arm64: signal: Report signal frame size to userspace via auxv

Stateful CPU architecture extensions may require the signal frame
to grow to a size that exceeds the arch's MINSIGSTKSZ #define.
However, changing this #define is an ABI break.

To allow userspace the option of determining the signal frame size
in a more forwards-compatible way, this patch adds a new auxv entry
tagged with AT_MINSIGSTKSZ, which provides the maximum signal frame
size that the process can observe during its lifetime.

If AT_MINSIGSTKSZ is absent from the aux vector, the caller can
assume that the MINSIGSTKSZ #define is sufficient.  This allows for
a consistent interface with older kernels that do not provide
AT_MINSIGSTKSZ.

The idea is that libc could expose this via sysconf() or some
similar mechanism.

There is deliberately no AT_SIGSTKSZ.  The kernel knows nothing
about userspace's own stack overheads and should not pretend to
know.

For arm64:

The primary motivation for this interface is the Scalable Vector
Extension, which can require at least 4KB or so of extra space
in the signal frame for the largest hardware implementations.

To determine the correct value, a "Christmas tree" mode (via the
add_all argument) is added to setup_sigframe_layout(), to simulate
addition of all possible records to the signal frame at maximum
possible size.

If this procedure goes wrong somehow, resulting in a stupidly large
frame layout and hence failure of sigframe_alloc() to allocate a
record to the frame, then this is indicative of a kernel bug.  In
this case, we WARN() and no attempt is made to populate
AT_MINSIGSTKSZ for userspace.

For arm64 SVE:

The SVE context block in the signal frame needs to be considered
too when computing the maximum possible signal frame size.

Because the size of this block depends on the vector length, this
patch computes the size based not on the thread's current vector
length but instead on the maximum possible vector length: this
determines the maximum size of SVE context block that can be
observed in any signal frame for the lifetime of the process.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 45 insertions, 7 deletions

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 154b7d30145d..e7da5dba7ba8 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -17,6 +17,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <linux/cache.h>
 #include <linux/compat.h>
 #include <linux/errno.h>
 #include <linux/kernel.h>
@@ -570,8 +571,15 @@ badframe:
 	return 0;
 }
 
-/* Determine the layout of optional records in the signal frame */
-static int setup_sigframe_layout(struct rt_sigframe_user_layout *user)
+/*
+ * Determine the layout of optional records in the signal frame
+ *
+ * add_all: if true, lays out the biggest possible signal frame for
+ *	this task; otherwise, generates a layout for the current state
+ *	of the task.
+ */
+static int setup_sigframe_layout(struct rt_sigframe_user_layout *user,
+				 bool add_all)
 {
 	int err;
 
@@ -581,7 +589,7 @@ static int setup_sigframe_layout(struct rt_sigframe_user_layout *user)
 		return err;
 
 	/* fault information, if valid */
-	if (current->thread.fault_code) {
+	if (add_all || current->thread.fault_code) {
 		err = sigframe_alloc(user, &user->esr_offset,
 				     sizeof(struct esr_context));
 		if (err)
@@ -591,8 +599,14 @@ static int setup_sigframe_layout(struct rt_sigframe_user_layout *user)
 	if (system_supports_sve()) {
 		unsigned int vq = 0;
 
-		if (test_thread_flag(TIF_SVE))
-			vq = sve_vq_from_vl(current->thread.sve_vl);
+		if (add_all || test_thread_flag(TIF_SVE)) {
+			int vl = sve_max_vl;
+
+			if (!add_all)
+				vl = current->thread.sve_vl;
+
+			vq = sve_vq_from_vl(vl);
+		}
 
 		err = sigframe_alloc(user, &user->sve_offset,
 				     SVE_SIG_CONTEXT_SIZE(vq));
@@ -603,7 +617,6 @@ static int setup_sigframe_layout(struct rt_sigframe_user_layout *user)
 	return sigframe_alloc_end(user);
 }
 
-
 static int setup_sigframe(struct rt_sigframe_user_layout *user,
 			  struct pt_regs *regs, sigset_t *set)
 {
@@ -701,7 +714,7 @@ static int get_sigframe(struct rt_sigframe_user_layout *user,
 	int err;
 
 	init_user_layout(user);
-	err = setup_sigframe_layout(user);
+	err = setup_sigframe_layout(user, false);
 	if (err)
 		return err;
 
@@ -936,3 +949,28 @@ asmlinkage void do_notify_resume(struct pt_regs *regs,
 		thread_flags = READ_ONCE(current_thread_info()->flags);
 	} while (thread_flags & _TIF_WORK_MASK);
 }
+
+unsigned long __ro_after_init signal_minsigstksz;
+
+/*
+ * Determine the stack space required for guaranteed signal devliery.
+ * This function is used to populate AT_MINSIGSTKSZ at process startup.
+ * cpufeatures setup is assumed to be complete.
+ */
+void __init minsigstksz_setup(void)
+{
+	struct rt_sigframe_user_layout user;
+
+	init_user_layout(&user);
+
+	/*
+	 * If this fails, SIGFRAME_MAXSZ needs to be enlarged.  It won't
+	 * be big enough, but it's our best guess:
+	 */
+	if (WARN_ON(setup_sigframe_layout(&user, true)))
+		return;
+
+	signal_minsigstksz = sigframe_size(&user) +
+		round_up(sizeof(struct frame_record), 16) +
+		16; /* max alignment padding */
+}