Re: [PATCH v2] gcc-plugins/stackleak: Use noinstr in favor of notrace

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Feb 03, 2022 at 12:17:54PM -0800, Kees Cook wrote:
> While the stackleak plugin was already using notrace, objtool is now a
> bit more picky. Update the notrace uses to noinstr. Silences the
> following objtool warnings when building with:
> 
> CONFIG_DEBUG_ENTRY=y
> CONFIG_STACK_VALIDATION=y
> CONFIG_VMLINUX_VALIDATION=y
> CONFIG_GCC_PLUGIN_STACKLEAK=y
> 
> vmlinux.o: warning: objtool: do_syscall_64()+0x9: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: do_int80_syscall_32()+0x9: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: exc_general_protection()+0x22: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: fixup_bad_iret()+0x20: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: do_machine_check()+0x27: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: .text+0x5346e: call to stackleak_erase() leaves .noinstr.text section
> vmlinux.o: warning: objtool: .entry.text+0x143: call to stackleak_erase() leaves .noinstr.text section
> vmlinux.o: warning: objtool: .entry.text+0x10eb: call to stackleak_erase() leaves .noinstr.text section
> vmlinux.o: warning: objtool: .entry.text+0x17f9: call to stackleak_erase() leaves .noinstr.text section
> 
> Note that the plugin's addition of calls to stackleak_track_stack()
> from noinstr functions is expected to be safe, as it isn't runtime
> instrumentation and is self-contained.
> 
> Cc: Alexander Popov <alex.popov@linux.com>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>

No, I didn't suggest this and it is actively wrong. noinstr *really*
should mean no instrumentation, nothing, nada, zip.