On Sun, Jan 7, 2018 at 8:07 PM, Sandra Loosemore
<san...@codesourcery.com> wrote:
> On 01/07/2018 03:58 PM, H.J. Lu wrote:
>>
>> This set of patches for GCC 8 mitigates variant #2 of the speculative
>> execution
>> vulnerabilities on x86 processors identified by CVE-2017-5715, aka
>> Spectre. They
>> convert indirect branches to call and return thunks to avoid speculative
>> execution
>> via indirect call and jmp.
>
>
> I have a general documentation issue with all the new command-line options
> and attributes added by this patch set: the documentation is very
> implementor-speaky and doesn't explain what user-level problem they're
> trying to solve.
Do you have any suggestions?
> E.g. to take just one example
>
>> +@item function_return("@var{choice}")
>> +@cindex @code{function_return} function attribute, x86
>> +On x86 targets, the @code{function_return} attribute causes the compiler
>> +to convert function return with @var{choice}. @samp{keep} keeps function
>> +return unmodified. @samp{thunk} converts function return to call and
>> +return thunk. @samp{thunk-inline} converts function return to inlined
>> +call and return thunk. @samp{thunk-extern} converts function return to
>> +external call and return thunk provided in a separate object file.
>
>
> Why would you want to mess with call and return code generation in this way?
> The documentation doesn't say.
>
> For thunk-extern, is the programmer supposed to provide the thunk? How
> would you go about implementing the missing bit of code? What should it do?
> I'm compiler implementor and I wouldn't even know how to use this feature by
> reading the manual; how would an ordinary application programmer who isn't
> familiar with GCC internals know how to use it?
This option was requested by Linux kernel people. Linux kernel may
choose different thunks at kernel load-time. If a program doesn't know
how to write external thunk, he/she shouldn't it.
> If the goal here is to tell GCC to produce code that is protected against
> the Spectre vulnerability, perhaps simplify this to adding just one option
> that controls all the things you've given separate options and attributes
> for?
-mindirect-branch=thunk does the job. Other options/choices are for
fine tuning.
Thanks.
--
H.J.
