[PATCH v1 RFC Zisslpcfi 17/20] riscv ucontext: adding shadow stack pointer field in ucontext

[Deepak Gupta <debug@rivosinc.com>]

Shadow stack needs to be saved and restored on signal delivery and
signal return.

ucontext structure on riscv has existing large padding for possible
future extension of uc_sigmask. This patch steals XLEN/8 bytes from
padding to keep structure size and offset of existing member fields
same.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
 arch/riscv/include/uapi/asm/ucontext.h | 32 +++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/include/uapi/asm/ucontext.h b/arch/riscv/include/uapi/asm/ucontext.h
index 516bd0bb0da5..72303e5618a1 100644
--- a/arch/riscv/include/uapi/asm/ucontext.h
+++ b/arch/riscv/include/uapi/asm/ucontext.h
@@ -21,9 +21,12 @@ struct ucontext {
 	 * at the end of this structure and explicitly state it can be
 	 * expanded, so we didn't want to box ourselves in here.
 	 */
-	__u8		  __unused[1024 / 8 - sizeof(sigset_t)];
-	/*
-	 * We can't put uc_sigmask at the end of this structure because we need
+	__u8		  __unused[1024 / 8 - sizeof(sigset_t)
+#ifdef CONFIG_USER_SHADOW_STACK
+				   - sizeof(unsigned long)
+#endif
+				  ];
+	/* We can't put uc_sigmask at the end of this structure because we need
 	 * to be able to expand sigcontext in the future.  For example, the
 	 * vector ISA extension will almost certainly add ISA state.  We want
 	 * to ensure all user-visible ISA state can be saved and restored via a
@@ -31,7 +34,30 @@ struct ucontext {
 	 * infinite extensibility.  Since we know this will be extended and we
 	 * assume sigset_t won't be extended an extreme amount, we're
 	 * prioritizing this.
+	 */				  
+
+	/*
+	 * Zisslpcfi will need state in ucontext to save and restore across
+	 * makecontext/setcontext. Such one state is shadow stack pointer. We may need
+	 * to save label (of the target function) as well (but that's to be decided).
+	 * Stealing 8 (64bit) / 4 (32bit) bytes from padding (__unused) reserved
+	 * for expanding sigset_t. We could've expanded the size of ucontext. But
+	 * shadow stack is something which by default would be enabled via ELF.
+	 * ucontext expansion makes more sense for situations like vector where
+	 * app is willingly opting in to get special functionality. Opt-in allows
+	 * for enlightening in ucontext restore. Second reason is shadow stack
+	 * doesn't need a lot of state and only shadow stack pointer. Tax on
+	 * ecosystem due to a small size change (8 bytes) of ucontext is more than
+	 * simply keeping the size same and shoving the ss pointer in here. Please
+	 * note that shadow stack pointer is pointing to a shadow stack address.
+	 * Shadow stack address has shadow stack restore token using which shadow
+	 * stack should be restored.
+	 * Please note that we're keeping uc_ss_ptr at that this location so that
+	 * every other offsets are same and thus works for compatibility.
 	 */
+#ifdef CONFIG_USER_SHADOW_STACK
+	unsigned long uc_ss_ptr;
+#endif
 	struct sigcontext uc_mcontext;
 };
 
-- 
2.25.1