> On Aug 25, 2025, at 04:13, Peter Zijlstra <pet...@infradead.org> wrote:
>
> On Fri, Aug 22, 2025 at 03:29:11PM -0700, Kees Cook wrote:
>> On Fri, Aug 22, 2025 at 08:29:16PM +0000, Qing Zhao wrote:
>>>> On Aug 22, 2025, at 15:02, Kees Cook <k...@kernel.org> wrote:
>>>> Right, and sometimes we have to explicitly perform a no-op
>>>> address-taking to make sure a symbol gets generated:
>>>>
>>>> /*
>>>> * Force the compiler to emit 'sym' as a symbol, so that we can reference
>>>> * it from inline assembler. Necessary in case 'sym' could be inlined
>>>> * otherwise, or eliminated entirely due to lack of references that are
>>>> * visible to the compiler.
>>>> */
>>>> #define ___ADDRESSABLE(sym, __attrs)
>>>> \
>>>> static void * __used __attrs
>>>> \
>>>> __UNIQUE_ID(__PASTE(__addressable_,sym)) = (void *)(uintptr_t)&sym;
>>>>
>>>> #define __ADDRESSABLE(sym) \
>>>> ___ADDRESSABLE(sym, __section(".discard.addressable"))
>>>>
>>>> $ git grep KCFI_REFERENCE
>>>> include/linux/compiler.h:#define KCFI_REFERENCE(sym) __ADDRESSABLE(sym)
>>>> arch/x86/include/asm/page_64.h:KCFI_REFERENCE(copy_page);
>>>> arch/x86/include/asm/string_64.h:KCFI_REFERENCE(__memset);
>>>> arch/x86/include/asm/string_64.h:KCFI_REFERENCE(__memmove);
>>>> arch/x86/kernel/alternative.c:KCFI_REFERENCE(__bpf_prog_runX);
>>>> arch/x86/kernel/alternative.c:KCFI_REFERENCE(__bpf_callback_fn);
>>>
>>> I am curious on why the compiler eliminates an external routine completely
>>> in the file if it's address-taken in that file.
>>> Why an additional no-op address-taken is needed here.
>>
>> If I am remembering correctly this is needed for rare cases where
>> a function built without a C definition is being used in Linux's
>> self-patching "alternatives" code swaps in one function for another,
>> and is being used indirectly. These cases end up not being visible to
>> compiler (so no address-taken), but the indirect call site is still
>> being instrumented. And the above list is the _entire_ list of such
>> corner cases: all really low-level things.
>>
>> Peter may remember this better than me...
>
> The above are all functions from assembly and JITs, the C compiler
> simply never sees the function definition, only the declaration. The
> above is used to force emit the __typeid symbol, such that assembly can
> reference it and it all links correctly.
Okay, I see. Thanks for the information.
Qing
