[PATCH] Always define referenced __start_SECNAME/__stop_SECNAME

[H.J. Lu]

On Tue, Jun 13, 2017 at 8:22 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Jun 13, 2017 at 7:28 AM, Alan Modra <amodra@gmail.com> wrote:
>> On Tue, Jun 13, 2017 at 05:38:45AM -0700, H.J. Lu wrote:
>>> On Tue, Jun 13, 2017 at 5:26 AM, Alan Modra <amodra@gmail.com> wrote:
>>> > On Mon, Jun 12, 2017 at 07:10:47PM -0700, H.J. Lu wrote:
>>> >> On Mon, Jun 12, 2017 at 6:54 PM, Alan Modra <amodra@gmail.com> wrote:
>>> >> > On Mon, Jun 12, 2017 at 05:18:58PM -0700, H.J. Lu wrote:
>>> >> >> On Mon, Jun 12, 2017 at 4:32 PM, Alan Modra <amodra@gmail.com> wrote:
>>> >> >> > On Sat, Jun 10, 2017 at 03:46:49PM -0700, H.J. Lu wrote:
>>> >> >> >> This patch changes linker to always define referenced __start_SECNAME and
>>> >> >> >> __stop_SECNAME if the input section name is the same output section name,
>>> >> >> >> which is always true for orphaned sections, and SECNAME is a C identifier.
>>> >> >> >
>>> >> >> > I think this change is reasonable.
>>> >> >> >
>>> >> >> >> Also __start_SECNAME and __stop_SECNAME symbols are marked as hidden by
>>> >> >> >> ELF linker.
>>> >> >> >
>>> >> >> > Why is this necessary?  Also, you make another change in behaviour
>>> >> >>
>>> >> >> It is to make sure that  __start_SECNAME and __stop_SECNAME
>>> >> >> symbols     for section SECNAME in different modules are unique.
>>> >> >
>>> >> > I understand, and it would have been good if these symbols were
>>> >> > defined that way in the beginning, but they weren't.  I'm concerned
>>> >> > that if you make this change as well we will find some code that
>>> >> > relies on the symbols being dynamic.
>>> >>
>>> >> My change fixes a real bug:
>>> >>
>>> >> https://sourceware.org/bugzilla/show_bug.cgi?id=20022
>>> >
>>> > Really, your change is a workaround not a fix.  You avoid exporting
>>> > __start or __stop symbols because doing so shows a problem with gc.  A
>>> > real fix for the PR would make ld do the right thing even in the
>>> > presense of exported dynamic symbols.
>>>
>>> Exporting __start_SECNAME and __stop_SECNAME as dynamic
>>> symbols may lead to unexpected behavior.  Reference to __start_SECNAME
>>> and __stop_SECNAME to section SECNAME within a DSO will be
>>> resolved to __start_SECNAME and __stop_SECNAME in another DSO
>>> or executable.  In most cases, it isn't the intended behavior and there are
>>> workaround if it is needed.
>>
>> You're correct regarding shared libraries.  I was thinking more about
>> an executable when I said the patch was a workaround.  OK, so I
>> suppose we should leave that in, and wait to see if we get complaints.
>>
>>> >> We can suggest workarounds if some codes depend on that.
>>> >>
>>> >> >>
>>> >> >> > that you don't mention:  __start and __stop symbols were previously
>>> >> >> > defined by ld -Ur, not just at final link.  You'll need to modify
>>> >> >>
>>> >> >> Is there a testcase for this behavior?
>>> >> >
>>> >> > Not that I'm aware of.
>>> >> >
>>> >>
>>> >> >From ld manual:
>>> >>
>>> >> '-Ur'
>>> >>      For anything other than C++ programs, this option is equivalent to
>>> >>      '-r': it generates relocatable output--i.e., an output file that
>>> >>      can in turn serve as input to 'ld'.  When linking C++ programs,
>>> >>      '-Ur' _does_ resolve references to constructors, unlike '-r'.  It
>>> >>      does not work to use '-Ur' on files that were themselves linked
>>> >>      with '-Ur'; once the constructor table has been built, it cannot be
>>> >>      added to.  Use '-Ur' only for the last partial link, and '-r' for
>>> >>      the others.
>>> >>
>>> >> It isn't intended for  __start_SECNAME and __stop_SECNAME.  Check
>>> >> config.build_constructors for __start_SECNAME and __stop_SECNAME
>>> >> was introduced by:
>>> >>
>>> >> commit 71bfc0aef6964c54b8e29466e97fb246cdeb2049
>>> >> Author: Alan Modra <amodra@gmail.com>
>>> >> Date:   Thu Sep 7 07:08:58 2000 +0000
>>> >>
>>> >>     Fix list handling for orphan section output statements.
>>> >
>>> > That's a long time ago.  Prior to that patch, ld defined __start and
>>> > __stop symbols for ld -r, which can lead to unexpected results for the
>>> > unwary.  I can't remember, but I suspect I changed ld to define the
>>> > symbols for -Ur so that people who wanted the previous ld -r behaviour
>>> > for __start and __stop could get that by using ld -Ur.
>>> >
>>>
>>> If it is the case, it isn't intended usage for ld -Ur and people shouldn't
>>> depend on it.
>>
>> Please leave that behaviour unchanged.
>>
>
> I am checking this patch to verify it and I will update my patch.
>

This is the patch I am checking in.

Thanks.

-- 
H.J.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0001-Always-define-referenced-__start_SECNAME-__stop_SECN.patch
Type: text/x-patch
Size: 42969 bytes
Desc: not available
URL: <https://sourceware.org/pipermail/binutils/attachments/20170613/5a9dff27/attachment.bin>