selftests/mm: Add new testcases for pkeys

Add a few new tests to exercise the signal handler flow, especially with
PKEY 0 disabled:

 - Verify that the SIGSEGV handler is invoked when pkey 0 is disabled.

 - Verify that a thread which disables PKEY 0 segfaults with PKUERR when
   accessing the stack.

 - Verify that the SIGSEGV handler that uses an alternate signal stack is
   correctly invoked when the thread disabled PKEY 0

 - Verify that the PKRU value set by the application is correctly restored
   upon return from signal handling.

 - Verify that sigreturn() is able to restore the altstack even if the
   thread had PKEY 0 disabled

[ Aruna: Adapted to upstream ]
[ tglx: Made it actually compile. Restored protection_keys compile. Added
  	useful info to the changelog instead of bare function names. ]

Signed-off-by: Keith Lucas <keith.lucas@oracle.com>
Signed-off-by: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240802061318.2140081-6-aruna.ramakrishna@oracle.com

Author: Keith Lucas

tools/testing/selftests/mm/Makefile

@@ -88,6 +88,7 @@ CAN_BUILD_X86_64 := $(shell ./../x86/check_cc.sh "$(CC)" ../x86/trivial_64bit_pr
 CAN_BUILD_WITH_NOPIE := $(shell ./../x86/check_cc.sh "$(CC)" ../x86/trivial_program.c -no-pie)
 
 VMTARGETS := protection_keys
+VMTARGETS += pkey_sighandler_tests
 BINARIES_32 := $(VMTARGETS:%=%_32)
 BINARIES_64 := $(VMTARGETS:%=%_64)
 

tools/testing/selftests/mm/pkey-helpers.h

@@ -79,7 +79,18 @@ extern void abort_hooks(void);
 	}					\
 } while (0)
 
-__attribute__((noinline)) int read_ptr(int *ptr);
+#define barrier() __asm__ __volatile__("": : :"memory")
+#ifndef noinline
+# define noinline __attribute__((noinline))
+#endif
+
+noinline int read_ptr(int *ptr)
+{
+	/* Keep GCC from optimizing this away somehow */
+	barrier();
+	return *ptr;
+}
+
 void expected_pkey_fault(int pkey);
 int sys_pkey_alloc(unsigned long flags, unsigned long init_val);
 int sys_pkey_free(unsigned long pkey);