linux-kernel - [PATCH] objtool: Deal with relative jump tables correctly

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Message-ID: <20241007224747.3973322-2-ardb+git@google.com>
Date: Tue,  8 Oct 2024 00:47:48 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: Ard Biesheuvel <ardb@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH] objtool: Deal with relative jump tables correctly

From: Ard Biesheuvel <ardb@...nel.org>

Relative jump tables contain entries that carry the offset between the
target of the jump and the start of the jump table. This permits the PIC
idiom of

    leaq    jump_table(%rip), %tbl
    movslq  (%tbl,%idx,4), %offset
    addq    %offset, %tbl
    jmp     *%tbl

The jump table entries are decorated with PC32 relocations, which record
the offset of the referenced symbol to the target of the relocation.
This means that only the first entry produces the correct value
directly; the subsequent ones need to be corrected to produce the offset
relative to the start of the table accurately, by applying an addend.

Given that the referenced symbols are already expressed in terms of
sections and addends, e.g., .text+0x5df9, the correction is incorporated
into the existing addend. The upshot of this is that chasing the
reference to find the target instruction needs to take this second
addend into account as well.

Cc: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 tools/objtool/arch/x86/special.c |  8 --------
 tools/objtool/check.c            | 14 ++++++++++++--
 2 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/tools/objtool/arch/x86/special.c b/tools/objtool/arch/x86/special.c
index 4ea0f9815fda..415e4d035e53 100644
--- a/tools/objtool/arch/x86/special.c
+++ b/tools/objtool/arch/x86/special.c
@@ -150,13 +150,5 @@ struct reloc *arch_find_switch_table(struct objtool_file *file,
 	if (!rodata_reloc)
 		return NULL;
 
-	/*
-	 * Use of RIP-relative switch jumps is quite rare, and
-	 * indicates a rare GCC quirk/bug which can leave dead
-	 * code behind.
-	 */
-	if (reloc_type(text_reloc) == R_X86_64_PC32)
-		file->ignore_unreachables = true;
-
 	return rodata_reloc;
 }
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 3cb3e9b5ad0b..db5c5069473f 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2111,13 +2111,15 @@ static int add_jump_table(struct objtool_file *file, struct instruction *insn,
 	 * instruction.
 	 */
 	for_each_reloc_from(table->sec, reloc) {
+		bool pcrel = reloc_type(reloc) == R_X86_64_PC32;
+		unsigned long addend = reloc_addend(reloc);
 
 		/* Check for the end of the table: */
 		if (reloc != table && reloc == next_table)
 			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 + (pcrel ? 4 : 8))
 			break;
 
 		/* Detect function pointers from contiguous objects: */
@@ -2125,7 +2127,15 @@ 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));
+		/*
+		 * Place-relative jump tables carry offsets relative to the
+		 * start of the jump table, not to the entry itself. So correct
+		 * the addend for the location of the entry in the table.
+		 */
+		if (pcrel)
+			addend -= reloc_offset(reloc) - reloc_offset(table);
+
+		dest_insn = find_insn(file, reloc->sym->sec, addend);
 		if (!dest_insn)
 			break;
 
-- 
2.47.0.rc0.187.ge670bccf7e-goog