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:
4356e9f841 ("work around gcc bugs with 'asm goto' with outputs")
68fb3ca0e4 ("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>
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