The CONFIG_X86_ESPFIX64 version of exc_double_fault() can return to its
caller, but the !CONFIG_X86_ESPFIX64 version never does. In the latter
case the compiler and/or objtool may consider it to be implicitly
noreturn.
However, due to the currently inflexible way objtool detects noreturns,
a function's noreturn status needs to be consistent across configs.
The current workaround for this issue is to suppress unreachable
warnings for exc_double_fault()'s callers. Unfortunately that can
result in ORC coverage gaps and potentially worse issues like inert
static calls and silently disabled CPU mitigations.
Instead, prevent exc_double_fault() from ever being implicitly marked
noreturn by forcing a return behind a never-taken conditional.
Until a more integrated noreturn detection method exists, this is likely
the least objectionable workaround.
Fixes: 55eeab2a8a11 ("objtool: Ignore exc_double_fault() __noreturn warnings")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
arch/x86/kernel/traps.c | 18 +++++++++++++++++-
tools/objtool/check.c | 31 +------------------------------
2 files changed, 18 insertions(+), 31 deletions(-)
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index f4263cb3d21e..9f88b8a78e50 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -461,6 +461,21 @@ __visible void __noreturn handle_stack_overflow(struct pt_regs *regs,
}
#endif
+/*
+ * Prevent the compiler and/or objtool from marking the !CONFIG_X86_ESPFIX64
+ * version of exc_double_fault() as noreturn. Otherwise the noreturn mismatch
+ * between configs triggers objtool warnings.
+ *
+ * This is a temporary hack until we have compiler or plugin support for
+ * annotating noreturns.
+ */
+#ifdef CONFIG_X86_ESPFIX64
+#define always_true() true
+#else
+bool always_true(void);
+bool __weak always_true(void) { return true; }
+#endif
+
/*
* Runs on an IST stack for x86_64 and on a special task stack for x86_32.
*
@@ -596,7 +611,8 @@ DEFINE_IDTENTRY_DF(exc_double_fault)
pr_emerg("PANIC: double fault, error_code: 0x%lx\n", error_code);
die("double fault", regs, error_code);
- panic("Machine halted.");
+ if (always_true())
+ panic("Machine halted.");
instrumentation_end();
}
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 814d40724069..3a3a1a905199 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -4441,35 +4441,6 @@ static int validate_sls(struct objtool_file *file)
return warnings;
}
-static bool ignore_noreturn_call(struct instruction *insn)
-{
- struct symbol *call_dest = insn_call_dest(insn);
-
- /*
- * FIXME: hack, we need a real noreturn solution
- *
- * Problem is, exc_double_fault() may or may not return, depending on
- * whether CONFIG_X86_ESPFIX64 is set. But objtool has no visibility
- * to the kernel config.
- *
- * Other potential ways to fix it:
- *
- * - have compiler communicate __noreturn functions somehow
- * - remove CONFIG_X86_ESPFIX64
- * - read the .config file
- * - add a cmdline option
- * - create a generic objtool annotation format (vs a bunch of custom
- * formats) and annotate it
- */
- if (!strcmp(call_dest->name, "exc_double_fault")) {
- /* prevent further unreachable warnings for the caller */
- insn->sym->warned = 1;
- return true;
- }
-
- return false;
-}
-
static int validate_reachable_instructions(struct objtool_file *file)
{
struct instruction *insn, *prev_insn;
@@ -4486,7 +4457,7 @@ static int validate_reachable_instructions(struct objtool_file *file)
prev_insn = prev_insn_same_sec(file, insn);
if (prev_insn && prev_insn->dead_end) {
call_dest = insn_call_dest(prev_insn);
- if (call_dest && !ignore_noreturn_call(prev_insn)) {
+ if (call_dest) {
WARN_INSN(insn, "%s() is missing a __noreturn annotation",
call_dest->name);
warnings++;
--
2.48.1