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SubjectRe: mmotm 2019-04-19-14-53 uploaded (objtool)
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> On Apr 23, 2019, at 1:24 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
>> On Fri, Apr 19, 2019 at 09:36:46PM -0700, Randy Dunlap wrote:
>>> On 4/19/19 2:53 PM, akpm@linux-foundation.org wrote:
>>> The mm-of-the-moment snapshot 2019-04-19-14-53 has been uploaded to
>>>
>>> http://www.ozlabs.org/~akpm/mmotm/
>>>
>>> mmotm-readme.txt says
>>>
>>> README for mm-of-the-moment:
>>>
>>> http://www.ozlabs.org/~akpm/mmotm/
>>>
>>> This is a snapshot of my -mm patch queue. Uploaded at random hopefully
>>> more than once a week.
>>
>> on x86_64:
>>
>> CC lib/strncpy_from_user.o
>> lib/strncpy_from_user.o: warning: objtool: strncpy_from_user()+0x315: call to __ubsan_handle_add_overflow() with UACCESS enabled
>> CC lib/strnlen_user.o
>> lib/strnlen_user.o: warning: objtool: strnlen_user()+0x337: call to __ubsan_handle_sub_overflow() with UACCESS enabled
>
> Lemme guess, you're using GCC < 8 ? That had a bug where UBSAN
> considered signed overflow UB when using -fno-strict-overflow or
> -fwrapv.
>
> Now, we could of course allow this symbol, but I found only the below
> was required to make allyesconfig build without issue.
>
> Andy, Linus?
>
> (note: the __put_user thing is from this one:
>
> drivers/gpu/drm/i915/i915_gem_execbuffer.c: if (unlikely(__put_user(offset, &urelocs[r-stack].presumed_offset))) {
>
> where (ptr) ends up non-trivial due to UBSAN)
>
> ---
>
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index 22ba683afdc2..c82abd6e4ca3 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -427,10 +427,11 @@ do { \
> ({ \
> __label__ __pu_label; \
> int __pu_err = -EFAULT; \
> - __typeof__(*(ptr)) __pu_val; \
> - __pu_val = x; \
> + __typeof__(*(ptr)) __pu_val = (x); \
> + __typeof__(ptr) __pu_ptr = (ptr); \

Hmm. I wonder if this forces the address calculation to be done before STAC, which means that gcc can’t use mov ..., %gs:(fancy stuff). It probably depends on how clever the optimizer is. Have you looked at the generated code?

Other than that, it seems reasonable to me.

> + __typeof__(size) __pu_size = (size); \
> __uaccess_begin(); \
> - __put_user_size(__pu_val, (ptr), (size), __pu_label); \
> + __put_user_size(__pu_val, __pu_ptr, __pu_size, __pu_label); \
> __pu_err = 0; \
> __pu_label: \
> __uaccess_end(); \
> diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
> index 58eacd41526c..07045bc4872e 100644
> --- a/lib/strncpy_from_user.c
> +++ b/lib/strncpy_from_user.c
> @@ -26,7 +26,7 @@
> static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
> {
> const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
> - long res = 0;
> + unsigned long res = 0;
>
> /*
> * Truncate 'max' to the user-specified limit, so that
> diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
> index 1c1a1b0e38a5..0729378ad3e9 100644
> --- a/lib/strnlen_user.c
> +++ b/lib/strnlen_user.c
> @@ -28,7 +28,7 @@
> static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
> {
> const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
> - long align, res = 0;
> + unsigned long align, res = 0;
> unsigned long c;
>
> /*
>

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