[llvm-dev] [cfe-dev] clang and -D_FORTIFY_SOURCE=1

Martin Storsjö via llvm-dev llvm-dev at lists.llvm.org
Tue Dec 3 11:48:00 PST 2019

On Tue, 3 Dec 2019, Serge Guelton via cfe-dev wrote:

> Hi folks (CCing llvm-dev, but that's probably more of a cfe-dev topic),
> As a follow-up to that old thread about -D_FORTIFY_SOURCE=n
>    http://lists.llvm.org/pipermail/cfe-dev/2015-November/045845.html
> And, more recently, to this fedora thread where clang/llvm -D_FORTIFY_SOURCE
> support is claimed to be only partial:
>    https://pagure.io/fesco/issue/2020
> I dig into the glibc headers in order to have a better understanding of what's
> going on, and wrote my notes here:
>    https://sergesanspaille.fedorapeople.org/fortify_source_requirements.rst
> TL;DR: clang does provide a similar compile-time checking as gcc, but no runtime
> checking. To assert that I wrote a small test suite:
>    https://github.com/serge-sans-paille/fortify-test-suite
> And indeed, clang doesn't pass it, mostly because it turns call to
> __builtin__(.*)_chk into calls to __builtin__\1.

I remember looking at the fortify macros recently, and iirc the issue was 
that the __builtin_object_size builtin, when used in an inline function, 
can't evaluate the size of the object in the context where it is inlined, 
which the glibc fortify macros/inline functions depend on.

This has been discussed before, e.g. here: 

// Martin

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