[cfe-dev] PIE/PIC Clang Defaults on Linux x86-64

[Eric Christopher via cfe-dev]

On Tue, Jun 2, 2020 at 3:38 PM Fangrui Song <maskray at google.com> wrote:

> On 2020-06-02, Eric Christopher wrote:
> >I'll elaborate on this a bit more since it may not be coming across
> clearly:
> >
> >a) gcc and clang differ here:
> >
> >clang++/g++  -o foo.x foo.c -fPIE
> >
> >gcc passes -pie to the linker and clang doesn't. This mostly seems like an
> >oversight that can be fixed.
> >
> >b) gcc and clang differ here
> >
> >clang++/g++ -o foo.x foo.c
> >
> >where gcc defaults to -pie and pie code generation and clang doesn't.
> >
> >This is something where we should at least have the discussion. For what
> >it's worth my thought is that we should move to PIE by default for linux
> >if, for not other reason, as the system compiler (gcc) already does this.
> >We also already do this on android and in some sanitizer cases.
> >
> >Anyone else have any thoughts?
> >
> >Thanks!
> >
> >-eric
>
> GCC 6 introduced a configure option --enable-default-pie (default -fPIE
> and -pie)
> (
> https://gcc.gnu.org/git/?p=gcc.git&a=commit;h=428b381275bc95032274440f02d7719225de2c17
> )
>
> Distributions can decide whether they want to enable it.
>
> There are two questions:
>
> (1) Do we want to make default-pie configurable (which requires a CMake
> option)
>


I have no strong opinions here.


> (2) Whether or not this is configurable, do we want
> Generic_GCC::isPIEDefault() to return true
>
>
At this point I think so.

+Craig Topper <craig.topper at gmail.com> for the x86 backend side of things
in case he wants to comment.

-eric


> At this point, it is probably that more distributions enable it by default.
> So switching Generic_GCC::isPIEDefault() probably makes sense.
> I want to avoid a CMake variable to reduce issues in testing.
> (These CLANG_DEFAULT_* are a source of test instability.)
>
> (I suspect some distributions already ship downstream patches to enable
> -fPIE and -pie by default.)
>
> >On Tue, Jun 2, 2020 at 2:44 PM Mitch Phillips via cfe-dev <
> >cfe-dev at lists.llvm.org> wrote:
> >
> >> Hi folks,
> >>
> >> I recently noticed that clang and GCC differ on the default PIE/PIC
> flags.
> >> In particular, clang doesn't default to PIE/PIC on Linux x86-64, and GCC
> >> does.
> >>
> >> $ clang++ a.cpp && readelf -l a.out | egrep '(LOAD|VirtAddr)'
> >>>   Type           Offset             VirtAddr           PhysAddr
> >>>   LOAD           0x0000000000000000 0x0000000000400000
> 0x0000000000400000
> >>>
> >>
> >> $ g++ a.cpp && readelf -l a.out | egrep '(LOAD|VirtAddr)'
> >>>   Type           Offset             VirtAddr           PhysAddr
> >>
> >>   LOAD           0x0000000000000000 0x0000000000000000
> 0x0000000000000000
> >>
> >>
> >> Are there any concerns about changing the clang default to emit PIC and
> >> pass PIE to the linker?
> >>
> >> Also, using `clang++ -fPIC -fPIE <...>` doesn't emit a PIE. Any
> opponents
> >> to making `-fPIE` imply passing `-pie` to the linker?
> >>
> >> - Mitch.
> >> _______________________________________________
> >> cfe-dev mailing list
> >> cfe-dev at lists.llvm.org
> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >>
>
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