[RFC PATCH v2 3/6] arm64: signal: factor out signal frame record allocation

[Dave Martin]

This patch factors out the allocator for signal frame optional
records into a separate function, to ensure consistency and
facilitate later expansion of the signal frame, so that the
allocation mechanism can be elaborated more easily in future
patches.

No overrun checking is currently done, because the allocation is in
user memory, and because the kernel statically does not attempt 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 table is added 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.

Signed-off-by: Dave Martin <Dave.Martin at arm.com>
---
 arch/arm64/include/uapi/asm/sigcontext.h | 19 ++++++++++++++
 arch/arm64/kernel/signal.c               | 43 ++++++++++++++++++++++++++------
 2 files changed, 55 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/include/uapi/asm/sigcontext.h b/arch/arm64/include/uapi/asm/sigcontext.h
index ee469be..1328a2c 100644
--- a/arch/arm64/include/uapi/asm/sigcontext.h
+++ b/arch/arm64/include/uapi/asm/sigcontext.h
@@ -34,6 +34,25 @@ struct sigcontext {
 };
 
 /*
+ * Allocation of __reserved[]:
+ * (Note: records do not necessarily occur in the order shown here.)
+ *
+ *	size		description
+ *
+ *	0x210		fpsimd_context
+ *	 0x10		esr_context
+ *	 0x10		terminator (null _aarch64_ctx)
+ *
+ *	0xdd0		(reserved for future allocation)
+ *
+ * New records that can exceed this space need to be opt-in for userspace, so
+ * that an expanded signal frame is not generated unexpectedly.  The mechanism
+ * for opting in will depend on the extension that generates each new record.
+ * The above table documents the maximum set and sizes of records than can be
+ * generated when userspace does not opt in for any such extension.
+ */
+
+/*
  * Header to be used at the beginning of structures extending the user
  * context. Such structures must be placed after the rt_sigframe on the stack
  * and be 16-byte aligned. The last structure must be a dummy one with the
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 6202753..47592af 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -75,6 +75,22 @@ 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)
 {
@@ -283,19 +299,32 @@ asmlinkage long sys_rt_sigreturn(struct pt_regs *regs)
 /* 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;
 }
 
 
-- 
2.1.4