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Re: [PATCH 0/5] x86: CVE-2017-5715, aka Spectre

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.

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?

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?


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