llvm dev - Dec 2014 - [LLVMdev] [lld] Handling multiple -init/-fini command line options

On Mon, Dec 8, 2014 at 8:39 PM, Shankar Easwaran
<shankare at codeaurora.org> wrote:
> On 12/8/2014 11:09 AM, Joerg Sonnenberger wrote:
>>
>> On Mon, Dec 08, 2014 at 10:21:49AM -0600, Shankar Easwaran wrote:
>>>
>>> The DT_INIT/DT_FINI correspond to one initializer function,where as
>>> DT_INIT_ARRAY/DT_FINI_ARRAY is used when there is more than one
>>> initalizer function/finalizer function respectively.
>>
>> This is not true. The difference is that the DT_INIT / DT_FINI function
>> is responsible for calling into the .init / .fini block and for
>> DT_INIT_ARRAY / DT_FINI_ARRAY, the block is explicitly defined as a
>> list of pointers and it is the responsibility of either the CRT logic
or
>> the dynamic linker to call them.
>>
>> That said, I don't think more than one DT_INIT / DT_FINI entry
should be
>> allowed as the latter entry will overwrite the earlier ones.
>
> Thanks Joerg, The snip of code that appears to run the init/fini was easily
> browseable with musl.
>
> http://git.musl-libc.org/cgit/musl/tree/src/ldso/dynlink.c (function:
> do_init_fini).
>
> That said, all of the code is essentially under NO_LEGACY_INITFINI, is the
> linker responsible to convert DT_INIT to DT_INITARRAY's ?
>
> If so, I think more than one init/fini option should be converted to
> DT_INIT_ARRAY (or) DT_FINI_ARRAY respectively ?
By the way, gnu linker support both DT_INIT and DT_INIT_ARRAY tags at
the same time. For example, if input objects contain .init_array
section and _init symbol, gnu linker emits both DT_INIT and
DT_INIT_ARRAY tags. And I still do not see any reason to assign more
complicated semantic to the -init/-fini options and introduce
incompatibility with gnu linker.

