On Tue, Oct 05, 2021 at 04:37PM +0200, Peter Zijlstra wrote:

On Tue, Oct 05, 2021 at 12:59:05PM +0200, Marco Elver wrote:

quoted

Teach objtool to turn instrumentation required for memory barrier
modeling into nops in noinstr text.

The __tsan_func_entry/exit calls are still emitted by compilers even
with the __no_sanitize_thread attribute. The memory barrier
instrumentation will be inserted explicitly (without compiler help), and
thus needs to also explicitly be removed.

How is arm64 and others using kernel/entry + noinstr going to fix this?

ISTR they fully rely on the compilers not emitting instrumentation,
since they don't have objtool to fix up stray issues like this.

So this is where I'd like to hear if the approach of:

 | #if !defined(CONFIG_ARCH_WANTS_NO_INSTR) || defined(CONFIG_STACK_VALIDATION)
 | ...
 | #else
 | #define kcsan_noinstr noinstr
 | static __always_inline bool within_noinstr(unsigned long ip)
 | {
 | 	return (unsigned long)__noinstr_text_start <= ip &&
 | 	       ip < (unsigned long)__noinstr_text_end;
 | }
 | #endif

and then (using the !STACK_VALIDATION definitions)

 | kcsan_noinstr void instrumentation_may_appear_in_noinstr(void)
 | {
 | 	if (within_noinstr(_RET_IP_))
 | 		return;

works for the non-x86 arches that select ARCH_WANTS_NO_INSTR.

If it doesn't I can easily just remove kcsan_noinstr/within_noinstr, and
add a "depends on !ARCH_WANTS_NO_INSTR || STACK_VALIDATION" to the
KCSAN_WEAK_MEMORY option.

Looking at a previous discussion [1], however, I was under the
impression that this would work.

[1] https://lkml.kernel.org/r/CANpmjNMAZiW-Er=2QDgGP+_3hg1LOvPYcbfGSPMv=aR6MVTB-g@mail.gmail.com