[RFC PATCH v2 28/28] arm64/sve: signal: Include SVE when computing AT_MINSIGSTKSZ

[Dave Martin]

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 at arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
Cc: Alex Bennée <alex.bennee at linaro.org>

---

Changes since v1
----------------

Changes related to Ard Biesheuvel's comments:

* Fix unbalanced ifelse bracing to conform to the kernel coding style.

Requested by Alex Bennée:

* Thin out BUG_ON()s:
Redundant BUG_ON()s and ones that just check invariants are removed.
Important sanity-checks are migrated to WARN_ON()s, with some
minimal best-effort patch-up code.
---
 arch/arm64/kernel/signal.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 7a3d6d2..c5ae575 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -594,8 +594,18 @@ 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;
+
+			/* Fail safe if something wasn't initialised */
+			if (WARN_ON(!sve_vl_valid(vl)))
+				vl = SVE_VL_MIN;
+
+			vq = sve_vq_from_vl(vl);
+		}
 
 		err = sigframe_alloc(user, &user->sve_offset,
 				     SVE_SIG_CONTEXT_SIZE(vq));
-- 
2.1.4