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SubjectRe: [PATCH 3/5] x86/ibrs: Add direct access support for MSR_IA32_SPEC_CTRL
On Fri, Jan 12, 2018 at 10:09:08AM +0000, David Woodhouse wrote:
> static_cpu_has() + asm-goto is NOT SUFFICIENT.
>
> It's still *possible* for a missed optimisation in GCC to still leave
> us with a conditional branch around the wrmsr, letting the CPU
> speculate around it too.

OK, so GCC would have to be bloody retarded to mess this up; but would
something like the below work for you?

The usage is like:

if (static_branch_unlikely(key)) {
arch_static_assert();
stuff();
}

And then objtool will fail things if the first instruction into that
branch is not immediately after a NOP/JMP patch site (on either the NOP
or the JMP+disp side of things).

---
diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index 8c0de4282659..6a1a893145ca 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -62,6 +62,15 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
return true;
}

+static __always_inline void arch_static_assert(void)
+{
+ asm volatile ("1:\n\t"
+ ".pushsection .discard.jump_assert, \"aw\" \n\t"
+ _ASM_ALIGN "\n\t"
+ _ASM_PTR "1b \n\t"
+ ".popsection \n\t");
+}
+
#ifdef CONFIG_X86_64
typedef u64 jump_label_t;
#else
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index f40d46e24bcc..657bfc706bb6 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -687,8 +687,17 @@ static int handle_jump_alt(struct objtool_file *file,
struct instruction *orig_insn,
struct instruction **new_insn)
{
- if (orig_insn->type == INSN_NOP)
+ struct instruction *next_insn = list_next_entry(orig_insn, list);
+
+ if (orig_insn->type == INSN_NOP) {
+ /*
+ * If orig_insn is a NOP, then new_insn is the branch target
+ * for when it would've been a JMP.
+ */
+ next_insn->br_static = true;
+ (*new_insn)->br_static = true;
return 0;
+ }

if (orig_insn->type != INSN_JUMP_UNCONDITIONAL) {
WARN_FUNC("unsupported instruction at jump label",
@@ -696,7 +705,16 @@ static int handle_jump_alt(struct objtool_file *file,
return -1;
}

- *new_insn = list_next_entry(orig_insn, list);
+ /*
+ * Otherwise, orig_insn is a JMP and it will have orig_insn->jump_dest.
+ * In this case we'll effectively NOP the alt by pointing new_insn at
+ * next_insn.
+ */
+ orig_insn->jump_dest->br_static = true;
+ next_insn->br_static = true;
+
+ *new_insn = next_insn;
+
return 0;
}

@@ -1067,6 +1085,50 @@ static int read_unwind_hints(struct objtool_file *file)
return 0;
}

+static int read_jump_assertions(struct objtool_file *file)
+{
+ struct section *sec, *relasec;
+ struct instruction *insn;
+ struct rela *rela;
+ int i;
+
+ sec = find_section_by_name(file->elf, ".discard.jump_assert");
+ if (!sec)
+ return 0;
+
+ relasec = sec->rela;
+ if (!relasec) {
+ WARN("missing .rela.discard.jump_assert section");
+ return -1;
+ }
+
+ if (sec->len % sizeof(unsigned long)) {
+ WARN("jump_assert size mismatch: %d %ld", sec->len, sizeof(unsigned long));
+ return -1;
+ }
+
+ for (i = 0; i < sec->len / sizeof(unsigned long); i++) {
+ rela = find_rela_by_dest(sec, i * sizeof(unsigned long));
+ if (!rela) {
+ WARN("can't find rela for jump_assert[%d]", i);
+ return -1;
+ }
+
+ insn = find_insn(file, rela->sym->sec, rela->addend);
+ if (!insn) {
+ WARN("can't find insn for jump_assert[%d]", i);
+ return -1;
+ }
+
+ if (!insn->br_static) {
+ WARN_FUNC("static assert FAIL", insn->sec, insn->offset);
+ return -1;
+ }
+ }
+
+ return 0;
+}
+
static int decode_sections(struct objtool_file *file)
{
int ret;
@@ -1105,6 +1167,10 @@ static int decode_sections(struct objtool_file *file)
if (ret)
return ret;

+ ret = read_jump_assertions(file);
+ if (ret)
+ return ret;
+
return 0;
}

diff --git a/tools/objtool/check.h b/tools/objtool/check.h
index dbadb304a410..12e0a3cf0350 100644
--- a/tools/objtool/check.h
+++ b/tools/objtool/check.h
@@ -45,6 +46,7 @@ struct instruction {
unsigned char type;
unsigned long immediate;
bool alt_group, visited, dead_end, ignore, hint, save, restore, ignore_alts;
+ bool br_static;
struct symbol *call_dest;
struct instruction *jump_dest;
struct list_head alts;
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