Re: [PATCH v9 20/39] arm64/gcs: Ensure that new threads have a GCS

diff --git a/arch/arm64/mm/gcs.c b/arch/arm64/mm/gcs.c
index b0a67efc522b..4a3ce8e3bdfb 100644
--- a/arch/arm64/mm/gcs.c
+++ b/arch/arm64/mm/gcs.c

@@ -8,6 +8,139 @@
 #include <asm/cpufeature.h>
 #include <asm/page.h>
 
+static unsigned long alloc_gcs(unsigned long addr, unsigned long size,
+			       unsigned long token_offset, bool set_res_tok)

+{
+	int flags = MAP_ANONYMOUS | MAP_PRIVATE;
+	struct mm_struct *mm = current->mm;
+	unsigned long mapped_addr, unused;
+
+	if (addr)
+		flags |= MAP_FIXED_NOREPLACE;
+
+	mmap_write_lock(mm);
+	mapped_addr = do_mmap(NULL, addr, size, PROT_READ | PROT_WRITE, flags,
+			      VM_SHADOW_STACK, 0, &unused, NULL);
+	mmap_write_unlock(mm);
+
+	return mapped_addr;
+}
+
+static unsigned long gcs_size(unsigned long size)
+{
+	if (size)
+		return PAGE_ALIGN(size);
+
+	/* Allocate RLIMIT_STACK/2 with limits of PAGE_SIZE..2G */
+	size = PAGE_ALIGN(min_t(unsigned long long,
+				rlimit(RLIMIT_STACK) / 2, SZ_2G));
+	return max(PAGE_SIZE, size);
+}
+
+static bool gcs_consume_token(struct mm_struct *mm, unsigned long user_addr)
+{
+	u64 expected = GCS_CAP(user_addr);
+	u64 val;
+	int ret;
+
+	/* This should really be an atomic cpmxchg.  It is not. */

+	ret = access_remote_vm(mm, user_addr, &val, sizeof(val),
+			       FOLL_FORCE);
+	if (ret != sizeof(val))
+		return false;
+
+	if (val != expected)
+		return false;
+
+	val = 0;
+	ret = access_remote_vm(mm, user_addr, &val, sizeof(val),
+			       FOLL_FORCE | FOLL_WRITE);
+	if (ret != sizeof(val))
+		return false;
+
+	return true;
+}