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[PATCH] Issue an error for GC on __patchable_function_entries section

H.J. Lu hjl.tools@gmail.com
Sun Feb 2 23:44:00 GMT 2020 
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.
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