linux-kernel - Re: [PATCH 5/7] signal: Add unsafe_copy_siginfo_to

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Date:   Tue, 15 Jun 2021 09:28:54 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/7] signal: Add unsafe_copy_siginfo_to_user()



Le 15/06/2021 à 09:21, Christoph Hellwig a écrit :
> On Tue, Jun 15, 2021 at 09:03:42AM +0200, Christophe Leroy wrote:
>>
>>
>> Le 15/06/2021 ?? 08:52, Christoph Hellwig a ??crit??:
>>> On Tue, Jun 15, 2021 at 06:41:01AM +0000, Christophe Leroy wrote:
>>>> +	unsafe_copy_to_user(__ucs_to, __ucs_from,			\
>>>> +			    sizeof(struct kernel_siginfo), label);	\
>>>> +	unsafe_clear_user(__ucs_expansion, SI_EXPANSION_SIZE, label);	\
>>>> +} while (0)
>>>
>>> unsafe_clear_user does not exist at this point, and even your later
>>> patch only adds it for powerpc.
>>>
>>
>> You missed below chunck I guess:
>>
>>> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
>>> index c05e903cef02..37073caac474 100644
>>> --- a/include/linux/uaccess.h
>>> +++ b/include/linux/uaccess.h
>>> @@ -398,6 +398,7 @@ long strnlen_user_nofault(const void __user *unsafe_addr, long count);
>>>    #define unsafe_put_user(x,p,e) unsafe_op_wrap(__put_user(x,p),e)
>>>    #define unsafe_copy_to_user(d,s,l,e) unsafe_op_wrap(__copy_to_user(d,s,l),e)
>>>    #define unsafe_copy_from_user(d,s,l,e) unsafe_op_wrap(__copy_from_user(d,s,l),e)
>>> +#define unsafe_clear_user(d, l, e) unsafe_op_wrap(__clear_user(d, l), e)
> 
> That doesn't help with architectures that define user_access_begin but
> do not define unsafe_clear_user. (i.e. x86).
> 

Yes, the day they want to use unsafe_copy_siginfo_to_user() they'll have to implement 
unsafe_clear_user().

Until that day, they don't need unsafe_clear_user() and I'm sure the result would be disastrous if a 
poor powerpc guy like me was trying to implement some low level x86 code.

Similar to unsafe_get_compat_sigset(), an arch wanting to use it has to implement 
unsafe_copy_from_user().