[PATCH] ld: Make --dynamic-list* override -Bsymbolic -Bsymbolic-functions

[Fangrui Song]

On 2020-05-23, H.J. Lu wrote:
>On Sat, May 23, 2020 at 6:05 PM Fangrui Song <maskray@google.com> wrote:
>>
>> On 2020-05-23, H.J. Lu wrote:
>> >On Sat, May 23, 2020 at 5:11 PM Fangrui Song <maskray@google.com> wrote:
>> >>
>> >> On 2020-05-23, Fangrui Song wrote:
>> >> >On 2020-05-23, H.J. Lu wrote:
>> >> >>On Fri, May 22, 2020 at 8:56 AM Fangrui Song <maskray@google.com> wrote:
>> >> >>>
>> >> >>>
>> >> >>>On 2020-05-22, H.J. Lu wrote:
>> >> >>>>On Wed, May 20, 2020 at 6:48 PM Fangrui Song via Binutils
>> >> >>>><binutils@sourceware.org> wrote:
>> >> >>>>>
>> >> >>>>> They aren't used together in reality, so it is safe to change the
>> >> >>>>> semantics. --dynamic-list is refined -Bsymbolic. -Bsymbolic-functions
>> >> >>>>> should be considered as a subset of -Bsymbolic, so --dynamic-list
>> >> >>>>> overridding -Bsymbolic implies that --dynamic-list overridding
>> >> >>>>> -Bsymbolic-functions.
>> >> >>>>
>> >> >>>>--dynamic-list=DYNAMIC-LIST-FILE'
>> >> >>>>     Specify the name of a dynamic list file to the linker.  This is
>> >> >>>>     typically used when creating shared libraries to specify a list of
>> >> >>>>     global symbols whose references shouldn't be bound to the
>> >> >>>>     definition within the shared library, or creating dynamically
>> >> >>>>     linked executables to specify a list of symbols which should be
>> >> >>>>     added to the symbol table in the executable.  This option is only
>> >> >>>>     meaningful on ELF platforms which support shared libraries.
>> >> >>>>
>> >> >>>>The --dynamic-list* options are intended for shared libraries.
>> >> >>>
>> >> >>>--dynamic-list* work for both executables and shared libraries.
>> >> >>>For an executable, export some symbols.
>> >> >>>For a shared library, specify preemptible symbols.
>> >> >>>
>> >> >>>>The goal
>> >> >>>>IS NOT put them in dynamic symbol table since all global symbols are in
>> >> >>>>dynamic symbol table already in a shared library.  The goal is to make
>> >> >>>>them PREEMPTIBLE.
>> >> >>>
>> >> >>>I agree with this sentence and the patch respects it.
>> >> >>>
>> >> >>>>So making the --dynamic-list* options override -Bsymbolic
>> >> >>>>and -Bsymbolic-functions is incorrect since there is NOTHING to override.
>> >> >>>
>> >> >>>I don't agree with this statement. --dynamic-list* have function overlay
>> >> >>>with -Bsymbolic. When two options overlap in functionality, many users
>> >> >>>expect the more fine-grained option to win, thus my thought that
>> >> >>>--dynamic-list* override -Bsymbolic and -Bsymbolic-functions.
>> >> >>>
>> >> >>>(FWIW LLD's preemptibility logic is
>> >> >>>   ...
>> >> >>>   if (config->hasDynamicList)
>> >> >>>     return sym.inDynamicList;
>> >> >>>   return !(config->bsymbolic || config->bsymbolicFunctions && sym.isFunc());
>> >> >>>   This works fine and I won't change it.
>> >> >>>)
>> >> >>>
>> >> >>>>To do it properly:
>> >> >>>>
>> >> >>>>1.  Extend  --dynamic-list* to executables.  Symbol binding is unchanged.
>> >> >>>>Only add symbols to dynamic symbol table.
>> >> >>>
>> >> >>>This was already implemented when you implemented --dynamic-list in
>> >> >>>2006. This patch does not change the fact.
>> >> >>>
>> >> >>>>2. Make --export-dynamic-symbol an alias of  --dynamic-list* for executables.
>> >> >>>>and ignore --export-dynamic-symbol for -shared.  It has to be done at the
>> >> >>>>end of command-line parsing.
>> >> >>
>> >> >>There is no need to change linker manual.  --dynamic-list* should work both
>> >> >>before and after -Bsymbolic and -Bsymbolic-functions.  Your patch used
>> >> >>libdl2d.so without creating it first.
>> >> >>
>> >> >>Here is the updated version of your patch.  I took the liberty to make
>> >> >>the changes above.
>> >> >>
>> >> >>--
>> >> >>H.J.
>> >> >
>> >> >Thanks for the fix (libdl4e.so->libdl2d.so)
>> >> >
>> >> >The updated patch looks good to me.
>> >>
>> >> Around lexsup.c:1162
>> >>
>> >> `case symbolic_functions:` should set link_info.dynamic_data and link_info.dynamic , otherwise
>> >> -Bsymbolic-functions is broken.
>> >>
>> >>
>> >> .globl _start, foo
>> >> _start:
>> >>    call foo
>> >> foo
>> >>
>> >> ld-new -shared -Bsymbolic-functions a.o -o a.so
>> >>
>> >> foo@plt is not created
>> >
>> >'-Bsymbolic-functions'
>> >     When creating a shared library, bind references to global function
>> >     symbols to the definition within the shared library, if any.  This
>> >     option is only meaningful on ELF platforms which support shared
>> >     libraries.
>> >
>> >There should be no foo@plt.
>>
>> Sorry, my example missed .type:
>>
>> .global _start, foo
>> .type foo, @function
>> _start:
>>    call foo
>> foo:
>>
>>
>> ld-new -shared -Bsymbolic-functions a.o -o a.so
>> foo@plt was created without the patch.
>
>On master branch, I got
>
>[hjl@gnu-cfl-2 pr26018]$ cat func.s
>.global _start, foo
>.type foo, %function
>.text
>_start:
>call foo
>foo:
>ret
>[hjl@gnu-cfl-2 pr26018]$ make func.so
>as   -o func.o func.s
>./ld -shared  -Bsymbolic-function -o func.so func.o
>[hjl@gnu-cfl-2 pr26018]$ objdump -dw func.so
>
>func.so:     file format elf64-x86-64
>
>
>Disassembly of section .text:
>
>0000000000001000 <_start>:
>    1000: e8 00 00 00 00        callq  1005 <foo>
>
>0000000000001005 <foo>:
>    1005: c3                    retq
>[hjl@gnu-cfl-2 pr26018]$
>
>It looks OK to me.

At master, foo is bound locally (intended).
With this patch, foo@plt is created

Fix:

>> >> `case symbolic_functions:` should set link_info.dynamic_data and link_info.dynamic , otherwise
>> >> -Bsymbolic-functions is broken.