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

[Fangrui Song via llvm-dev]

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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>
>
>
>
>    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:
>
>        NetApp Security WARNING: This is an external email. Do not click links or open attachments unless you recognize the sender and know the content is safe.
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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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