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

[Hendrik Leppkes]

On Sat, Mar 12, 2016 at 3:09 AM, Michael Niedermayer
<michael at niedermayer.cc> wrote:
> On Thu, Mar 10, 2016 at 07:57:53PM +0100, Hendrik Leppkes wrote:
>> 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).
>
> as i was the one applying it IIRC, i should probably comment ...
> iam not against this being partially or fully revered/mode conditional
> if it causes more problems than it does good.
>
>

I was hoping Alex would comment, but not being able to debug using GDB
is a serious issue which people have reported ever since it was
commited, we just never knew the cause of it.

- Hendrik