-- 
Simon Atanasyan

If we want to be able to add functions to init_array using command line we
can introduce new options like -init-array / -fini-array
On Dec 8, 2014 10:22 PM, "Simon Atanasyan" <simon at
atanasyan.com> wrote:
> On Mon, Dec 8, 2014 at 8:39 PM, Shankar Easwaran
> <shankare at codeaurora.org> wrote:
> > On 12/8/2014 11:09 AM, Joerg Sonnenberger wrote:
> >>
> >> On Mon, Dec 08, 2014 at 10:21:49AM -0600, Shankar Easwaran wrote:
> >>>
> >>> The DT_INIT/DT_FINI correspond to one initializer
function,where as
> >>> DT_INIT_ARRAY/DT_FINI_ARRAY is used when there is more than
one
> >>> initalizer function/finalizer function respectively.
> >>
> >> This is not true. The difference is that the DT_INIT / DT_FINI
function
> >> is responsible for calling into the .init / .fini block and for
> >> DT_INIT_ARRAY / DT_FINI_ARRAY, the block is explicitly defined as
a
> >> list of pointers and it is the responsibility of either the CRT
logic or
> >> the dynamic linker to call them.
> >>
> >> That said, I don't think more than one DT_INIT / DT_FINI entry
should be
> >> allowed as the latter entry will overwrite the earlier ones.
> >
> > Thanks Joerg, The snip of code that appears to run the init/fini was
> easily
> > browseable with musl.
> >
> > http://git.musl-libc.org/cgit/musl/tree/src/ldso/dynlink.c (function:
> > do_init_fini).
> >
> > That said, all of the code is essentially under NO_LEGACY_INITFINI, is
> the
> > linker responsible to convert DT_INIT to DT_INITARRAY's ?
> >
> > If so, I think more than one init/fini option should be converted to
> > DT_INIT_ARRAY (or) DT_FINI_ARRAY respectively ?
>
> By the way, gnu linker support both DT_INIT and DT_INIT_ARRAY tags at
> the same time. For example, if input objects contain .init_array
> section and _init symbol, gnu linker emits both DT_INIT and
> DT_INIT_ARRAY tags. And I still do not see any reason to assign more
> complicated semantic to the -init/-fini options and introduce
> incompatibility with gnu linker.
>
> --
> Simon Atanasyan
>
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On 12/8/2014 1:22 PM, Simon Atanasyan wrote:
> On Mon, Dec 8, 2014 at 8:39 PM, Shankar Easwaran
> <shankare at codeaurora.org> wrote:
>> On 12/8/2014 11:09 AM, Joerg Sonnenberger wrote:
>>> On Mon, Dec 08, 2014 at 10:21:49AM -0600, Shankar Easwaran wrote:
>>>> The DT_INIT/DT_FINI correspond to one initializer
function,where as
>>>> DT_INIT_ARRAY/DT_FINI_ARRAY is used when there is more than one
>>>> initalizer function/finalizer function respectively.
>>> This is not true. The difference is that the DT_INIT / DT_FINI
function
>>> is responsible for calling into the .init / .fini block and for
>>> DT_INIT_ARRAY / DT_FINI_ARRAY, the block is explicitly defined as a
>>> list of pointers and it is the responsibility of either the CRT
logic or
>>> the dynamic linker to call them.
>>>
>>> That said, I don't think more than one DT_INIT / DT_FINI entry
should be
>>> allowed as the latter entry will overwrite the earlier ones.
>> Thanks Joerg, The snip of code that appears to run the init/fini was
easily
>> browseable with musl.
>>
>> http://git.musl-libc.org/cgit/musl/tree/src/ldso/dynlink.c (function:
>> do_init_fini).
>>
>> That said, all of the code is essentially under NO_LEGACY_INITFINI, is
the
>> linker responsible to convert DT_INIT to DT_INITARRAY's ?
>>
>> If so, I think more than one init/fini option should be converted to
>> DT_INIT_ARRAY (or) DT_FINI_ARRAY respectively ?
> By the way, gnu linker support both DT_INIT and DT_INIT_ARRAY tags at
> the same time. For example, if input objects contain .init_array
> section and _init symbol, gnu linker emits both DT_INIT and
> DT_INIT_ARRAY tags. And I still do not see any reason to assign more
> complicated semantic to the -init/-fini options and introduce
> incompatibility with gnu linker.
The dynamic loader handles only one entry for DT_INIT. If there is more 
than one init option, we could convert this as an .init_array instead ?

If that doesnot work, we can come up with a .init_array option but I am 
not sure about how will you handle priority with init_array's ? Do you 
set linker defined init/fini symbols the last in priority ?

Shankar Easwaran

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the
Linux Foundation

On Mon, Dec 8, 2014 at 10:57 PM, Shankar Easwaran
<shankare at codeaurora.org> wrote:
> The dynamic loader handles only one entry for DT_INIT. If there is more
than
> one init option, we could convert this as an .init_array instead ?
>
> If that doesnot work, we can come up with a .init_array option but I am not
> sure about how will you handle priority with init_array's ? Do you set
> linker defined init/fini symbols the last in priority ?
My idea:
1. -init/-fini options affects DT_INIT/DT_FINI tags only. If there is
no any option, the linker searches _init/_finit symbols and configure
DT_INIT/DT_FINI tags. This behaviour exactly corresponds to the GNU
linker. By the way symbols specified in the -init/-fini options can be
in any sections, not only .init/.fini.

2. -init-array/-fini-array options (I still not sure we need this
functionality) affect .init_array/.fini_array and
DT_INIT_ARRAY/DT_FINI_ARRAY tags. The order of these options define
priority.

The only problem is that we cannot setup priority of DT_INIT vs DT_INIT_ARRAY.

-- 
Simon Atanasyan