[PATCH v2] Support for SHF_GNU_RETAIN ELF Section Flag

[H.J. Lu]

On Tue, Sep 29, 2020 at 12:38 PM Fangrui Song <i@maskray.me> wrote:
>

> >>
> >>   // a.h
> >>   __attribute__((section("sec")))
> >>   inline void bar() { ... }
> >>   // a.c
> >>   #include "a.h"
> >>   __attribute__((section("sec"), retain))
> >>   void foo() {
> >>   }
> >>
> >> compiles to
> >>
> >>   .section sec,"a",@progbits
> >>   ...
> >>   .section sec,"aR",@progbits
> >>   ...
> >>
> >> You will get a gas error for changing section flags
> >> (https://sourceware.org/pipermail/binutils/2020-February/109945.html)
> >>
> >
> >There is no error in this case.
> >The original patch handled it, and it has been further improved in v2.
> >There's even a testcase testing exactly the functionality you say is
> >broken.
> >
> >  gas/testsuite/gas/elf/section22.s:
> >      .section .bss,"aw"
> >    ...
> >      .section .bss,"awR",%nobits
> >
> >I already gave a detailed response to your reloc proposal:
> >https://sourceware.org/pipermail/binutils/2020-September/113450.html
>
> Letting the following assembly creates two .bss sections adds an exception to
> "gas: error for section type, attr, or entsize change"
> (https://sourceware.org/pipermail/binutils/2020-February/109945.html)
> for which I have achieved consensus with Alan Modra and some LLVM
> toolchain folks. That is why I hope we could re-consider this design
> choice.
>
> .section        .bss,"aw"
> .section        .bss,"awR",%nobits
>

FWIW, GNU assembler has been able to create multiple sections with
the same section name for a while.  For SHF_GNU_RETAIN, we added

+/* Create unique input sections for sections with the same name, but different
+   values for the flags in this mask.  */
+#define SEC_ASSEMBLER_SHF_MASK SHF_GNU_RETAIN

to control when to create a new section with the same section name.
For other cases, no new section is created.

-- 
H.J.