On Fri, Sep 04, 2020 at 03:55:10AM +0100, Al Viro wrote:

On Thu, Sep 03, 2020 at 04:22:40PM +0200, Christoph Hellwig wrote:

quoted

diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
index c8a85b512796e1..94f7be4971ed04 100644
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S

@@ -35,10 +35,19 @@
 #include <asm/smap.h>
 #include <asm/export.h>
 
+#ifdef CONFIG_X86_5LEVEL
+#define LOAD_TASK_SIZE_MINUS_N(n) \
+	ALTERNATIVE "mov $((1 << 47) - 4096 - (n)),%rdx", \
+		    "mov $((1 << 56) - 4096 - (n)),%rdx", X86_FEATURE_LA57
+#else
+#define LOAD_TASK_SIZE_MINUS_N(n) \
+	mov $(TASK_SIZE_MAX - (n)),%_ASM_DX
+#endif

Wait a sec... how is that supposed to build with X86_5LEVEL?  Do you mean

#define LOAD_TASK_SIZE_MINUS_N(n) \
	ALTERNATIVE __stringify(mov $((1 << 47) - 4096 - (n)),%rdx), \
		    __stringify(mov $((1 << 56) - 4096 - (n)),%rdx), X86_FEATURE_LA57

there?

Don't ask me about the how, but it builds and works with X86_5LEVEL,
and the style is copied from elsewhere..