[patch 06/38] objtool: Allow GS relative relocs

Thomas Gleixner
2022-07-16T23:17:42Z

From: Peter Zijlstra <peterz@infradead.org>

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

Allow this.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
tools/objtool/arch/x86/decode.c | 26 +++++++++++++++++++++-----
tools/objtool/check.c | 6 ++----
tools/objtool/include/objtool/arch.h | 4 +---
tools/objtool/include/objtool/check.h | 20 +++++++++++---------
4 files changed, 35 insertions(+), 21 deletions(-)

--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -103,24 +103,37 @@ unsigned long arch_jump_destination(stru
#define rm_is_mem(reg) (mod_is_mem() && !is_RIP() && rm_is(reg))
#define rm_is_reg(reg) (mod_is_reg() && modrm_rm == (reg))

-static bool has_notrack_prefix(struct insn *insn)
+static bool has_prefix(struct insn *insn, u8 prefix)
{
int i;

for (i = 0; i < insn->prefixes.nbytes; i++) {
- if (insn->prefixes.bytes[i] == 0x3e)
+ if (insn->prefixes.bytes[i] == prefix)
return true;
}

return false;
}

+static bool has_notrack_prefix(struct insn *insn)
+{
+ return has_prefix(insn, 0x3e);
+}
+
+static bool has_gs_prefix(struct insn *insn)
+{
+ return has_prefix(insn, 0x65);
+}
+
int arch_decode_instruction(struct objtool_file *file, const struct section *sec,
unsigned long offset, unsigned int maxlen,
- unsigned int *len, enum insn_type *type,
- unsigned long *immediate,
- struct list_head *ops_list)
+ struct instruction *instruction)
{
+ struct list_head *ops_list = &instruction->stack_ops;
+ unsigned long *immediate = &instruction->immediate;
+ enum insn_type *type = &instruction->type;
+ unsigned int *len = &instruction->len;
+
const struct elf *elf = file->elf;
struct insn insn;
int x86_64, ret;
@@ -149,6 +162,9 @@ int arch_decode_instruction(struct objto
if (insn.vex_prefix.nbytes)
return 0;

+ if (has_gs_prefix(&insn))
+ instruction->alt_reloc_safe = 1;
+
prefix = insn.prefixes.bytes[0];

op1 = insn.opcode.bytes[0];
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -396,9 +396,7 @@ static int decode_instructions(struct ob

ret = arch_decode_instruction(file, sec, offset,
sec->sh.sh_size - offset,
- &insn->len, &insn->type,
- &insn->immediate,
- &insn->stack_ops);
+ insn);
if (ret)
goto err;

@@ -1620,7 +1618,7 @@ static int handle_group_alt(struct objto
* accordingly.
*/
alt_reloc = insn_reloc(file, insn);
- if (alt_reloc &&
+ if (alt_reloc && !insn->alt_reloc_safe &&
!arch_support_alt_relocation(special_alt, insn, alt_reloc)) {

WARN_FUNC("unsupported relocation in alternatives section",
--- a/tools/objtool/include/objtool/arch.h
+++ b/tools/objtool/include/objtool/arch.h
@@ -73,9 +73,7 @@ void arch_initial_func_cfi_state(struct

int arch_decode_instruction(struct objtool_file *file, const struct section *sec,
unsigned long offset, unsigned int maxlen,
- unsigned int *len, enum insn_type *type,
- unsigned long *immediate,
- struct list_head *ops_list);
+ struct instruction *insn);

bool arch_callee_saved_reg(unsigned char reg);

--- a/tools/objtool/include/objtool/check.h
+++ b/tools/objtool/include/objtool/check.h
@@ -47,15 +47,17 @@ struct instruction {
unsigned long immediate;

u16 dead_end : 1,
- ignore : 1,
- ignore_alts : 1,
- hint : 1,
- save : 1,
- restore : 1,
- retpoline_safe : 1,
- noendbr : 1,
- entry : 1;
- /* 7 bit hole */
+ ignore : 1,
+ ignore_alts : 1,
+ hint : 1,
+ save : 1,
+ restore : 1,
+ retpoline_safe : 1,
+ noendbr : 1,
+ entry : 1,
+ alt_reloc_safe : 1;
+
+ /* 6 bit hole */

s8 instr;
u8 visited;