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SubjectRe: [PATCH 2/2] vmlinux.lds.h: add HEADERED_SECTION_* macros
Date
> From: Jim Cromie <jim.cromie@gmail.com>
> Date: Wed, 16 Nov 2022 17:20:22 -0700
>
> > These macros elaborate on BOUNDED_SECTION_(PRE|POST)_LABEL macros,
> > prepending an optional KEEP(.gnu.linkonce##_sec_) reservation, and a
> > linker-symbol to address it.
> >
> > This allows a developer to define a header struct (which must fit with
> > the section's base struct-type), and could contain:
>
> [...]
>
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index 85d5d5b203dc..a3b6aa30a525 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -214,6 +214,21 @@
> >
> > #define BOUNDED_SECTION(_sec) BOUNDED_SECTION_BY(_sec, _sec)
> >

Also, those two pasting ops below:

> > +#define HEADERED_SECTION_PRE_LABEL(_sec_, _label_, _BEGIN_, _END_, _HDR_) \
> > + _HDR_##_label_ = .; \
> > + KEEP(*(.gnu.linkonce.##_sec_)) \

^^

> > + BOUNDED_SECTION_PRE_LABEL(_sec_, _label_, _BEGIN_, _END_)
> > +
> > +#define HEADERED_SECTION_POST_LABEL(_sec_, _label_, _BEGIN_, _END_, _HDR_) \
> > + _label_##_HDR_ = .; \
> > + KEEP(*(.gnu.linkonce.##_sec_)) \

^^

will produce incorrect results when @_sec_ starts with a dot (not
a rare thing for section names):

HEADERED_SECTION_BY(.entry_sites, _entry_sites)

vmlinux.map:

.entry_sites 0xffffffff83c89ca0 0x37118 load address 0x0000000003c89ca0
0xffffffff83c89ca0 __header_entry_sites = .
*(.gnu.linkonce. .entry_sites)

here ^^^

.entry_sites 0xffffffff83c89ca0 0x37118 vmlinux.o
0xffffffff83cc0db8 __start_entry_sites = .
*(.entry_sites)
0xffffffff83cc0db8 __stop_entry_sites = .

^ as a result, all .entry_sites entries went to the "header" instead
of the "body".

Sorta dangerous stuff as for me, maybe that could be handled
differently?

> > + BOUNDED_SECTION_POST_LABEL(_sec_, _label_, _BEGIN_, _END_)
> > +
> > +#define HEADERED_SECTION_BY(_sec_, _label_) \
> > + HEADERED_SECTION_PRE_LABEL(_sec_, _label_, __start, __stop)
>
> Now HEADERED_SECTION_PRE_LABEL() takes 5 arguments, but this line
> passes only 4 to it. This went unnoticed probably due to that the
> macro is not used anywhere, thus can't trigger a compiler error.
> Would you prefer to fix it yourself or me to send the fix?[*]
>
> > +
> > +#define HEADERED_SECTION(_sec) HEADERED_SECTION_BY(_sec, _sec)
> > +
> > #ifdef CONFIG_TRACE_BRANCH_PROFILING
> > #define LIKELY_PROFILE() \
> > BOUNDED_SECTION_BY(_ftrace_annotated_branch, _annotated_branch_profile)
> > --
> > 2.38.1
>
> [*] If it needs fixing at all -- some people over the MLs say that
> if there's no trigger, then there's nothing to fix :clownface:
>
> Thanks,
> Olek

Thanks,
Olek

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