Fixing dlopen("libpthread.so")

[Daniel Eischen]

On Wed, 7 Jan 2015, Konstantin Belousov wrote:

> On Tue, Jan 06, 2015 at 11:01:30PM -0500, Daniel Eischen wrote:
>> On Wed, 7 Jan 2015, Konstantin Belousov wrote:
>>
>>> On Tue, Jan 06, 2015 at 09:50:46PM +0100, Jilles Tjoelker wrote:
>>>> On Mon, Jan 05, 2015 at 04:37:08AM +0200, Konstantin Belousov wrote:
>>>>> Next natural question is about the __error calls through PLT in the
>>>>> .cerror asm. Before the work was committed, libthr interposed __error,
>>>>> which was required for correct operation. Now, we need __error exported,
>>>>> but do we need support its interposing ? This is an implementation
>>>>> detail for errno, I do not see any loss from not allowing to override
>>>>> errno location.
>>>>
>>>> Indeed, there is no need to allow interposing __error (as with many
>>>> libc-internal calls to its exported symbols). Additionally, the current
>>>> namespace.h mechanism could be used to redirect __error calls from C
>>>> code.
>>>>
>>>> Glibc uses macro trickery with the asm labels compiler feature, making
>>>> libc code see things like
>>>>   int *__error(void) asm("__hidden__error");
>>>> and defining the hidden alias somewhere. This also works for
>>>> compiler-generated calls like to memcpy(). I'm not sure whether it is a
>>>> good idea to add this extra trickery and depend on the compiler feature.
>>> I might look at this after the current change is committed.
>>
>> I don't quite follow all the above about __error(), but non-C
>> programs need to interface to an __error() that works regardless
>> of threading or not.
>
> C programs also need to be able to call __error(), to have working
> errno. Discussion above talks about the need for the programs to be
> able to interpose __error(), which I and Jilles agree is excessive. My
> proposal is to remove PLT indirection in the calls to __error() from
> _inside_ libc, which makes errno functioning in libc both more reliable
> and slightly faster.

Fair enough - thanks for the explanation!

-- 
DE