[tip: x86/core] objtool: Allow !PC relative relocations

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     61c6065ef7ec0447a280179d04b2d81c80c2f479
Gitweb:        https://git.kernel.org/tip/61c6065ef7ec0447a280179d04b2d81c80c2f479
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 15 Sep 2022 13:11:07 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 17 Oct 2022 16:41:06 +02:00

objtool: Allow !PC relative relocations

Objtool doesn't currently much like per-cpu usage in alternatives:

arch/x86/entry/entry_64.o: warning: objtool: .altinstr_replacement+0xf: unsupported relocation in alternatives section
  f:   65 c7 04 25 00 00 00 00 00 00 00 80     movl   $0x80000000,%gs:0x0      13: R_X86_64_32S        __x86_call_depth

Since the R_X86_64_32S relocation is location invariant (it's
computation doesn't include P - the address of the location itself),
it can be trivially allowed.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20220915111145.806607235@xxxxxxxxxxxxx
---
 tools/objtool/arch/x86/decode.c      | 24 ++++++++++++++++++++++++
 tools/objtool/check.c                |  2 +-
 tools/objtool/include/objtool/arch.h |  2 ++
 3 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index 1c253b4..f094383 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -73,6 +73,30 @@ unsigned long arch_jump_destination(struct instruction *insn)
 	return insn->offset + insn->len + insn->immediate;
 }
 
+bool arch_pc_relative_reloc(struct reloc *reloc)
+{
+	/*
+	 * All relocation types where P (the address of the target)
+	 * is included in the computation.
+	 */
+	switch (reloc->type) {
+	case R_X86_64_PC8:
+	case R_X86_64_PC16:
+	case R_X86_64_PC32:
+	case R_X86_64_PC64:
+
+	case R_X86_64_PLT32:
+	case R_X86_64_GOTPC32:
+	case R_X86_64_GOTPCREL:
+		return true;
+
+	default:
+		break;
+	}
+
+	return false;
+}
+
 #define ADD_OP(op) \
 	if (!(op = calloc(1, sizeof(*op)))) \
 		return -1; \
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 43ec14c..7174bba 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1645,7 +1645,7 @@ static int handle_group_alt(struct objtool_file *file,
 		 * accordingly.
 		 */
 		alt_reloc = insn_reloc(file, insn);
-		if (alt_reloc &&
+		if (alt_reloc && arch_pc_relative_reloc(alt_reloc) &&
 		    !arch_support_alt_relocation(special_alt, insn, alt_reloc)) {
 
 			WARN_FUNC("unsupported relocation in alternatives section",
diff --git a/tools/objtool/include/objtool/arch.h b/tools/objtool/include/objtool/arch.h
index beb2f3a..fe2ea4b 100644
--- a/tools/objtool/include/objtool/arch.h
+++ b/tools/objtool/include/objtool/arch.h
@@ -93,4 +93,6 @@ bool arch_is_rethunk(struct symbol *sym);
 
 int arch_rewrite_retpolines(struct objtool_file *file);
 
+bool arch_pc_relative_reloc(struct reloc *reloc);
+
 #endif /* _ARCH_H */