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x86/traps: Make exc_double_fault() consistently noreturn

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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: 55eeab2a8a ("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
This commit is contained in:
Josh Poimboeuf
2025-03-14 12:28:59 -07:00
committed by Peter Zijlstra
parent e20ab7d454
commit 8085fcd78c
2 changed files with 18 additions and 31 deletions
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18
arch/x86/kernel/traps.c
View File

@@ -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();
}

31
tools/objtool/check.c
View File

@@ -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++;