linux-kernel - Re: [PATCH 2/2] vmlinux.lds.h: add HEADERED_SECTION

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Date:   Fri, 16 Dec 2022 19:11:07 +0100
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Jim Cromie <jim.cromie@...il.com>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>, jbaron@...mai.com,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] vmlinux.lds.h: add HEADERED_SECTION_* macros

> From: Jim Cromie <jim.cromie@...il.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