 1, RSEQ_SIG);
+	unsigned int size = __rseq_size;
+	if (__rseq_size < 32)
+		size = 32;
+	syscall(__NR_rseq, (void *)((char *)thread_pointer() + __rseq_offset), size, 1, RSEQ_SIG);
 }
 #else
 static inline void unregister_old_rseq(void)