Re: [RFC PATCH 1/3] static_call: Add static call infrastructure

[Ard Biesheuvel]

On 12 November 2018 at 04:07, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> On Sat, Nov 10, 2018 at 08:09:17AM -0500, Steven Rostedt wrote:
>> On Sat, 10 Nov 2018 12:58:08 +0100
>> Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
>>
>>
>> > > The complaint is on:
>> > >
>> > >         DEFINE_STATIC_CALL(__tp_func_##name, __tracepoint_iter_##name);
>> > >
>> > > And the previous definition is on:
>> > >
>> > >         DECLARE_STATIC_CALL(__tp_func_##name, __tracepoint_iter_##name); \
>> > >
>> >
>> > Does the DECLARE really need the __ADDRESSABLE? Its purpose is to
>> > ensure that symbols with static linkage are not optimized away, but
>> > since the reference is from a header file, the symbol should have
>> > external linkage anyway.
>
> Yes, DECLARE needs the __ADDRESSABLE.  In the case where DECLARE
> is used, but DEFINE is not, GCC strips the symbol.
>

I assume DECLARE() is intended for use in header files, and DEFINE()
for source files, no?

Doesn't that mean that whatever symbol __ADDRESSABLE() refers to
should have external linkage, in which case it it addressable anyway?
Or are we talking about some LTO / --gc-sections use case here?

>> I applied the following change and it compiled fine:
>>
>> diff --git a/include/linux/static_call.h b/include/linux/static_call.h
>> index 90b580b95303..5f8a0f0e77be 100644
>> --- a/include/linux/static_call.h
>> +++ b/include/linux/static_call.h
>> @@ -108,8 +108,6 @@ extern void arch_static_call_poison_tramp(unsigned long insn);
>>  #define DECLARE_STATIC_CALL(key, func)                                       \
>>       extern struct static_call_key key;                              \
>>       extern typeof(func) STATIC_CALL_TRAMP(key);                     \
>> -     /* Preserve the ELF symbol so objtool can access it: */         \
>> -     __ADDRESSABLE(key)
>>
>>  #define DEFINE_STATIC_CALL(key, _func)                                       \
>>       DECLARE_STATIC_CALL(key, _func);                                \
>> @@ -117,7 +115,9 @@ extern void arch_static_call_poison_tramp(unsigned long insn);
>>               .func = _func,                                          \
>>               .site_mods = LIST_HEAD_INIT(key.site_mods),             \
>>       };                                                              \
>> -     ARCH_STATIC_CALL_TEMPORARY_TRAMP(key)
>> +     ARCH_STATIC_CALL_TEMPORARY_TRAMP(key);                          \
>> +     /* Preserve the ELF symbol so objtool can access it: */         \
>> +     __ADDRESSABLE(key)
>>
>>  #define static_call(key, args...) STATIC_CALL_TRAMP(key)(args)
>
> Adding __ADDRESSABLE to the DEFINE macro doesn't do any good.  By
> definition, the key is defined in the .o file, so the symbol already
> exists.
>
> The issue you're seeing is really an issue with the __ADDRESSABLE macro
> not creating unique symbol names.  This should fix it:
>
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 06396c1cf127..4bb73fd918b5 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -282,7 +282,7 @@ unsigned long read_word_at_a_time(const void *addr)
>   */
>  #define __ADDRESSABLE(sym) \
>         static void * __section(".discard.addressable") __used \
> -               __PASTE(__addressable_##sym, __LINE__) = (void *)&sym;
> +               __UNIQUE_ID(__addressable_##sym) = (void *)&sym;
>
>  /**
>   * offset_to_ptr - convert a relative memory offset to an absolute pointer

Not sure if it matters, but we'll end up with a lot more stuff in
.discard.addressable this way.