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Re: [PATCH] x86: Generate PLT relocations for -z now

On Thu, May 11, 2017 at 10:33 AM, Carlos O'Donell <> wrote:
> On 05/11/2017 01:09 PM, H.J. Lu wrote:
>> On Thu, May 11, 2017 at 9:44 AM, Carlos O'Donell <> wrote:
>>> On 05/11/2017 10:51 AM, H.J. Lu wrote:
>>>> The run-time R_X86_64_GLOB_DAT relocation against foo is
>>>> required.  There is no need to add another run-time
>>>> R_X86_64_JUMP_SLOT relocation.  We can add a linker option
>>>> to generate the extra R_X86_64_JUMP_SLOT.  But I don't think
>>>> it should be the default.
>>> I don't disagree with your analysis, I disagree with the execution
>>> of the plan.
>>> The plan should be:
>>> (a) Analyze uses of the PLT entries.
>>> In this case LD_AUDIT, LD_PROFILE, and ltrace all require them to
>>> operate correctly.
>> As I stated before, glibc and GCC have been doing everything we
>> can to avoid PLT.
>>> (b) Develop plan for migrating developer tooling, like LD_AUDIT,
>>>     which is a part of glibc and is well supported.
>>> (c) Choose one of:
>>>     (C.1) Make PLT elision optional e.g. -z noplt
>>>     or
>>>     (C.2) Make LD_AUDIT work with optimizations.
>> This is the only option.
> Why is this the only option?
> Is it the only option because we want to reduce the number of dynamic
> relocs as much as possible?

Yes and a function can be called via GOT.

> Is it because we have benchmarks that show reducing relocations helps
> some application?
> Or is it an abstract engineering decision that a minimum number of
> relocations is always a good thing? Even though their cost might be
> hidden in subsequent load/store latencies?

What did you mean by hidden cost?

> If (C.2) is truly the only option, then what are our solutions?
> (a) Always generate the PLT entries, even if they might be unused.

That is not "Make LD_AUDIT work with optimizations."

> (b) If LD_AUDIT is in effect resolve all R_X86_64_GLOB_DAT which
>     would otherwise have resolved to an STT_FUNC (or equivalent)
>     to the appropriate PLT entry for the function in question.
>     This would bind all such functions to run the profiled resolution
>     function for the lifetime of the process because potentially with
>     -z now, and -z relro this decision is binding
>     once made (and the GOT entries relocated).

This also works with functions called via function pointer.

> In (b) how do we find which PLT entry (index) to use for which GOT
> entry? If we had a 1:1 correspondence then we could make the mapping
> work. may need to create PLT on the fly for each GLOB_DAT relocation
against function and use its PLT entry as function address.


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