From 4356e9f841f7fbb945521cef3577ba394c65f3fc Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Fri, 9 Feb 2024 12:39:31 -0800 Subject: work around gcc bugs with 'asm goto' with outputs We've had issues with gcc and 'asm goto' before, and we created a 'asm_volatile_goto()' macro for that in the past: see commits 3f0116c3238a ("compiler/gcc4: Add quirk for 'asm goto' miscompilation bug") and a9f180345f53 ("compiler/gcc4: Make quirk for asm_volatile_goto() unconditional"). Then, much later, we ended up removing the workaround in commit 43c249ea0b1e ("compiler-gcc.h: remove ancient workaround for gcc PR 58670") because we no longer supported building the kernel with the affected gcc versions, but we left the macro uses around. Now, Sean Christopherson reports a new version of a very similar problem, which is fixed by re-applying that ancient workaround. But the problem in question is limited to only the 'asm goto with outputs' cases, so instead of re-introducing the old workaround as-is, let's rename and limit the workaround to just that much less common case. It looks like there are at least two separate issues that all hit in this area: (a) some versions of gcc don't mark the asm goto as 'volatile' when it has outputs: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98619 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110420 which is easy to work around by just adding the 'volatile' by hand. (b) Internal compiler errors: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110422 which are worked around by adding the extra empty 'asm' as a barrier, as in the original workaround. but the problem Sean sees may be a third thing since it involves bad code generation (not an ICE) even with the manually added 'volatile'. but the same old workaround works for this case, even if this feels a bit like voodoo programming and may only be hiding the issue. Reported-and-tested-by: Sean Christopherson Link: https://lore.kernel.org/all/20240208220604.140859-1-seanjc@google.com/ Cc: Nick Desaulniers Cc: Uros Bizjak Cc: Jakub Jelinek Cc: Andrew Pinski Signed-off-by: Linus Torvalds --- include/linux/compiler_types.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'include/linux/compiler_types.h') diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h index 6f1ca49306d2..663d8791c871 100644 --- a/include/linux/compiler_types.h +++ b/include/linux/compiler_types.h @@ -362,8 +362,8 @@ struct ftrace_likely_data { #define __member_size(p) __builtin_object_size(p, 1) #endif -#ifndef asm_volatile_goto -#define asm_volatile_goto(x...) asm goto(x) +#ifndef asm_goto_output +#define asm_goto_output(x...) asm goto(x) #endif #ifdef CONFIG_CC_HAS_ASM_INLINE -- cgit v1.2.3-70-g09d2 From 68fb3ca0e408e00db1c3f8fccdfa19e274c033be Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Thu, 15 Feb 2024 11:14:33 -0800 Subject: update workarounds for gcc "asm goto" issue In commit 4356e9f841f7 ("work around gcc bugs with 'asm goto' with outputs") I did the gcc workaround unconditionally, because the cause of the bad code generation wasn't entirely clear. In the meantime, Jakub Jelinek debugged the issue, and has come up with a fix in gcc [2], which also got backported to the still maintained branches of gcc-11, gcc-12 and gcc-13. Note that while the fix technically wasn't in the original gcc-14 branch, Jakub says: "while it is true that no GCC 14 snapshots until today (or whenever the fix will be committed) have the fix, for GCC trunk it is up to the distros to use the latest snapshot if they use it at all and would allow better testing of the kernel code without the workaround, so that if there are other issues they won't be discovered years later. Most userland code doesn't actually use asm goto with outputs..." so we will consider gcc-14 to be fixed - if somebody is using gcc snapshots of the gcc-14 before the fix, they should upgrade. Note that while the bug goes back to gcc-11, in practice other gcc changes seem to have effectively hidden it since gcc-12.1 as per a bisect by Jakub. So even a gcc-14 snapshot without the fix likely doesn't show actual problems. Also, make the default 'asm_goto_output()' macro mark the asm as volatile by hand, because of an unrelated gcc issue [1] where it doesn't match the documented behavior ("asm goto is always volatile"). Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979 [1] Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113921 [2] Link: https://lore.kernel.org/all/20240208220604.140859-1-seanjc@google.com/ Requested-by: Jakub Jelinek Cc: Uros Bizjak Cc: Nick Desaulniers Cc: Sean Christopherson Cc: Andrew Pinski Signed-off-by: Linus Torvalds --- include/linux/compiler-gcc.h | 7 ++++--- include/linux/compiler_types.h | 9 ++++++++- init/Kconfig | 9 +++++++++ 3 files changed, 21 insertions(+), 4 deletions(-) (limited to 'include/linux/compiler_types.h') diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h index c1a963be7d28..75bd1692d2e3 100644 --- a/include/linux/compiler-gcc.h +++ b/include/linux/compiler-gcc.h @@ -67,10 +67,9 @@ /* * GCC 'asm goto' with outputs miscompiles certain code sequences: * - * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110420 - * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110422 + * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113921 * - * Work it around via the same compiler barrier quirk that we used + * Work around it via the same compiler barrier quirk that we used * to use for the old 'asm goto' workaround. * * Also, always mark such 'asm goto' statements as volatile: all @@ -80,8 +79,10 @@ * * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98619 */ +#ifdef CONFIG_GCC_ASM_GOTO_OUTPUT_WORKAROUND #define asm_goto_output(x...) \ do { asm volatile goto(x); asm (""); } while (0) +#endif #if defined(CONFIG_ARCH_USE_BUILTIN_BSWAP) #define __HAVE_BUILTIN_BSWAP32__ diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h index 663d8791c871..0caf354cb94b 100644 --- a/include/linux/compiler_types.h +++ b/include/linux/compiler_types.h @@ -362,8 +362,15 @@ struct ftrace_likely_data { #define __member_size(p) __builtin_object_size(p, 1) #endif +/* + * Some versions of gcc do not mark 'asm goto' volatile: + * + * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979 + * + * We do it here by hand, because it doesn't hurt. + */ #ifndef asm_goto_output -#define asm_goto_output(x...) asm goto(x) +#define asm_goto_output(x...) asm volatile goto(x) #endif #ifdef CONFIG_CC_HAS_ASM_INLINE diff --git a/init/Kconfig b/init/Kconfig index deda3d14135b..8426d59cc634 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -89,6 +89,15 @@ config CC_HAS_ASM_GOTO_TIED_OUTPUT # Detect buggy gcc and clang, fixed in gcc-11 clang-14. def_bool $(success,echo 'int foo(int *x) { asm goto (".long (%l[bar]) - .": "+m"(*x) ::: bar); return *x; bar: return 0; }' | $CC -x c - -c -o /dev/null) +config GCC_ASM_GOTO_OUTPUT_WORKAROUND + bool + depends on CC_IS_GCC && CC_HAS_ASM_GOTO_OUTPUT + # Fixed in GCC 14, 13.3, 12.4 and 11.5 + # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113921 + default y if GCC_VERSION < 110500 + default y if GCC_VERSION >= 120000 && GCC_VERSION < 120400 + default y if GCC_VERSION >= 130000 && GCC_VERSION < 130300 + config TOOLS_SUPPORT_RELR def_bool $(success,env "CC=$(CC)" "LD=$(LD)" "NM=$(NM)" "OBJCOPY=$(OBJCOPY)" $(srctree)/scripts/tools-support-relr.sh) -- cgit v1.2.3-70-g09d2