linux-kernel - Re: [PATCH v2 10/14] objtool: Add support for relative switch tables

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Date:   Thu, 22 Jun 2023 13:48:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        Naveen N Rao <naveen@...nel.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 10/14] objtool: Add support for relative switch tables

On Thu, Jun 22, 2023 at 12:54:32PM +0200, Christophe Leroy wrote:
> On powerpc, switch tables are relative, than means the address of the
> table is added to the value of the entry in order to get the pointed
> address: (r10 is the table address, r4 the index in the table)
> 
>       lis     r10,0		<== Load r10 with upper part of .rodata address
>           R_PPC_ADDR16_HA     .rodata
>       addi    r10,r10,0		<== Add lower part of .rodata address
>           R_PPC_ADDR16_LO     .rodata
>       lwzx    r8,r10,r4		<== Read table entry at r10 + r4 into r8
>       add     r10,r8,r10	<== Add table address to read value
>       mtctr   r10		<== Save calculated address in CTR
>       bctr			<== Branch to address in CTR
> 
> But for c_jump_tables it is not the case, they contain the
> pointed address directly:
> 
>       lis     r28,0		<== Load r28 with upper .rodata..c_jump_table
>           R_PPC_ADDR16_HA   .rodata..c_jump_table
>       addi    r28,r28,0		<== Add lower part of .rodata..c_jump_table
>           R_PPC_ADDR16_LO   .rodata..c_jump_table
>       lwzx    r10,r28,r10	<== Read table entry at r10 + r28 into r10
>       mtctr   r10		<== Save read value in CTR
>       bctr			<== Branch to address in CTR
> 
> Add support to objtool for relative tables, with a flag in order to
> tell table by table if that table uses relative or absolute addressing.
> 
> And use correct size for 'long' instead of hard coding a size of '8'.

Again, see tip/objtool/core...

This one is going to be a little hard. It would be very good if you can
avoid growing struct reloc; Josh went through a ton of effort to shrink
it.

Would it work to use reloc->sym->is_rel_jumptable ? That still has a few
spare bits in it's bitfield.

> diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
> index e1ca588eb69d..64aac87a4825 100644
> --- a/tools/objtool/include/objtool/elf.h
> +++ b/tools/objtool/include/objtool/elf.h
> @@ -80,6 +80,7 @@ struct reloc {
>  	s64 addend;
>  	int idx;
>  	bool jump_table_start;
> +	bool jump_table_is_rel;
>  };