Re: [PATCH v2 10/14] objtool: Add support for relative switch tables
From: Peter Zijlstra <peterz@infradead.org>
Date: 2023-06-22 11:48:29
Also in:
lkml
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.
quoted hunk ↗ jump to hunk
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; };