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

[H.J. Lu]

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.

-- 
H.J.