Re: [PATCH 2/2] x86/asm: Fix inline asm call constraints for clang

From: Alexander Potapenko
Date: Tue Sep 19 2017 - 18:26:15 EST

On Tue, Sep 19, 2017 at 2:55 PM, Alexander Potapenko <glider@xxxxxxxxxx> wrote:
> On Tue, Sep 19, 2017 at 11:45 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> For inline asm statements which have a CALL instruction, we list the
>> stack pointer as a constraint to convince GCC to ensure the frame
>> pointer is set up first:
>>
>> static inline void foo()
>> {
>> register void *__sp asm(_ASM_SP);
>> asm("call bar" : "+r" (__sp))
>> }
>>
>> Unfortunately, that pattern causes clang to corrupt the stack pointer.
>>
>> There's actually an easier way to achieve the same goal in GCC, without
>> causing trouble for clang. If we declare the stack pointer register
>> variable as a global variable, and remove the constraint altogether,
>> that convinces GCC to always set up the frame pointer before inserting
>> *any* inline asm.
>>
>> It basically acts as if *every* inline asm statement has a CALL
>> instruction. It's a bit overkill, but the performance impact should be
>> negligible.
>>
>> Here are the vmlinux .text size differences with the following configs
>> on GCC:
>>
>> - defconfig
>> - defconfig without frame pointers
>> - Fedora distro config
>> - Fedora distro config without frame pointers
>>
>> defconfig defconfig-nofp distro distro-nofp
>> before 9796300 9466764 9076191 8789745
>> after 9796941 9462859 9076381 8785325
>>
>> With frame pointers, the text size increases slightly. Without frame
>> pointers, the text size decreases, and a little more significantly.
>>
>> Reported-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
>> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>> ---
>> arch/x86/include/asm/alternative.h | 3 +--
>> arch/x86/include/asm/asm.h | 9 ++++++++
>> arch/x86/include/asm/mshyperv.h | 27 ++++++++++--------------
>> arch/x86/include/asm/paravirt_types.h | 14 ++++++------
>> arch/x86/include/asm/preempt.h | 15 +++++--------
>> arch/x86/include/asm/processor.h | 6 ++----
>> arch/x86/include/asm/rwsem.h | 6 +++---
>> arch/x86/include/asm/uaccess.h | 5 ++---
>> arch/x86/include/asm/xen/hypercall.h | 5 ++---
>> arch/x86/kvm/emulate.c | 3 +--
>> arch/x86/kvm/vmx.c | 4 +---
>> arch/x86/mm/fault.c | 3 +--
>> tools/objtool/Documentation/stack-validation.txt | 20 +++++++++++++-----
>> 13 files changed, 60 insertions(+), 60 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
>> index 1b020381ab38..7aeb1cef4204 100644
>> --- a/arch/x86/include/asm/alternative.h
>> +++ b/arch/x86/include/asm/alternative.h
>> @@ -218,10 +218,9 @@ static inline int alternatives_text_reserved(void *start, void *end)
>> #define alternative_call_2(oldfunc, newfunc1, feature1, newfunc2, feature2, \
>> output, input...) \
>> { \
>> - register void *__sp asm(_ASM_SP); \
>> asm volatile (ALTERNATIVE_2("call %P[old]", "call %P[new1]", feature1,\
>> "call %P[new2]", feature2) \
>> - : output, "+r" (__sp) \
>> + : output \
>> : [old] "i" (oldfunc), [new1] "i" (newfunc1), \
>> [new2] "i" (newfunc2), ## input); \
>> }
>> diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
>> index 676ee5807d86..ff8921d4615b 100644
>> --- a/arch/x86/include/asm/asm.h
>> +++ b/arch/x86/include/asm/asm.h
>> @@ -132,4 +132,13 @@
>> /* For C file, we already have NOKPROBE_SYMBOL macro */
>> #endif
>>
>> +#ifndef __ASSEMBLY__
>> +/*
>> + * This variable declaration has the side effect of forcing GCC to always set
>> + * up the frame pointer before inserting any inline asm. This is necessary
>> + * because some inline asm statements have CALL instructions.
>> + */
>> +register unsigned int __sp_unused asm("esp");
> Shouldn't this be "asm(_ASM_SP)"?
Answering my own question, this can't be _ASM_SP because of the
realmode code, which is built with -m16 whereas _ASM_SP is "rsp".
The patch works fine with "esp" though - most certainly declaring a
ESP variable is enough to reserve the whole RSP in an x86_64 build.
