arm64: signal: factor out signal frame record allocation

This patch factors out the allocator for signal frame optional
records into a separate function, to ensure consistency and
facilitate later expansion.

No overrun checking is currently done, because the allocation is in
user memory and anyway the kernel never tries to allocate enough
space in the signal frame yet for an overrun to occur.  This
behaviour will be refined in future patches.

The approach taken in this patch to allocation of the terminator
record is not very clean: this will also be replaced in subsequent
patches.

For future extension, a comment is added in sigcontext.h
documenting the current static allocations in __reserved[].  This
will be important for determining under what circumstances
userspace may or may not see an expanded signal frame.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 36 insertions, 7 deletions

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index eaef530579f8..fa787e6ac7c2 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -79,6 +79,22 @@ static size_t sigframe_size(struct rt_sigframe_user_layout const *user)
 	return round_up(max(user->size, sizeof(struct rt_sigframe)), 16);
 }
 
+/*
+ * Allocate space for an optional record of <size> bytes in the user
+ * signal frame.  The offset from the signal frame base address to the
+ * allocated block is assigned to *offset.
+ */
+static int sigframe_alloc(struct rt_sigframe_user_layout *user,
+			  unsigned long *offset, size_t size)
+{
+	size_t padded_size = round_up(size, 16);
+
+	*offset = user->size;
+	user->size += padded_size;
+
+	return 0;
+}
+
 static void __user *apply_user_offset(
 	struct rt_sigframe_user_layout const *user, unsigned long offset)
 {
@@ -287,19 +303,32 @@ badframe:
 /* Determine the layout of optional records in the signal frame */
 static int setup_sigframe_layout(struct rt_sigframe_user_layout *user)
 {
-	user->fpsimd_offset = user->size;
-	user->size += round_up(sizeof(struct fpsimd_context), 16);
+	int err;
+
+	err = sigframe_alloc(user, &user->fpsimd_offset,
+			     sizeof(struct fpsimd_context));
+	if (err)
+		return err;
 
 	/* fault information, if valid */
 	if (current->thread.fault_code) {
-		user->esr_offset = user->size;
-		user->size += round_up(sizeof(struct esr_context), 16);
+		err = sigframe_alloc(user, &user->esr_offset,
+				     sizeof(struct esr_context));
+		if (err)
+			return err;
 	}
 
-	/* set the "end" magic */
-	user->end_offset = user->size;
+	/*
+	 * Allocate space for the terminator record.
+	 * HACK: here we undo the reservation of space for the end record.
+	 * This bodge should be replaced with a cleaner approach later on.
+	 */
+	user->limit = offsetof(struct rt_sigframe, uc.uc_mcontext.__reserved) +
+		sizeof(user->sigframe->uc.uc_mcontext.__reserved);
 
-	return 0;
+	err = sigframe_alloc(user, &user->end_offset,
+			     sizeof(struct _aarch64_ctx));
+	return err;
 }