From 9f9d68c308cb63de67d35171925ce3875d076d4f Mon Sep 17 00:00:00 2001 From: Heiko Carstens Date: Fri, 9 Jan 2026 16:31:44 +0100 Subject: [PATCH] s390/bug: Prevent tail-call optimization For the exception based __WARN_trap() implementation it is technically not necessary to prevent tail-call optimization, however it may be confusing to see warning messages like: WARNING: arch/s390/kernel/setup.c:1017 at foobar+0x2c/0x50, CPU#0: swapper/0/0 together with a disassembly of a different function caused by tail-call optimization for the __WARN_trap() call. Prevent that by adding an empty asm statement. This generates slightly worse code, but should hopefully avoid confusion. With this the output looks like: WARNING: arch/s390/kernel/setup.c:1017 at foobar+0x2c/0x50, CPU#0: swapper/0/0 ... Krnl PSW : 0704c00180000000 000003ffe0119788 (foobar+0x38/0x50) ... Krnl Code: 000003ffe0119776: e3e0f0980024 stg %r14,152(%r15) 000003ffe011977c: c02000b8992a larl %r2,000003ffe182c9d0 *000003ffe0119782: c0e5007270b7 brasl %r14,000003ffe0f678f0 >000003ffe0119788: ebeff0a00004 lmg %r14,%r15,160(%r15) 000003ffe011978e: 07fe bcr 15,%r14 000003ffe0119790: 47000700 bc 0,1792 000003ffe0119794: 0707 bcr 0,%r7 000003ffe0119796: 0707 bcr 0,%r7 Call Trace: [<000003ffe0119788>] foobar+0x38/0x50 [<000003ffe185bc2e>] arch_cpu_finalize_init+0x26/0x60 [<000003ffe185654c>] start_kernel+0x53c/0x5d8 [<000003ffe010002e>] startup_continue+0x2e/0x40 A better solution would be to replace or patch the branch instruction to __WARN_trap() with the monitor call instruction, similar to what is done for x86 [1]. However s390 does not support static_cond_calls(). Therefore use the simple approach for the time being. [1] commit 860238af7a33 ("x86_64/bug: Inline the UD1") Reviewed-by: Sven Schnelle Signed-off-by: Heiko Carstens --- arch/s390/include/asm/bug.h | 2 ++ 1 file changed, 2 insertions(+) diff --git a/arch/s390/include/asm/bug.h b/arch/s390/include/asm/bug.h index 1fbcbdbc595fe..59017fd3d9358 100644 --- a/arch/s390/include/asm/bug.h +++ b/arch/s390/include/asm/bug.h @@ -99,6 +99,8 @@ do { \ int __flags = (flags) | BUGFLAG_WARNING | BUGFLAG_ARGS; \ \ __WARN_trap(__WARN_bug_entry(__flags, format), ## arg); \ + /* prevent tail-call optimization */ \ + asm(""); \ } while (0) #define __WARN_printf(taint, fmt, arg...) \ -- 2.47.3