Re: [BUG] objtool complains about missing __noreturn__ on x64_sys_call() and ia32_sys_call()

[Josh Poimboeuf]

On Thu, Apr 18, 2024 at 05:22:26PM -0700, Paul E. McKenney wrote:
> Hello!
> 
> Recent -next kernels, including next-20240418, get the following objtool
> build errors:
> 
> vmlinux.o: warning: objtool: x64_sys_call+0x2804: __x64_sys_exit() is missing a __noreturn annotation
> vmlinux.o: warning: objtool: ia32_sys_call+0x29b6: __ia32_sys_exit_group() is missing a __noreturn annotation
> 
> These functions appear to have been added to -next and mainline by
> commit 1e3ad78334a6 ("x86/syscall: Don't force use of indirect calls
> for system calls").  But the diagnostic does not make much sense because
> both functions always return unless the system call itself doesn't return.

These warnings are (perhaps confusingly yet still correctly) calling out
the fact that the exit syscalls __x64_sys_exit() and
__ia32_sys_exit_group() don't return but are not annotated as such.

It probably doesn't matter much for syscalls and I could get objtool to
just ignore such scenarios.

Or we could actually annotate them as __noreturn.  Does this fix it?


diff --git a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c
index 96ea1f8a1d3f..ff36a993a07e 100644
--- a/arch/x86/entry/syscall_64.c
+++ b/arch/x86/entry/syscall_64.c
@@ -8,9 +8,13 @@
 #include <asm/syscall.h>
 
 #define __SYSCALL(nr, sym) extern long __x64_##sym(const struct pt_regs *);
+#define __SYSCALL_NORETURN(nr, sym) extern long __noreturn __x64_##sym(const struct pt_regs *);
 #include <asm/syscalls_64.h>
 #undef __SYSCALL
 
+#undef __SYSCALL_NORETURN
+#define __SYSCALL_NORETURN __SYSCALL
+
 #define __SYSCALL(nr, sym) __x64_##sym,
 const sys_call_ptr_t sys_call_table[] = {
 #include <asm/syscalls_64.h>
diff --git a/arch/x86/entry/syscall_x32.c b/arch/x86/entry/syscall_x32.c
index 5aef4230faca..4221ecce6e68 100644
--- a/arch/x86/entry/syscall_x32.c
+++ b/arch/x86/entry/syscall_x32.c
@@ -8,9 +8,13 @@
 #include <asm/syscall.h>
 
 #define __SYSCALL(nr, sym) extern long __x64_##sym(const struct pt_regs *);
+#define __SYSCALL_NORETURN(nr, sym) extern long __noreturn __x64_##sym(const struct pt_regs *);
 #include <asm/syscalls_x32.h>
 #undef __SYSCALL
 
+#undef __SYSCALL_NORETURN
+#define __SYSCALL_NORETURN __SYSCALL
+
 #define __SYSCALL(nr, sym) __x64_##sym,
 const sys_call_ptr_t x32_sys_call_table[] = {
 #include <asm/syscalls_x32.h>
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 7e8d46f4147f..f99e9e4ad671 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -68,7 +68,7 @@
 57	common	fork			sys_fork
 58	common	vfork			sys_vfork
 59	64	execve			sys_execve
-60	common	exit			sys_exit
+60	common	exit			sys_exit	0	noreturn
 61	common	wait4			sys_wait4
 62	common	kill			sys_kill
 63	common	uname			sys_newuname
@@ -239,7 +239,7 @@
 228	common	clock_gettime		sys_clock_gettime
 229	common	clock_getres		sys_clock_getres
 230	common	clock_nanosleep		sys_clock_nanosleep
-231	common	exit_group		sys_exit_group
+231	common	exit_group		sys_exit_group		0	noreturn
 232	common	epoll_wait		sys_epoll_wait
 233	common	epoll_ctl		sys_epoll_ctl
 234	common	tgkill			sys_tgkill
diff --git a/scripts/syscalltbl.sh b/scripts/syscalltbl.sh
index 6abe143889ef..16487d47e06a 100755
--- a/scripts/syscalltbl.sh
+++ b/scripts/syscalltbl.sh
@@ -54,7 +54,7 @@ nxt=0
 
 grep -E "^[0-9]+[[:space:]]+$abis" "$infile" | {
 
-	while read nr abi name native compat ; do
+	while read nr abi name native compat noreturn; do
 
 		if [ $nxt -gt $nr ]; then
 			echo "error: $infile: syscall table is not sorted or duplicates the same syscall number" >&2
@@ -66,7 +66,9 @@ grep -E "^[0-9]+[[:space:]]+$abis" "$infile" | {
 			nxt=$((nxt + 1))
 		done
 
-		if [ -n "$compat" ]; then
+		if [ -n "$noreturn" ]; then
+			echo "__SYSCALL_NORETURN($nr, $native)"
+		elif [ -n "$compat" ]; then
 			echo "__SYSCALL_WITH_COMPAT($nr, $native, $compat)"
 		elif [ -n "$native" ]; then
 			echo "__SYSCALL($nr, $native)"
diff --git a/tools/objtool/noreturns.h b/tools/objtool/noreturns.h
index 7ebf29c91184..1e8141ef1b15 100644
--- a/tools/objtool/noreturns.h
+++ b/tools/objtool/noreturns.h
@@ -7,12 +7,16 @@
  * Yes, this is unfortunate.  A better solution is in the works.
  */
 NORETURN(__fortify_panic)
+NORETURN(__ia32_sys_exit)
+NORETURN(__ia32_sys_exit_group)
 NORETURN(__kunit_abort)
 NORETURN(__module_put_and_kthread_exit)
 NORETURN(__reiserfs_panic)
 NORETURN(__stack_chk_fail)
 NORETURN(__tdx_hypercall_failed)
 NORETURN(__ubsan_handle_builtin_unreachable)
+NORETURN(__x64_sys_exit)
+NORETURN(__x64_sys_exit_group)
 NORETURN(arch_cpu_idle_dead)
 NORETURN(bch2_trans_in_restart_error)
 NORETURN(bch2_trans_restart_error)