> -----Original Message----- > From: Florian Weimer [mailto:fwei...@redhat.com] > Sent: Wednesday, September 27, 2017 10:52 AM > To: Sandra Loosemore <san...@codesourcery.com>; Tsimbalist, Igor V > <igor.v.tsimbal...@intel.com>; Uros Bizjak <ubiz...@gmail.com> > Cc: gcc-patches@gcc.gnu.org > Subject: Re: 0005-Part-5.-Add-x86-CET-documentation > > On 09/27/2017 05:40 AM, Sandra Loosemore wrote: > >> > >> +@emph{x86 implementation:} when @option{-fcf-protection} option is > >> +specified the compiler inserts an ENDBR instruction at function's > >> +prologue if the function's type does not have the @code{nocf_check} > >> +attribute and addresses to which indirect control-flow transfer can > >> +happen. The instruction triggers the HW check if a control-flow > >> +transfer to the address of ENDBR instruction is valid. > > > > Implementation details like this should be comments in the code, not > > included in the user-facing documentation. > > This is part of the ABI GCC implements, so it has to be documented > somewhere, and not just as part of the GCC source code.
A question for both Sandra and Florian - What is your suggestion where the text should go? > CET is not properly described in the ABI supplement and I don't think this > will > change, so detailed documentation in the GCC manual is very much > desirable. > > That being said, the implementation notes above need some clarification. > It's not clear to me what the conditions are under which the ENDBR > instruction is emitted (and we probably should use @code{endbr} in the > manual), what it is trying to achieve, and how the x86 calling convention > changes. I assume it is somehow related to what we call internally “the > suffix We are diving into implementation details but it's simple enough. - endbr is generated for every function, which does not have nocf_check attribute. Optimization can be done later to exclude functions, whose address was not taken. - there is no change in calling convention Thanks, Igor > problem”: without control flow integrity, an attacker might skip over > precondition/hardening checks, directly to the critical changes we want to > protect, executing only the suffix of a function (hence the name). > > Thanks, > Florian