[cfe-dev] clang and -D_FORTIFY_SOURCE=1

[Serge Guelton via cfe-dev]

> Are you sure you've diagnosed the issue correctly? __builtin___memcpy_chk
works correctly, as far as I know.

100% sure. Let's have a look at the output of

  #include <string.h>
  static char dest[10];
  char* square(int n) {
    memcpy(dest, "hello", n);
    return dest;
  }

compiled with -D_FORTIFY_SOURCE=1 -O1 : https://godbolt.org/z/UvABWp

Clang issues a call to memcpy, while gcc issues a call to __memcpy_chk.
The call to __memcpy_chk performs extra runtime checks memcpy doesn't,
and clang doesn't generate the extra checks inline either. This is a
separate
concern from the accuracy of __builtin_object_size, just a different
runtime behavior.

Clang could generate the call to __memcpy_chk if its declaration is
available, which is the case for the glibc.

On Tue, Dec 3, 2019 at 8:41 PM Eli Friedman <efriedma at quicinc.com> wrote:

> Are you sure you've diagnosed the issue correctly? __builtin___memcpy_chk
> works correctly, as far as I know.
>
> -Eli
>
> > -----Original Message-----
> > From: cfe-dev <cfe-dev-bounces at lists.llvm.org> On Behalf Of Serge
> Guelton via
> > cfe-dev
> > Sent: Tuesday, December 3, 2019 2:07 AM
> > To: cfe-dev at lists.llvm.org
> > Cc: llvm-dev at lists.llvm.org
> > Subject: [EXT] [cfe-dev] clang and -D_FORTIFY_SOURCE=1
> >
> > 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.
> >
> > We need to support the runtime behavior of the following builtins:
> >
> > - __builtin___memcpy_chk
> > - __builtin___memmove_chk
> > - __builtin___mempcpy_chk
> > - __builtin___memset_chk
> > - __builtin___snprintf_chk
> > - __builtin___sprintf_chk
> > - __builtin___stpcpy_chk
> > - __builtin___strcat_chk
> > - __builtin___strcpy_chk
> > - __builtin___strncat_chk
> > - __builtin___strncpy_chk
> > - __builtin___vsnprintf_chk
> > - __builtin___vsprintf_chk
> >
> > And I'd like to implement them at clang level, leveraging their existing
> > implementation. Is that the right way to go / any comments / issue with
> that
> > approach ?
> > _______________________________________________
> > cfe-dev mailing list
> > cfe-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
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