[FFmpeg-devel] [PATCH 1/3] configure: Force mingw's ld to keep the reloc section

[Hendrik Leppkes]

On Mon, Sep 21, 2015 at 6:41 AM, Alex Smith <alex.smith at warpsharp.info> wrote:
> From: Alex Smith <alex.smith at warpsharp.info>
>
> Binutils will always strip the relocation information from executable
> files even if it needs it (dynamicbase/ASLR).  We can work around this
> by using the pic-executable flag combined with setting the correct entry
> point since apparently ld forgets what that should be.  This problem
> affects both 32 and 64-bit mingw-w64.
>
> We can combine the nxcompat/dynamicbase check because they were added to
> binutils at the same time.
>
> Signed-off-by: Alex Smith <alex.smith at warpsharp.info>
>
> Conflicts:
>         configure
> ---
>  configure | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/configure b/configure
> index d2a25bb..d6ab35a 100755
> --- a/configure
> +++ b/configure
> @@ -4367,8 +4367,6 @@ case $target_os in
>              LIBTARGET=arm-wince
>          fi
>          enabled shared && ! enabled small && check_cmd $windres --version && enable gnu_windres
> -        check_ldflags -Wl,--nxcompat
> -        check_ldflags -Wl,--dynamicbase
>          enabled x86_32 && check_ldflags -Wl,--large-address-aware
>          shlibdir_default="$bindir_default"
>          SLIBPREF=""
> @@ -4392,6 +4390,17 @@ case $target_os in
>          objformat="win32"
>          ranlib=:
>          enable dos_paths
> +        check_ldflags -Wl,--nxcompat,--dynamicbase
> +        # Lets work around some stupidity in binutils.
> +        # ld will strip relocations from executables even though we need them
> +        # for dynamicbase (ASLR).  Using -pie does retain the reloc section
> +        # however ld then forgets what the entry point should be (oops) so we
> +        # have to manually (re)set it.
> +        if enabled x86_32; then
> +            add_ldexeflags -Wl,--pic-executable,-e,_mainCRTStartup
> +        elif enabled x86_64; then
> +            add_ldexeflags -Wl,--pic-executable,-e,mainCRTStartup
> +        fi
>          ;;
>      win32|win64)
>          disable symver

This patch (the relocations part) broke debugging mingw-w64 ffmpeg
builds with gdb, you can't set breakpoints anymore when its applied.
It should either be reverted or made dependent on
--enable/disable-debug (I would favor the first, honestly, since its a
rather ugly hack in itself).

Did the binutils/mingw guys ever comment anything useful on this issue?

- Hendrik