>> +#endif
>> +
>> #endif /* _ASM_X86_ASM_H */
>> diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
>> index 63cc96f064dc..1c913ae27f99 100644
>> --- a/arch/x86/include/asm/mshyperv.h
>> +++ b/arch/x86/include/asm/mshyperv.h
>> @@ -179,16 +179,15 @@ static inline u64 hv_do_hypercall(u64 control, void *input, void *output)
>> u64 input_address = input ? virt_to_phys(input) : 0;
>> u64 output_address = output ? virt_to_phys(output) : 0;
>> u64 hv_status;
>> - register void *__sp asm(_ASM_SP);
>>
>> #ifdef CONFIG_X86_64
>> if (!hv_hypercall_pg)
>> return U64_MAX;
>>
>> - __asm__ __volatile__("mov %4, %%r8\n"
>> - "call *%5"
>> - : "=a" (hv_status), "+r" (__sp),
>> - "+c" (control), "+d" (input_address)
>> + __asm__ __volatile__("mov %3, %%r8\n"
>> + "call *%4"
>> + : "=a" (hv_status), "+c" (control),
>> + "+d" (input_address)
>> : "r" (output_address), "m" (hv_hypercall_pg)
>> : "cc", "memory", "r8", "r9", "r10", "r11");
>> #else
>> @@ -200,9 +199,8 @@ static inline u64 hv_do_hypercall(u64 control, void *input, void *output)
>> if (!hv_hypercall_pg)
>> return U64_MAX;
>>
>> - __asm__ __volatile__("call *%7"
>> - : "=A" (hv_status),
>> - "+c" (input_address_lo), "+r" (__sp)
>> + __asm__ __volatile__("call *%6"
>> + : "=A" (hv_status), "+c" (input_address_lo)
>> : "A" (control),
>> "b" (input_address_hi),
>> "D"(output_address_hi), "S"(output_address_lo),
>> @@ -224,13 +222,12 @@ static inline u64 hv_do_hypercall(u64 control, void *input, void *output)
>> static inline u64 hv_do_fast_hypercall8(u16 code, u64 input1)
>> {
>> u64 hv_status, control = (u64)code | HV_HYPERCALL_FAST_BIT;
>> - register void *__sp asm(_ASM_SP);
>>
>> #ifdef CONFIG_X86_64
>> {
>> - __asm__ __volatile__("call *%4"
>> - : "=a" (hv_status), "+r" (__sp),
>> - "+c" (control), "+d" (input1)
>> + __asm__ __volatile__("call *%3"
>> + : "=a" (hv_status), "+c" (control),
>> + "+d" (input1)
>> : "m" (hv_hypercall_pg)
>> : "cc", "r8", "r9", "r10", "r11");
>> }
>> @@ -239,10 +236,8 @@ static inline u64 hv_do_fast_hypercall8(u16 code, u64 input1)
>> u32 input1_hi = upper_32_bits(input1);
>> u32 input1_lo = lower_32_bits(input1);
>>
>> - __asm__ __volatile__ ("call *%5"
>> - : "=A"(hv_status),
>> - "+c"(input1_lo),
>> - "+r"(__sp)
>> + __asm__ __volatile__ ("call *%4"
>> + : "=A"(hv_status), "+c"(input1_lo)
>> : "A" (control),
>> "b" (input1_hi),
>> "m" (hv_hypercall_pg)
>> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
>> index 42873edd9f9d..b8966ed71ba5 100644
>> --- a/arch/x86/include/asm/paravirt_types.h
>> +++ b/arch/x86/include/asm/paravirt_types.h
>> @@ -459,8 +459,8 @@ int paravirt_disable_iospace(void);
>> */
>> #ifdef CONFIG_X86_32
>> #define PVOP_VCALL_ARGS \
>> - unsigned long __eax = __eax, __edx = __edx, __ecx = __ecx; \
