From: Will Deacon

Sent: 01 April 2021 09:31

...

quoted

+/*
+ * These macros must be used during syscall entry when interrupts and
+ * preempt are disabled, and after user registers have been stored to
+ * the stack.
+ */
+#define add_random_kstack_offset() do {					\
+	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
+				&randomize_kstack_offset)) {		\
+		u32 offset = __this_cpu_read(kstack_offset);		\
+		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
+		asm volatile("" : "=m"(*ptr) :: "memory");		\

Using the "m" constraint here is dangerous if you don't actually evaluate it
inside the asm. For example, if the compiler decides to generate an
addressing mode relative to the stack but with writeback (autodecrement), then
the stack pointer will be off by 8 bytes. Can you use "o" instead?

Is it allowed to use such a mode?
It would have to know that the "m" was substituted exactly once.
I think there are quite a few examples with 'strange' uses of memory
asm arguments.

However, in this case, isn't it enough to ensure the address is 'saved'?
So:
	asm volatile("" : "=r"(ptr) );
should be enough.

	David

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