[llvm-dev] LLD : __start_ and __end_ symbols for orphan sections

[Petr Hosek via llvm-dev]

In our case, we'd prefer the opposite that is for __start/__stop symbols to
be always hidden (which is something Rafael also pointed out in
https://reviews.llvm.org/D44566#1041984), currently this also requires a
linker script on our side which is unfortunate. I have created a change
https://reviews.llvm.org/D55682 to provide the -z hidden-start-stop-symbol
but this was rejected. I'm wondering if it'd make sense to extend that
change to instead provide -z start-stop-symbol-visibility= which could be
used to override visibility for __start/__stop symbol (i.e. default or
hidden) which would match the frontend -fvisibility= flag. Would that be
acceptable?

On Tue, Jun 2, 2020 at 10:05 AM Moshtaghi, Alireza via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Okay
> Thank you
>
> On 6/2/20, 8:37 AM, "Fangrui Song" <maskray at google.com> wrote:
>
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>
>
>
>     On 2020-06-02, Moshtaghi, Alireza wrote:
>     >Sorry for the cryptic code but I had to modify stuff from original
>     >In the following example see the difference when you comment or
> uncomment the line in the linker script:
>     >============ test.c ============= :
>     >struct orphan_dummy_anno_s {
>     > void (*func)(void);
>     >};
>     >
>     >static void dummy_export_dbg_log_init_f (void) __attribute__
> ((unused));
>     >static struct orphan_dummy_anno_s const
>     >  dummy_export_dbg_log_init_linker_set =
>     >  { dummy_export_dbg_log_init_f, };
>     >__asm__ (".globl " "__start_set_orphan_dummy_anno");
>     >__asm__ (".globl " "__stop_set_orphan_dummy_anno");
>     >static void const
>     >  *__set_orphan_dummy_anno_sym_dummy_export_dbg_log_init_linker_set
>     >  __attribute__ ((__section__ ("set_" "orphan_dummy_anno")))
>     >  __attribute__ ((__used__)) = &dummy_export_dbg_log_init_linker_set;
>     >============ linker_script ============= :
>     >SECTIONS {
>     >   data : { *(data) }
>     ># when commented, the __start and __stop symbols will be protected.
> How can I keep it from setting it as protected?
>     >#   set_orphan_dummy_anno : { PROVIDE(__start_set_orphan_dummy_anno =
> . ); *(set_orphan_dummy_anno) PROVIDE (__stop_set_orphan_dummy_anno = . );}
>     >}
>     >============ howto ============= :
>     >LDFLAGS="-Bshareable -T ./linker_script"
>     >clang -fPIC -c test.c
>     >ld.lld $LDFLAGS test.o -o test.so
>     >objdump -tT test.so
>
>     Making __start_* __stop_* STV_PROTECTED is a deliberate choice. See
>     https://reviews.llvm.org/D44566
>     Such symbols can still be exported to .dynsym (can be looked up from
>     another shared object with dlopen) but prevent accidental symbol
> preemption..
>     >
>     >On 6/2/20, 12:00 AM, "llvm-dev on behalf of Moshtaghi, Alireza via
> llvm-dev" <llvm-dev-bounces at lists.llvm.org on behalf of
> llvm-dev at lists.llvm.org> wrote:
>     >
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> links or open attachments unless you recognize the sender and know the
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>     >
>     >
>     >
>     >
>     >    You are right it creates them but sets the protected flag
> (STV_PROTECTED) which seems to be the cause of my problem.
>     >    How can I tell it to set the flag as STV_DEFAULT?
>     >
>     >    Thanks
>     >    A
>     >
>     >    On 5/28/20, 11:30 PM, "Fangrui Song" <maskray at google.com> wrote:
>     >
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>     >
>     >
>     >
>     >
>     >        On 2020-05-28, Moshtaghi, Alireza via llvm-dev wrote:
>     >        >lld does not seem to create the __start and __end symbols
> for orphan sections.
>     >        >I would like to keep my linker script as generic as possible
> so how can I tell lld to create these symbols without having to add them in
> the linker script?
>     >        >
>     >        >Thanks
>     >        >A
>     >
>     >        It works for me.
>     >
>     >        SECTIONS {
>     >           data : { *(data) }
>     >        }
>     >
>     >        .section data,"aw"
>     >        .quad __start_orphan
>     >        .quad __stop_orphan
>     >
>     >        .section orphan,"aw"
>     >        .quad __start_data
>     >        .quad __stop_data
>     >
>     >
>     >        Can you give an example __start_ or __end_ symbols are not
> defined for orphan sections?
>     >
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