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Re: [ARM] PR ld/20608
On 28/09/16 00:57, Christophe Lyon wrote:
> Sorry for the delay, I've just committed the patch as approved by Nick.
> Tristan, is it OK to backport this patch to 2.27 ? 2.26 ?
> On 23 September 2016 at 01:25, Alan Modra <firstname.lastname@example.org> wrote:
>> On Thu, Sep 22, 2016 at 04:39:30PM +0100, Richard Earnshaw (lists) wrote:
>>> So this patch got me wondering. If the PLT slot is so far away that we
>>> need an indirect jump to get there, why don't we clone the PLT code
>>> sequence at the veneer location? As I recall there's nothing
>>> architectural about placing all the PLT slots together, it's just more
>>> convenient to do that. We could even have the clone in Thumb-2 code if
>>> that's appropriate, so that it's compatible with tail calls.
> If there are multiple PLT slots for the same function,
> doesn't this break the uniqueness of a function pointer?
Yes, you have to ensure that only one plt stub is used for function
pointers (this is true globally for the program, so each shared library
with a PLT for a function f() has to be able to sort this out), but
that's a relatively small issue that's well within the linkers control.
Duplicate stubs would be entirely private and can't leak into function
>> Yeah, that's what ppc64 does. It does have one downside in that
>> synthetic plt symbols are not so easy to add.
>> /* FIXME: It would be very much nicer to put sym@plt on the
>> stub rather than on the glink branch table entry. The
>> objdump disassembler would then use a sensible symbol
>> name on plt calls. The difficulty in doing so is
>> a) finding the stubs, and,
>> b) matching stubs against plt entries, and,
>> c) there can be multiple stubs for a given plt entry.
>> Solving (a) could be done by code scanning, but older
>> ppc64 binaries used different stubs to current code.
>> (b) is the tricky one since you need to known the toc
>> pointer for at least one function that uses a pic stub to
>> be able to calculate the plt address referenced.
>> (c) means gdb would need to set multiple breakpoints (or
>> find the glink branch itself) when setting breakpoints
>> for pending shared library loads. */
>> We avoid the problem by enabling --emit-stub-syms by default.
>> Alan Modra
>> Australia Development Lab, IBM