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Re: linker script, /DISCARD/ section and SHT_GROUP
On 2019-11-14, Serge Guelton wrote:
On Thu, Nov 14, 2019@12:30:00PM +0000, Nick Clifton wrote:
Hi Serge,
> I find this behavior relatively strange with respect to group semantic: we end
> up with a group with one of its member stripped, and the others happily living
> around.
>
> Surely, the /DISCARD/ section is incomplete, but I would have expected a
> warning, if not an error, in that case.
>
> Is my understanding correct or am I missing something?
No this is a bug, or possibly a feature. Either the linker documentation
needs to be updated to say that the /DISCARD/ section is allowed to break
ELF section group semantics (a bad idea imho), or else we need to fix the
linker to generate either an error or a warning.
My feeling is that the correct behaviour would be to discard the entire
group, but to also generate a warning indicating that only part of the
group was covered by the /DISCARD/ section.
That's my feeling too. Or an error and suggest to discard the group?
But there is another complication. What should happen if one or more
of the sections are explicitly kept ? Ie:
% cat b.script
SECTIONS
{
/DISCARD/ : { *(.myanote.foo) }
.mytext.foo : { KEEP (*(.mytext.foo)) }
.mynote.foo : { *(.mynote.foo) }
}
% ld -T b.script a.o
Now we have one section in the group that is deliberately discarded,
one that is deliberately kept, one that can be discarded if it is not
referenced, and one that is an orphan. What is the poor linker to do ?
Complain to the user and say that the script cannot be honoured would
be my suggestion, but what do you think ?
Yeah, an error looks like the way to go.
I'd consider it more of a feature. The ELF specification is written
without garbage collection in mind. With the reading that garbage
collection is *orthogonal* to section group selection, we can address some
use cases where particular sections need to be stripped. For example,
cat > a.s <<e
.globl anote_foo, foo, bss_foo, note_foo
.section .myanote.foo,"aG",%note,foo,comdat
anote_foo:
.byte 0
.section .mytext.foo,"axG",%progbits,foo,comdat
foo:
.fnstart
.cantunwind
bx lr
.fnend
.section .mybss.foo,"awG",%nobits,foo,comdat
bss_foo:
.byte 0
.section .mynote.foo,"G",%note,foo,comdat
note_foo:
.byte 0
e
cat > a.script <<e
SECTIONS { /DISCARD/ : { *(.ARM.exidx*) }}
e
# a.o -> a.o
ld --gc-sections a.o -T a.script -e foo -o a
The intention is to have no .ARM.exidx* in the output, though input
object files may include them.
If we place .debug_info or other metadata (whether or not have the
SHF_ALLOC flag) in the group, we can strip them individually without
affecting the whole group.
I am in favor of not having a warning or error, so that such use cases
can work out-of-the-box. Such information could probably be moved to
the --verbose output, if we do find that useful.