linux-kernel - RE: [PATCH 3/3] rseq: optimise for 64bit arches

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Message-ID: <56c227c1defe4b9ca9c6a75537234e20@AcuMS.aculab.com>
Date:   Tue, 13 Apr 2021 15:06:00 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mathieu Desnoyers' <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     Eric Dumazet <eric.dumazet@...il.com>,
        Ingo Molnar <mingo@...nel.org>, paulmck <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Arjun Roy <arjunroy@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Eric Dumazet" <edumazet@...gle.com>
Subject: RE: [PATCH 3/3] rseq: optimise for 64bit arches

From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Sent: 13 April 2021 15:22
...
> >	David
> >
> >> So I suppose that if we're going to #ifdef this, we might as well do the
> >> whole thing.
> >>
> >> Mathieu; did I forget a reason why this cannot work?
> 
> The only difference it brings on 32-bit is that the truncation of high bits
> will be done before the following validation:
> 
>         if (!ptr) {
>                 memset(rseq_cs, 0, sizeof(*rseq_cs));
>                 return 0;
>         }
>         if (ptr >= TASK_SIZE)
>                 return -EINVAL;
> 
> The question is whether we really want to issue a segmentation fault if 32-bit
> user-space has set non-zero high bits, or if silently ignoring those high
> bits is acceptable.

Well, 32bit programs running on 64bit kernels better zero the 'padding'.

It is a shame that the 32bit compilers don't have an attribute(64bit)
for pointers.

> ...
> I am always reluctant to use long/unsigned long type as type for the get/put_user
> (x) argument, because it hides the cast deep within architecture-specific macros.
> I understand that in this specific case it happens that on 64-bit archs we end up
> casting a u64 to unsigned long (same size), and on 32-bit archs we end up casting a
> u32 to unsigned long (also same size), so there is no practical concern about type
> promotion and sign-extension, but I think it would be better to have something
> explicit, e.g.:

If the ABI passes small structures in registers (most modern ones)
then user pointers probably ought to be encapsulated in a structure.
Possibly as a pointer to an unknown structure.
That would force all the required type checking.
Unfortunately the fallout/churn would be massive.

> #ifdef CONFIG_64BIT
> static int rseq_get_cs_ptr(struct rseq_cs __user **uptrp, struct rseq_cs *rseq_cs)
> {
>     u64 ptr;
> 
>     if (get_user(ptr, &rseq_cs->ptr64))
>         return -EFAULT;
>     *ptrp = (struct rseq_cs __user *)ptr;
>     return 0;
> }
> #else
> static int rseq_get_cs_ptr(struct rseq_cs __user **uptrp, struct rseq_cs *rseq_cs)
> {
>     u32 ptr;
> 
>     if (get_user(ptr, &rseq_cs->ptr.ptr32))
>         return -EFAULT;
>     *ptrp = (struct rseq_cs __user *)ptr;
>     return 0;
> }
> #endif

Hmmm... too much replication.
You could do:
#ifdef CONFIG_64BIT
#define PTR_TYPE u64
#define PTR_FLD ptr64
#else
#define PTR_TYPE u32
#define PTR_FLD ptr32
#endif

Then have one copy of the code and the #undefs.
...

	David

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