>> - register void *__sp asm("esp")
>> + unsigned long __eax = __eax, __edx = __edx, __ecx = __ecx;
>> +
>> #define PVOP_CALL_ARGS PVOP_VCALL_ARGS
>>
>> #define PVOP_CALL_ARG1(x) "a" ((unsigned long)(x))
>> @@ -480,8 +480,8 @@ int paravirt_disable_iospace(void);
>> /* [re]ax isn't an arg, but the return val */
>> #define PVOP_VCALL_ARGS \
>> unsigned long __edi = __edi, __esi = __esi, \
>> - __edx = __edx, __ecx = __ecx, __eax = __eax; \
>> - register void *__sp asm("rsp")
>> + __edx = __edx, __ecx = __ecx, __eax = __eax;
>> +
>> #define PVOP_CALL_ARGS PVOP_VCALL_ARGS
>>
>> #define PVOP_CALL_ARG1(x) "D" ((unsigned long)(x))
>> @@ -532,7 +532,7 @@ int paravirt_disable_iospace(void);
>> asm volatile(pre \
>> paravirt_alt(PARAVIRT_CALL) \
>> post \
>> - : call_clbr, "+r" (__sp) \
>> + : call_clbr \
>> : paravirt_type(op), \
>> paravirt_clobber(clbr), \
>> ##__VA_ARGS__ \
>> @@ -542,7 +542,7 @@ int paravirt_disable_iospace(void);
>> asm volatile(pre \
>> paravirt_alt(PARAVIRT_CALL) \
>> post \
>> - : call_clbr, "+r" (__sp) \
>> + : call_clbr \
>> : paravirt_type(op), \
>> paravirt_clobber(clbr), \
>> ##__VA_ARGS__ \
>> @@ -569,7 +569,7 @@ int paravirt_disable_iospace(void);
>> asm volatile(pre \
>> paravirt_alt(PARAVIRT_CALL) \
>> post \
>> - : call_clbr, "+r" (__sp) \
>> + : call_clbr \
>> : paravirt_type(op), \
>> paravirt_clobber(clbr), \
>> ##__VA_ARGS__ \
>> diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h
>> index ec1f3c651150..f66b189a1142 100644
>> --- a/arch/x86/include/asm/preempt.h
>> +++ b/arch/x86/include/asm/preempt.h
>> @@ -100,19 +100,14 @@ static __always_inline bool should_resched(int preempt_offset)
>>
>> #ifdef CONFIG_PREEMPT
>> extern asmlinkage void ___preempt_schedule(void);
>> -# define __preempt_schedule() \
>> -({ \
>> - register void *__sp asm(_ASM_SP); \
>> - asm volatile ("call ___preempt_schedule" : "+r"(__sp)); \
>> -})
>> +# define __preempt_schedule() \
>> + asm volatile ("call ___preempt_schedule")
>>
>> extern asmlinkage void preempt_schedule(void);
>> extern asmlinkage void ___preempt_schedule_notrace(void);
>> -# define __preempt_schedule_notrace() \
>> -({ \
>> - register void *__sp asm(_ASM_SP); \
>> - asm volatile ("call ___preempt_schedule_notrace" : "+r"(__sp)); \
>> -})
>> +# define __preempt_schedule_notrace() \
>> + asm volatile ("call ___preempt_schedule_notrace")
>> +
>> extern asmlinkage void preempt_schedule_notrace(void);
>> #endif
>>
>> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
>> index 3fa26a61eabc..7a7969211527 100644
>> --- a/arch/x86/include/asm/processor.h
>> +++ b/arch/x86/include/asm/processor.h
>> @@ -677,8 +677,6 @@ static inline void sync_core(void)
>> * Like all of Linux's memory ordering operations, this is a
>> * compiler barrier as well.
