x86/traps: Make exc_double_fault() consistently noreturn

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Link: https://lore.kernel.org/r/d1f4026f8dc35d0de6cc61f2684e0cb6484009d1.1741975349.git.jpoimboe@kernel.org

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 2dbadf347b5f4f66625c4f49b76c41b412270d57..5e3e036e6e537f70b3770122523db41a8170e586 100644 (file)
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -379,6 +379,21 @@ __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.
  *
@@ -514,7 +529,8 @@ 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 7dbf22c6da9d4681a6011f51b35499b5190f094e..12bf6c1f5071d02f53651efe0fa15bbd9375237b 100644 (file)
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -4460,35 +4460,6 @@ 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;
@@ -4505,7 +4476,7 @@ 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++;