From: Rusty Russell <rusty@rustcorp.com.au>
To: Miroslav Benes <mbenes@suse.cz>
Cc: linux-kernel@vger.kernel.org, Miroslav Benes <mbenes@suse.cz>
Subject: Re: [PATCH] module: keep percpu symbols in module's symtab
In-Reply-To: <1448024043-29578-1-git-send-email-mbenes@suse.cz>
References: <1448024043-29578-1-git-send-email-mbenes@suse.cz>
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Date: Wed, 25 Nov 2015 12:09:13 +1030
Message-ID: <87a8q2yi0u.fsf@rustcorp.com.au>
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Miroslav Benes <mbenes@suse.cz> writes:
> Currently, percpu symbols from .data..percpu ELF section of a module are
> not copied over and stored in final symtab array of struct module.
> Consequently such symbol cannot be returned via kallsyms API (for
> example kallsyms_lookup_name). This can be especially confusing when the
> percpu symbol is exported. Only its __ksymtab et al. are present in its
> symtab.
>
> The culprit is in layout_and_allocate() function where SHF_ALLOC flag is
> dropped for .data..percpu section. There is in fact no need to copy the
> section to final struct module, because kernel module loader allocates
> extra percpu section by itself. Unfortunately only symbols from
> SHF_ALLOC sections are copied (see is_core_symbol()).
>
> The patch restores SHF_ALLOC flag for original percpu section. The
> section with its symbols is thus copied over, but not otherwise used.
> st_value of percpu symbols points to correct newly allocated section
> thanks to correction in simplify_symbols().
>
> Signed-off-by: Miroslav Benes <mbenes@suse.cz>
> ---
>
> I don't deem the solution nice. The other one I came up with was to hack
> is_core_symbol() to copy percpu symbols. There is a catch though.
> Elf_Sym's st_shndx is an index to an associated section. If we do not
> preserve .data..percpu section the index would be invalid. But this is
> similar to other symbols as well I guess. The index is never valid after
> move_module(), right? The only relevant check I found in the kernel is
> in get_ksymbol() - 'st_shndx == SHN_UNDEF'. So it could be harmless.

Yes, you should do this instead, I think.

Cheers,
Rusty.
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