[PATCH] Issue an error for GC on __patchable_function_entries section

Fangrui Song i@maskray.me
Mon Feb 3 00:57:00 GMT 2020


On 2020-02-02, H.J. Lu wrote:
>On Sat, Feb 1, 2020 at 10:21 AM Fangrui Song <i@maskray.me> wrote:
>>
>> On 2020-02-01, H.J. Lu wrote:
>> >On Sat, Feb 1, 2020 at 9:34 AM Fangrui Song <i@maskray.me> wrote:
>> >>
>> >> On 2020-02-01, H.J. Lu wrote:
>> >> >After all text sections have been garbage collected, if a
>> >> >__patchable_function_entries section references a section which
>> >> >wasn't marked, mark it with SEC_EXCLUDE and return NULL.  Otherwise,
>> >> >keep it.
>> >> >
>> >> >Should it be handled in _bfd_elf_gc_mark_extra_sections?
>> >>
>> >> Thanks for paying attention to these feature requests.
>> >>
>> >> I referenced GNU as and ld requests at
>> >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93492#c2
>> >> If we
>> >>
>> >> * implement SHF_LINK_ORDER
>> >
>> >I am not sure if overloading (abusing?) SHF_LINK_ORDER is a good idea.
>>
>> It is an extension, but it is agreed by multiple parties on
>> https://groups.google.com/d/msg/generic-abi/_CbBM6T6WeM/eGF9A0AnAAAJ ,
>> including HP-UX and Solaris developers.
>>
>> >> * allow multiple sections with the same name ("unique")
>> >
>> >This is orthogonal to this.  I have a question on assembly syntax:
>> >
>> >https://sourceware.org/bugzilla/show_bug.cgi?id=25380#c1
>> >
>> >> * teach GCC to use SHF_LINK_ORDER and "unique" (see https://gcc.gnu.org/ml/gcc/2020-01/msg00067.html)
>> >>
>> >> An ad-hoc gc marking will be unnecessary.
>> >
>> >We need to scan relocations in _patchable_function_entries section for
>> >references to garbage collected sections.   We can either check section
>> >name or a SHF_XXX.  But I don't know if SHF_LINK_ORDER is a good
>> >approach.
>>
>> We don't need to if we use multiple __patchable_function_entries
>> sections. Multiple sections are a must due to
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93195#c1 (COMDAT)
>>
>> > A symbol table entry with STB_LOCAL binding that is defined relative
>> > to one of a group's sections, and that is contained in a symbol table
>> > section that is not part of the group, must be discarded if the group
>> > members are discarded. References to this symbol table entry from
>> > outside the group are not allowed.
>>
>> The __patchable_function_entries creation logic I implemented in clang:
>>
>> foreach function foo
>>    find the first function label defined in foo's section, name it $associated
>>      ($associated can have 2 reasonable values, w/ or w/o -ffunction-sections)
>>    get or create an id for $associated, say, $unique
>>
>>    if foo is in a COMDAT named $comdat
>>      .section __patchable_function_entries,"awo",@progbits,$associated,comdat,$comdat,unique,$unique
>>    else
>>      .section __patchable_function_entries,"awo",@progbits,$associated,unique,$unique
>>
>> This approach uses feature requests I referenced in *direct* links of
>> https://gcc.gnu.org/ml/gcc/2020-01/msg00067.html plus
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93492#c2 ,
>> and addresses all bugs I filed.
>>
>
>Here is a linker patch to issue an error to avoid corrupt
>linker output.
>
>-- 
>H.J.

A typo in the description.

- .section __patchable_function_entries,"awo",%progbits,_start
+ .section __patchable_function_entries,"aw",%progbits

GCC's output is the below (no SHF_LINK_ORDER).

I don't have an opinion whether !SHF_LINK_ORDER should be worked around in GNU ld.
CC Szabolcs who fixed AArch64 BTI https://gcc.gnu.org/ml/gcc-patches/2020-01/msg01942.html



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