arm64: Expose ESR_EL1 information to user when SIGSEGV/SIGBUS

This information is useful for instruction emulators to detect
read/write and access size without having to decode the faulting
instruction. The current patch exports it via sigcontext (struct
esr_context) and is only valid for SIGSEGV and SIGBUS.
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/uapi/asm/sigcontext.h

@@ -53,5 +53,12 @@ struct fpsimd_context {
 	__uint128_t vregs[32];
 };
 
+/* ESR_EL1 context */
+#define ESR_MAGIC	0x45535201
+
+struct esr_context {
+	struct _aarch64_ctx head;
+	u64 esr;
+};
 
 #endif /* _UAPI__ASM_SIGCONTEXT_H */

arch/arm64/kernel/signal.c

@@ -194,6 +194,16 @@ static int setup_sigframe(struct rt_sigframe __user *sf,
 		aux += sizeof(*fpsimd_ctx);
 	}
 
+	/* fault information, if valid */
+	if (current->thread.fault_code) {
+		struct esr_context *esr_ctx =
+			container_of(aux, struct esr_context, head);
+		__put_user_error(ESR_MAGIC, &esr_ctx->head.magic, err);
+		__put_user_error(sizeof(*esr_ctx), &esr_ctx->head.size, err);
+		__put_user_error(current->thread.fault_code, &esr_ctx->esr, err);
+		aux += sizeof(*esr_ctx);
+	}
+
 	/* set the "end" magic */
 	end = aux;
 	__put_user_error(0, &end->magic, err);