>> */
>> - register void *__sp asm(_ASM_SP);
>> -
>> #ifdef CONFIG_X86_32
>> asm volatile (
>> "pushfl\n\t"
>> @@ -686,7 +684,7 @@ static inline void sync_core(void)
>> "pushl $1f\n\t"
>> "iret\n\t"
>> "1:"
>> - : "+r" (__sp) : : "memory");
>> + : : : "memory");
>> #else
>> unsigned int tmp;
>>
>> @@ -703,7 +701,7 @@ static inline void sync_core(void)
>> "iretq\n\t"
>> UNWIND_HINT_RESTORE
>> "1:"
>> - : "=&r" (tmp), "+r" (__sp) : : "cc", "memory");
>> + : "=&r" (tmp) : : "cc", "memory");
>> #endif
>> }
>>
>> diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
>> index a34e0d4b957d..f8f127a2a300 100644
>> --- a/arch/x86/include/asm/rwsem.h
>> +++ b/arch/x86/include/asm/rwsem.h
>> @@ -103,10 +103,9 @@ static inline bool __down_read_trylock(struct rw_semaphore *sem)
>> ({ \
>> long tmp; \
>> struct rw_semaphore* ret; \
>> - register void *__sp asm(_ASM_SP); \
>> \
>> asm volatile("# beginning down_write\n\t" \
>> - LOCK_PREFIX " xadd %1,(%4)\n\t" \
>> + LOCK_PREFIX " xadd %1,(%3)\n\t" \
>> /* adds 0xffff0001, returns the old value */ \
>> " test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t" \
>> /* was the active mask 0 before? */\
>> @@ -114,7 +113,8 @@ static inline bool __down_read_trylock(struct rw_semaphore *sem)
>> " call " slow_path "\n" \
>> "1:\n" \
>> "# ending down_write" \
>> - : "+m" (sem->count), "=d" (tmp), "=a" (ret), "+r" (__sp) \
>> + : "+m" (sem->count), "=d" (tmp), \
>> + "=a" (ret) \
>> : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS) \
>> : "memory", "cc"); \
>> ret; \
>> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
>> index 184eb9894dae..113ddb8ce0d8 100644
>> --- a/arch/x86/include/asm/uaccess.h
>> +++ b/arch/x86/include/asm/uaccess.h
>> @@ -166,11 +166,10 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
>> ({ \
>> int __ret_gu; \
>> register __inttype(*(ptr)) __val_gu asm("%"_ASM_DX); \
>> - register void *__sp asm(_ASM_SP); \
>> __chk_user_ptr(ptr); \
>> might_fault(); \
>> - asm volatile("call __get_user_%P4" \
>> - : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp) \
>> + asm volatile("call __get_user_%P3" \
>> + : "=a" (__ret_gu), "=r" (__val_gu) \
>> : "0" (ptr), "i" (sizeof(*(ptr)))); \
>> (x) = (__force __typeof__(*(ptr))) __val_gu; \
>> __builtin_expect(__ret_gu, 0); \
>> diff --git a/arch/x86/include/asm/xen/hypercall.h b/arch/x86/include/asm/xen/hypercall.h
>> index 9606688caa4b..6264dba01896 100644
>> --- a/arch/x86/include/asm/xen/hypercall.h
>> +++ b/arch/x86/include/asm/xen/hypercall.h
>> @@ -113,10 +113,9 @@ extern struct { char _entry[32]; } hypercall_page[];
>> register unsigned long __arg2 asm(__HYPERCALL_ARG2REG) = __arg2; \
>> register unsigned long __arg3 asm(__HYPERCALL_ARG3REG) = __arg3; \
>> register unsigned long __arg4 asm(__HYPERCALL_ARG4REG) = __arg4; \
>> - register unsigned long __arg5 asm(__HYPERCALL_ARG5REG) = __arg5; \
>> - register void *__sp asm(_ASM_SP);
>> + register unsigned long __arg5 asm(__HYPERCALL_ARG5REG) = __arg5;
>>
>> -#define __HYPERCALL_0PARAM "=r" (__res), "+r" (__sp)
>> +#define __HYPERCALL_0PARAM "=r" (__res)
>> #define __HYPERCALL_1PARAM __HYPERCALL_0PARAM, "+r" (__arg1)
>> #define __HYPERCALL_2PARAM __HYPERCALL_1PARAM, "+r" (__arg2)
>> #define __HYPERCALL_3PARAM __HYPERCALL_2PARAM, "+r" (__arg3)
>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>> index 16bf6655aa85..5e82e0821c1f 100644
>> --- a/arch/x86/kvm/emulate.c
>> +++ b/arch/x86/kvm/emulate.c
>> @@ -5296,7 +5296,6 @@ static void fetch_possible_mmx_operand(struct x86_emulate_ctxt *ctxt,
>>
>> static int fastop(struct x86_emulate_ctxt *ctxt, void (*fop)(struct fastop *))
>> {
>> - register void *__sp asm(_ASM_SP);
>> ulong flags = (ctxt->eflags & EFLAGS_MASK) | X86_EFLAGS_IF;
>>
>> if (!(ctxt->d & ByteOp))
>> @@ -5304,7 +5303,7 @@ static int fastop(struct x86_emulate_ctxt *ctxt, void (*fop)(struct fastop *))
