llvm dev - May 2017 - [lld][ELF] Add option to make .dynamic read only

Petr Hosek via llvm-dev <llvm-dev at lists.llvm.org>
writes:
> One of the design principles we're trying to follow is to make
everything
> read-only, unless it has be writable. The only reason for .dynamic to be
> writable is DT_DEBUG which is something we never intend to support. FWIW in
> Fuchsia all we need is a read-only .dynamic without emitting DT_DEBUG
> altogether, but we wanted to make sure that this flag is also usable
> elsewhere hence implementing DT_DEBUG_INDIRECT which is already supported
> by musl as Jake pointed out.
It might be easier to have an option for making .dynamic be read only
and the logic for emitting DT_DEBUG becomes ".dynamic is rw or mips".

BTW, dynamic linkers really never relocate the .d_ptr entries?

What are you guys using instead of DT_DEBUG?

Cheers,
Rafael

On Tue, May 23, 2017 at 3:19 PM Rafael Avila de Espindola <
rafael.espindola at gmail.com> wrote:
> Petr Hosek via llvm-dev <llvm-dev at lists.llvm.org> writes:
> > One of the design principles we're trying to follow is to make
everything
> > read-only, unless it has be writable. The only reason for .dynamic to
be
> > writable is DT_DEBUG which is something we never intend to support.
FWIW
> in
> > Fuchsia all we need is a read-only .dynamic without emitting DT_DEBUG
> > altogether, but we wanted to make sure that this flag is also usable
> > elsewhere hence implementing DT_DEBUG_INDIRECT which is already
supported
> > by musl as Jake pointed out.
>
> It might be easier to have an option for making .dynamic be read only
> and the logic for emitting DT_DEBUG becomes ".dynamic is rw or
mips".
>
> BTW, dynamic linkers really never relocate the .d_ptr entries?
>
musl's dynamic linker doesn't and neither ours. Older versions of
glibc's
dynamic linker do, but I'm not sure about the newer ones.

> What are you guys using instead of DT_DEBUG?
>
We use other OS-specific runtime means to register that address.

> Cheers,
> Rafael
>
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On Tue, May 23, 2017 at 4:17 PM, Petr Hosek <phosek at chromium.org>
wrote:
> On Tue, May 23, 2017 at 3:19 PM Rafael Avila de Espindola <
> rafael.espindola at gmail.com> wrote:
>
>> Petr Hosek via llvm-dev <llvm-dev at lists.llvm.org> writes:
>> > One of the design principles we're trying to follow is to make
>> everything
>> > read-only, unless it has be writable. The only reason for .dynamic
to be
>> > writable is DT_DEBUG which is something we never intend to
support.
>> FWIW in
>> > Fuchsia all we need is a read-only .dynamic without emitting
DT_DEBUG
>> > altogether, but we wanted to make sure that this flag is also
usable
>> > elsewhere hence implementing DT_DEBUG_INDIRECT which is already
>> supported
>> > by musl as Jake pointed out.
>>
>> It might be easier to have an option for making .dynamic be read only
>> and the logic for emitting DT_DEBUG becomes ".dynamic is rw or
mips".
>>
>> BTW, dynamic linkers really never relocate the .d_ptr entries?
>>
>
> musl's dynamic linker doesn't and neither ours. Older versions of
glibc's
> dynamic linker do, but I'm not sure about the newer ones.
>
>
>> What are you guys using instead of DT_DEBUG?
>>
>
> We use other OS-specific runtime means to register that address.
>
Do the OS-specific means make sense for other OSes?
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Petr Hosek <phosek at chromium.org> writes:
>> BTW, dynamic linkers really never relocate the .d_ptr entries?
>>
>
> musl's dynamic linker doesn't and neither ours. Older versions of
glibc's
> dynamic linker do, but I'm not sure about the newer ones.
Could you check what current glibc does by testing a simple program with
your patch to make .dynamic ro?

Cheers,
Rafael