# git rev-parse -q --verify f2f6a8e8871725035959b90bac048cde555aa0e9^{commit} f2f6a8e8871725035959b90bac048cde555aa0e9 already have revision, skipping fetch # git checkout -q -f -B kisskb f2f6a8e8871725035959b90bac048cde555aa0e9 # git clean -qxdf # < git log -1 # commit f2f6a8e8871725035959b90bac048cde555aa0e9 # Author: Mark Rutland # Date: Thu Jul 18 13:06:47 2024 +0100 # # init/Kconfig: remove CONFIG_GCC_ASM_GOTO_OUTPUT_WORKAROUND # # Several versions of GCC mis-compile asm goto with outputs. We try to # workaround this, but our workaround is demonstrably incomplete and # liable to result in subtle bugs, especially on arm64 where get_user() # has recently been moved over to using asm goto with outputs. # # From discussion(s) with Linus at: # # https://lore.kernel.org/linux-arm-kernel/Zpfv2tnlQ-gOLGac@J2N7QTR9R3.cambridge.arm.com/ # https://lore.kernel.org/linux-arm-kernel/ZpfxLrJAOF2YNqCk@J2N7QTR9R3.cambridge.arm.com/ # # ... it sounds like the best thing to do for now is to remove the # workaround and make CC_HAS_ASM_GOTO_OUTPUT depend on working compiler # versions. # # The issue was originally reported to GCC by Sean Christopherson: # # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113921 # # ... and Jakub Jelinek fixed this for GCC 14, with the fix backported to # 13.3.0, 12.4.0, and 11.5.0. # # In the kernel, we tried to workaround broken compilers in commits: # # 4356e9f841f7 ("work around gcc bugs with 'asm goto' with outputs") # 68fb3ca0e408 ("update workarounds for gcc "asm goto" issue") # # ... but the workaround of adding an empty asm("") after the asm volatile # goto(...) demonstrably does not always avoid the problem, as can be seen # in the following test case: # # | #define asm_goto_output(x...) \ # | do { asm volatile goto(x); asm (""); } while (0) # | # | #define __good_or_bad(__val, __key) \ # | do { \ # | __label__ __failed; \ # | unsigned long __tmp; \ # | asm_goto_output( \ # | " cbnz %[key], %l[__failed]\n" \ # | " mov %[val], #0x900d\n" \ # | : [val] "=r" (__tmp) \ # | : [key] "r" (__key) \ # | : \ # | : __failed); \ # | (__val) = __tmp; \ # | break; \ # | __failed: \ # | (__val) = 0xbad; \ # | } while (0) # | # | unsigned long get_val(unsigned long key); # | unsigned long get_val(unsigned long key) # | { # | unsigned long val = 0xbad; # | # | __good_or_bad(val, key); # | # | return val; # | } # # GCC 13.2.0 (at -O2) compiles this to: # # | cbnz x0, .Lfailed # | mov x0, #0x900d # | .Lfailed: # | ret # # GCC 14.1.0 (at -O2) compiles this to: # # | cbnz x0, .Lfailed # | mov x0, #0x900d # | ret # | .Lfailed: # | mov x0, #0xbad # | ret # # Note that GCC 13.2.0 erroneously omits the assignment to 'val' in the # error path (even though this does not depend on an output of the asm # goto). GCC 14.1.0 correctly retains the assignment. # # This problem can be seen within the kernel with the following test case: # # | #include # | #include # | # | noinline unsigned long test_unsafe_get_user(unsigned long __user *ptr); # | noinline unsigned long test_unsafe_get_user(unsigned long __user *ptr) # | { # | unsigned long val; # | # | unsafe_get_user(val, ptr, Efault); # | return val; # | # | Efault: # | val = 0x900d; # | return val; # | } # # GCC 13.2.0 (arm64 defconfig) compiles this to: # # | and x0, x0, #0xff7fffffffffffff # | ldtr x0, [x0] # | .Lextable_fixup: # | ret # # GCC 13.2.0 (x86_64 defconfig + MITIGATION_RETPOLINE=n) compiles this to: # # | endbr64 # | mov (%rdi),%rax # | .Lextable_fixup: # | ret # # ... omitting the assignment to 'val' in the error path, and leaving # garbage in the result register returned by the function (which happens # to contain the faulting address in the generated code). # # GCC 14.1.0 (arm64 defconfig) compiles this to: # # | and x0, x0, #0xff7fffffffffffff # | ldtr x0, [x0] # | ret # | .Lextable_fixup: # | mov x0, #0x900d // #36877 # | ret # # GCC 14.1.0 (x86_64 defconfig + MITIGATION_RETPOLINE=n) compiles this to: # # | endbr64 # | mov (%rdi),%rax # | ret # | .Lextable_fixup: # | mov $0x900d,%eax # | ret # # ... retaining the expected assignment to 'val' in the error path. # # We don't have a complete and reasonable workaround. While placing empty # asm("") blocks after each goto label *might* be sufficient, we don't # know for certain, this is tedious and error-prone, and there doesn't # seem to be a neat way to wrap this up (which is especially painful for # cases with multiple goto labels). # # Avoid this issue by disabling CONFIG_CC_HAS_ASM_GOTO_OUTPUT for # known-broken compiler versions and removing the workaround (along with # the CONFIG_GCC_ASM_GOTO_OUTPUT_WORKAROUND config option). # # For the moment I've left the default implementation of asm_goto_output() # unchanged. This should now be redundant since any compiler with the fix # for the clobbering issue whould also have a fix for the (earlier) # volatile issue, but it's far less churny to leave it around, which makes # it easier to backport this patch if necessary. # # Signed-off-by: Mark Rutland # Cc: Alex Coplan # Cc: Catalin Marinas # Cc: Jakub Jelinek # Cc: Peter Zijlstra # Cc: Sean Christopherson # Cc: Szabolcs Nagy # Cc: Will Deacon # Cc: linux-arm-kernel@lists.infradead.org # Cc: linux-kernel@vger.kernel.org # Signed-off-by: Linus Torvalds # < /opt/cross/kisskb/korg/gcc-13.1.0-nolibc/powerpc64-linux/bin/powerpc64-linux-gcc --version # < /opt/cross/kisskb/korg/gcc-13.1.0-nolibc/powerpc64-linux/bin/powerpc64-linux-ld --version # < git log --format=%s --max-count=1 f2f6a8e8871725035959b90bac048cde555aa0e9 # make -s -j 160 ARCH=powerpc O=/kisskb/build/linus_skiroot_defconfig_powerpc-gcc13 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-13.1.0-nolibc/powerpc64-linux/bin/powerpc64-linux- skiroot_defconfig # < make -s -j 160 ARCH=powerpc O=/kisskb/build/linus_skiroot_defconfig_powerpc-gcc13 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-13.1.0-nolibc/powerpc64-linux/bin/powerpc64-linux- help # make -s -j 160 ARCH=powerpc O=/kisskb/build/linus_skiroot_defconfig_powerpc-gcc13 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-13.1.0-nolibc/powerpc64-linux/bin/powerpc64-linux- olddefconfig # make -s -j 160 ARCH=powerpc O=/kisskb/build/linus_skiroot_defconfig_powerpc-gcc13 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-13.1.0-nolibc/powerpc64-linux/bin/powerpc64-linux- Completed OK # rm -rf /kisskb/build/linus_skiroot_defconfig_powerpc-gcc13 # Build took: 0:01:51.941905