linux-kernel - Re: [RFC][PATCH] fix csum_and_copy_..._user() idiocy. Re: AW: [PATCH] amd64: Fix csum_partial_copy

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Date:   Tue, 24 Oct 2023 14:31:01 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-arch@...r.kernel.org, gus Gusenleitner Klaus <gus@...a.com>,
        Al Viro <viro@....linux.org.uk>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        "dsahern@...nel.org" <dsahern@...nel.org>,
        "kuba@...nel.org" <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [RFC][PATCH] fix csum_and_copy_..._user() idiocy.  Re: AW:
 [PATCH] amd64: Fix csum_partial_copy_generic()

On Tue, Oct 24 2023 at 05:26, Al Viro wrote:
> On Mon, Oct 23, 2023 at 12:37:58PM +0200, Thomas Gleixner wrote:
>> On Sun, Oct 22 2023 at 20:46, Al Viro wrote:
>> > -	return checksum;
>> > +	return from64to16 (checksum);
>> 
>>   from64to16(checksum); all over the place
>
> Umm...  Is that about whitespace?

Yes, my parser choked on that :)
  
>> >  /*
>> > - * We report fault by returning 0 csum - impossible in normal case, since
>> > - * we start with 0xffffffff for initial sum.
>> > + * We report fault by returning ~0ULL csum
>> >   */
>> 
>> There is also a stale comment a few lines further up.
>
> Umm...
>  *  Returns : r0:r1 = checksum:0 on success or -1:-1 on fault  
> perhaps?

Looks good.

>> > +static inline bool wsum_fault_check(__wsum_fault v)
>> > +{
>> > +#if defined(CONFIG_64BIT) || defined(__LITTLE_ENDIAN__)
>> > +	return (__force s64)v < 0;
>> > +#else
>> > +	return (int)(__force u32)v < 0;
>> 
>> Why not __force s32 right away?
>
> Mostly to keep the reader within more familiar cases
> of conversion - u64 to u32 is "throw the upper 32 bits
> away", u32 to s32 - "treat MSB as sign".

Fair enough.

> It's still a nasal demon country, of course - the proper
> solution is
>
> static inline bool wsum_fault_check(__wsum_fault v)
> {
> #if defined(CONFIG_64BIT) || defined(__LITTLE_ENDIAN__)
> 	return (__force u64)v & (1ULL << 63);
> #else
> 	return (__force u32)v & (1ULL << 31);
> #endif
> }
>
> Incidentally, in this case we really want a cast to u32
> rather than u64 - gcc is smart enough to figure out that
> checking MSB in 32bit can be done as signed 32bit comparison
> with 0, but bit 31 in 64bit is not special as far as it's
> concerned, even though it's a bit 31 of 32bit register...

Indeed.
  
>> As the callers just check for != 0 such a partial copy is considered
>> success, no?
>
> Check the callers...
>
> static __always_inline __must_check
> bool csum_and_copy_from_iter_full(void *addr, size_t bytes,
>                                   __wsum *csum, struct iov_iter *i)
> {
>         size_t copied = csum_and_copy_from_iter(addr, bytes, csum, i);
>         if (likely(copied == bytes))
>                 return true;

Duh. I think I stared at a caller of csum_and_copy_from_iter_full()
instead...

Thanks,

        tglx