linux-kernel - [PATCH v3 10/15] objtool: Add support for relative switch tables

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Date:   Fri, 23 Jun 2023 19:05:21 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        Naveen N Rao <naveen@...nel.org>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v3 10/15] objtool: Add support for relative switch tables

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

      RELOCATION RECORDS FOR [.rodata]:
      OFFSET   TYPE              VALUE
      00000000 R_PPC_REL32       .text+0x0000054c
      00000004 R_PPC_REL32       .text+0x000003d0
	...

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

      RELOCATION RECORDS FOR [.rodata..c_jump_table]:
      OFFSET   TYPE              VALUE
      00000000 R_PPC_ADDR32      .text+0x00000dc8
      00000004 R_PPC_ADDR32      .text+0x00000dc8
	...

Add support to objtool for relative tables, based on the relocation
type which is R_PPC_REL32 for switch tables and R_PPC_ADDR32 for
C jump tables. Do the comparison using R_ABS32 and R_ABS64 which are
architecture agnostic.

And use correct size for 'long' instead of hard coding a size of '8'.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 tools/objtool/check.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index ae4a1608d97e..05d789e6d3b6 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1988,7 +1988,7 @@ static int add_jump_table(struct objtool_file *file, struct instruction *insn,
 	struct symbol *pfunc = insn_func(insn)->pfunc;
 	struct reloc *table = insn_jump_table(insn);
 	struct instruction *dest_insn;
-	unsigned int prev_offset = 0;
+	unsigned int offset, prev_offset = 0;
 	struct reloc *reloc = table;
 	struct alternative *alt;
 
@@ -2003,7 +2003,7 @@ static int add_jump_table(struct objtool_file *file, struct instruction *insn,
 			break;
 
 		/* Make sure the table entries are consecutive: */
-		if (prev_offset && reloc_offset(reloc) != prev_offset + 8)
+		if (prev_offset && reloc_offset(reloc) != prev_offset + elf_addr_size(file->elf))
 			break;
 
 		/* Detect function pointers from contiguous objects: */
@@ -2011,7 +2011,12 @@ static int add_jump_table(struct objtool_file *file, struct instruction *insn,
 		    reloc_addend(reloc) == pfunc->offset)
 			break;
 
-		dest_insn = find_insn(file, reloc->sym->sec, reloc_addend(reloc));
+		if (reloc_type(reloc) == R_ABS32 || reloc_type(reloc) == R_ABS64)
+			offset = reloc_addend(reloc);
+		else
+			offset = reloc_addend(reloc) + reloc_offset(table) - reloc_offset(reloc);
+
+		dest_insn = find_insn(file, reloc->sym->sec, offset);
 		if (!dest_insn)
 			break;
 
-- 
2.40.1