Buildresult: linus/x86_64-allnoconfig/x86_64-gcc8 built on Jul 19, 04:12

kisskb

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Status:	OK
Date/Time:	Jul 19, 04:12
Duration:	0:00:23.300626
Builder:	ka4
Revision:	init/Kconfig: remove CONFIG_GCC_ASM_GOTO_OUTPUT_WORKAROUND (`f2f6a8e8871725035959b90bac048cde555aa0e9)`
Target:	linus/x86_64-allnoconfig/x86_64-gcc8
Branch:	linus
Compiler:	x86_64-gcc8 (x86_64-linux-gcc (GCC) 8.5.0 / GNU ld (GNU Binutils) 2.36.1)
Config:	allnoconfig (download)
Log:	Download original No warnings found in log. Full Log # 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 <mark.rutland@arm.com> # 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 <linux/uaccess.h> # \| #include <linux/types.h> # \| # \| 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 <mark.rutland@arm.com> # Cc: Alex Coplan <alex.coplan@arm.com> # Cc: Catalin Marinas <catalin.marinas@arm.com> # Cc: Jakub Jelinek <jakub@gcc.gnu.org> # Cc: Peter Zijlstra <peterz@infradead.org> # Cc: Sean Christopherson <seanjc@google.com> # Cc: Szabolcs Nagy <szabolcs.nagy@arm.com> # Cc: Will Deacon <will@kernel.org> # Cc: linux-arm-kernel@lists.infradead.org # Cc: linux-kernel@vger.kernel.org # Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> # < /opt/cross/kisskb/korg/gcc-8.5.0-nolibc/x86_64-linux/bin/x86_64-linux-gcc --version # < /opt/cross/kisskb/korg/gcc-8.5.0-nolibc/x86_64-linux/bin/x86_64-linux-ld --version # < git log --format=%s --max-count=1 f2f6a8e8871725035959b90bac048cde555aa0e9 # make -s -j 40 ARCH=x86_64 O=/kisskb/build/linus_allnoconfig_x86_64-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.5.0-nolibc/x86_64-linux/bin/x86_64-linux- allnoconfig # < make -s -j 40 ARCH=x86_64 O=/kisskb/build/linus_allnoconfig_x86_64-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.5.0-nolibc/x86_64-linux/bin/x86_64-linux- help # make -s -j 40 ARCH=x86_64 O=/kisskb/build/linus_allnoconfig_x86_64-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.5.0-nolibc/x86_64-linux/bin/x86_64-linux- olddefconfig # make -s -j 40 ARCH=x86_64 O=/kisskb/build/linus_allnoconfig_x86_64-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.5.0-nolibc/x86_64-linux/bin/x86_64-linux- Completed OK # rm -rf /kisskb/build/linus_allnoconfig_x86_64-gcc8 # Build took: 0:00:23.300626

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