[PATCH] Always define referenced __start_SECNAME/__stop_SECNAME

[H.J. Lu]

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

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.

diff --git a/ld/ChangeLog b/ld/ChangeLog
index e9ca684..0292f83 100644
--- a/ld/ChangeLog
+++ b/ld/ChangeLog
@@ -1,3 +1,10 @@
+2000-09-07  Alan Modra  <alan@linuxcare.com.au>
+
+  * emultempl/elf32.em (gld${EMULATION_NAME}_place_orphan): Fix
+  broken list handling.  Create __start_SECNAME and __stop_SECNAME
+  when no place-holder.  Add some comments.  Test both SEC_CODE and
+  SEC_READONLY for hold_text to prevent .rodata orphan poisoning.

Do you remember if it was done on purpose to generate
__start_SECNAME and __stop_SECNAME for -Ur? If yes,
we can add your use case as a testcase.

Here is the updated patch to address

https://sourceware.org/bugzilla/show_bug.cgi?id=21571


-- 
H.J.
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