From: Cary Coutant <ccoutant@gmail.com>
To: "H.J. Lu" <hjl.tools@gmail.com>
Cc: Joe Groff <jgroff@apple.com>, Binutils <binutils@sourceware.org>
Subject: Re: Preventing preemption of 'protected' symbols in GNU ld 2.26
Date: Thu, 24 Mar 2016 18:31:00 -0000 [thread overview]
Message-ID: <CAJimCsFffshMvsDoRq_33Ss8u9Y_Z4y2NKsqDbxJQuO6SyJNbg@mail.gmail.com> (raw)
In-Reply-To: <CAMe9rOqqPVeaRZ8SPD-uoxRnHFOCGV3xXFNnDY5ez6xY8uG6hw@mail.gmail.com>
>>> What relocation do you propose to access external protected
>>> symbol on x86 for non-PIC code?
>>
>> Non-PIC code can still use a GOT, can't it?
>
> Yes.
Right.
Some additional thoughts:
- This has nothing to do with PIE. Non-PIC-non-PIE code has been
living with this restriction for at least 18 years (since visiblity
was introduced into the gABI).
- This has nothing to do with Ulrich's diatribe against protected
visibility, which applies only to function symbols (and really only to
one platform, due to how function pointer comparison works).
- You don't need to go full-on PIC to reference a protected data
symbol in a shared library. You can always statically initialize a
pointer variable, and go indirect through that ("poor-man's PIC").
- We could add support for __attribute__((dllimport)) or
__declspec(dllimport) when declaring an external variable that is
expected to be defined in a shared library (ideally, that attribute
ought to be placed in the library's public interface). The compiler
can generate a PIC-style reference for that variable without
penalizing all the other variables in the main program. This is how
HP-UX compilers work on PA and Itanium (using #pragma external).
- Compilers could also generate tentative PIC-style references, with
sufficient relocations to allow the linker to convert the indirect
reference to a direct reference when possible (changing the second
load into a nop or copy). HP-UX also does this.
- Indirect references from PIC code to a protected symbol penalize the
common case (referencing a symbol within its own module) to support
the uncommon case, while introducing the nasty side effects of a COPY
relocation.
- COPY relocations are evil. They bind an application to a specific
version of a shared library's ABI, and introduce a hidden startup
cost. If we're going to make any changes, we should be moving towards
elimination of COPY relocations, rather than disabling features that
were designed to improve performance.
- Arguing that protected means that the definition is in the same
module but its address might be external is absurd. The *only* reason
for the gABI to make that guarantee is so the compilers can optimize
the code based on the knowledge that the symbol can't be pre-empted.
-cary
next prev parent reply other threads:[~2016-03-24 18:31 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-24 0:00 Joe Groff
2016-03-24 0:45 ` H.J. Lu
2016-03-24 0:52 ` Joe Groff
2016-03-24 1:25 ` H.J. Lu
2016-03-24 15:01 ` Cary Coutant
2016-03-24 15:07 ` H.J. Lu
2016-03-24 16:06 ` Cary Coutant
2016-03-24 16:42 ` H.J. Lu
2016-03-24 16:56 ` Cary Coutant
2016-03-24 17:05 ` H.J. Lu
2016-03-24 17:06 ` Joe Groff
2016-03-24 17:09 ` H.J. Lu
2016-03-24 18:31 ` Cary Coutant [this message]
2016-03-27 16:26 ` Rafael Espíndola
2016-03-28 12:12 ` H.J. Lu
2016-03-28 22:12 ` Cary Coutant
[not found] ` <BC969B3B-87A2-4238-90C8-DA2E166707AF@apple.com>
2016-03-28 17:03 ` Joe Groff
2016-03-28 17:17 ` H.J. Lu
2016-03-28 22:22 ` Cary Coutant
2016-03-28 22:24 ` Joe Groff
2016-03-28 22:38 ` Cary Coutant
2016-03-28 22:41 ` Joe Groff
2016-03-28 23:21 ` Alan Modra
2016-03-29 0:29 ` Cary Coutant
2016-03-29 15:44 ` H.J. Lu
2016-03-29 12:40 ` Maciej W. Rozycki
[not found] <AB592ABD-D6D7-4D2F-A0D6-45738F168DC4@apple.com>
2016-03-29 19:31 ` Fwd: " Joe Groff
2016-03-29 19:33 ` H.J. Lu
2016-03-29 19:36 ` Joe Groff
2016-03-29 19:43 ` H.J. Lu
2016-03-29 19:51 ` Joe Groff
2016-03-29 19:54 ` H.J. Lu
2016-03-29 22:05 ` H.J. Lu
2016-03-30 1:44 ` Alan Modra
2016-03-30 1:46 ` Cary Coutant
2016-03-30 4:04 ` Jeff Law
2016-03-30 7:20 ` Cary Coutant
2016-03-30 7:34 ` Cary Coutant
2016-03-30 14:44 ` Alan Modra
2016-03-31 0:45 ` Cary Coutant
2016-03-31 0:40 ` Cary Coutant
2016-03-31 0:53 ` Jeff Law
2016-03-31 13:27 ` Ramana Radhakrishnan
2016-03-31 15:05 ` H.J. Lu
2016-04-15 16:10 ` Szabolcs Nagy
2016-04-01 19:51 ` Jeff Law
2016-04-02 2:53 ` Alan Modra
2016-04-15 16:16 H.J. Lu
2016-04-15 16:36 ` Jeff Law
2016-04-15 16:45 ` H.J. Lu
2016-04-15 16:43 ` Szabolcs Nagy
2016-04-15 23:59 ` Maciej W. Rozycki
2016-04-16 1:08 ` Szabolcs Nagy
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