>>
>> asm("push %[flags]; popf; call *%[fastop]; pushf; pop %[flags]\n"
>> : "+a"(ctxt->dst.val), "+d"(ctxt->src.val), [flags]"+D"(flags),
>> - [fastop]"+S"(fop), "+r"(__sp)
>> + [fastop]"+S"(fop)
>> : "c"(ctxt->src2.val));
>>
>> ctxt->eflags = (ctxt->eflags & ~EFLAGS_MASK) | (flags & EFLAGS_MASK);
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 06c0c6d0541e..a693362f41af 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -9036,7 +9036,6 @@ static void vmx_complete_atomic_exit(struct vcpu_vmx *vmx)
>> static void vmx_handle_external_intr(struct kvm_vcpu *vcpu)
>> {
>> u32 exit_intr_info = vmcs_read32(VM_EXIT_INTR_INFO);
>> - register void *__sp asm(_ASM_SP);
>>
>> if ((exit_intr_info & (INTR_INFO_VALID_MASK | INTR_INFO_INTR_TYPE_MASK))
>> == (INTR_INFO_VALID_MASK | INTR_TYPE_EXT_INTR)) {
>> @@ -9063,9 +9062,8 @@ static void vmx_handle_external_intr(struct kvm_vcpu *vcpu)
>> "call *%[entry]\n\t"
>> :
>> #ifdef CONFIG_X86_64
>> - [sp]"=&r"(tmp),
>> + [sp]"=&r"(tmp)
>> #endif
>> - "+r"(__sp)
>> :
>> [entry]"r"(entry),
>> [ss]"i"(__KERNEL_DS),
>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
>> index b836a7274e12..4457e41378e4 100644
>> --- a/arch/x86/mm/fault.c
>> +++ b/arch/x86/mm/fault.c
>> @@ -806,7 +806,6 @@ no_context(struct pt_regs *regs, unsigned long error_code,
>> if (is_vmalloc_addr((void *)address) &&
>> (((unsigned long)tsk->stack - 1 - address < PAGE_SIZE) ||
>> address - ((unsigned long)tsk->stack + THREAD_SIZE) < PAGE_SIZE)) {
>> - register void *__sp asm("rsp");
>> unsigned long stack = this_cpu_read(orig_ist.ist[DOUBLEFAULT_STACK]) - sizeof(void *);
>> /*
>> * We're likely to be running with very little stack space
>> @@ -821,7 +820,7 @@ no_context(struct pt_regs *regs, unsigned long error_code,
>> asm volatile ("movq %[stack], %%rsp\n\t"
>> "call handle_stack_overflow\n\t"
>> "1: jmp 1b"
>> - : "+r" (__sp)
>> + :
>> : "D" ("kernel stack overflow (page fault)"),
>> "S" (regs), "d" (address),
>> [stack] "rm" (stack));
>> diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt
>> index 6a1af43862df..18af4aa7488d 100644
>> --- a/tools/objtool/Documentation/stack-validation.txt
>> +++ b/tools/objtool/Documentation/stack-validation.txt
>> @@ -192,12 +192,22 @@ they mean, and suggestions for how to fix them.
>> use the manual unwind hint macros in asm/unwind_hints.h.
>>
>> If it's a GCC-compiled .c file, the error may be because the function
>> - uses an inline asm() statement which has a "call" instruction. An
>> - asm() statement with a call instruction must declare the use of the
>> - stack pointer in its output operand. For example, on x86_64:
>> + uses an inline asm() statement which has a "call" instruction. To
>> + fix this, either:
>>
>> - register void *__sp asm("rsp");
>> - asm volatile("call func" : "+r" (__sp));
>> + a) list the stack pointer as an input/output constraint;
>> +
>> + register void *__sp asm("rsp");
>> + asm volatile("call func" : "+r" (__sp));
>> +
>> + or
>> +
>> + b) declare a global register variable for the stack pointer.
>> +
>> + arch/x86/include/asm/asm.h:register unsigned int __sp_unused asm("esp");
>> +
>> + This is the strategy used on x86_64. It ensures that GCC always
>> + sets up the frame pointer before inserting *any* inline asm.
>>
>> Otherwise the stack frame may not get created before the call.
Tested-by: Alexander Potapenko <glider@xxxxxxxxxx>
>> --
>> 2.13.5
>>
>
>
>
> --
> Alexander Potapenko
> Software Engineer
>
> Google Germany GmbH
> Erika-Mann-StraÃe, 33
> 80636 MÃnchen
>
> GeschÃftsfÃhrer: Paul Manicle, Halimah DeLaine Prado
> Registergericht und -nummer: Hamburg, HRB 86891
> Sitz der Gesellschaft: Hamburg



--
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-StraÃe, 33
80636 MÃnchen

GeschÃftsfÃhrer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg