Date: Thu, 18 Jul 2019 00:07:00 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Alan Somers <asomers@freebsd.org> Cc: Bruce Evans <brde@optusnet.com.au>, John Baldwin <jhb@freebsd.org>, "Conrad E. Meyer" <cem@freebsd.org>, src-committers <src-committers@freebsd.org>, svn-src-all <svn-src-all@freebsd.org>, svn-src-head <svn-src-head@freebsd.org> Subject: Re: svn commit: r349391 - head/sys/kern Message-ID: <20190717235525.L1726@besplex.bde.org> In-Reply-To: <CAOtMX2gVQs7j%2BNnszvSeaEA5MTDLKS7%2BRz1W-xaq_DR=1VHgXg@mail.gmail.com> References: <201906251944.x5PJiNaA093352@repo.freebsd.org> <CAG6CVpUscojbGvA0=BmZPhSR%2B5Yt5ah8SQKrBaouUNDpu9tw9A@mail.gmail.com> <3b2ade4e-d19d-31e4-7ee2-c0085a00e53e@FreeBSD.org> <20190716180618.C962@besplex.bde.org> <CAOtMX2gVQs7j%2BNnszvSeaEA5MTDLKS7%2BRz1W-xaq_DR=1VHgXg@mail.gmail.com>
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On Tue, 16 Jul 2019, Alan Somers wrote:
> On Tue, Jul 16, 2019 at 3:48 AM Bruce Evans <brde@optusnet.com.au> wrote:
>>
>> On Mon, 15 Jul 2019, John Baldwin wrote:
>>> ...
>>> I'm not sure which variants are most readable:
>> ...
>>> C)
>>> if (fp->f_seqcount + howmany(resid, 16384) < fp->f_seqcount)
>>> fp->f_seqcount = IO_SEQMAX;
>>> else
>>> fp->f_seqcount = lmin(IO_SEQMAX,
>>> fp->f_seqcount + howmany(resid, 16384));
>>>
>>> I'm probably not a fan of C).
>>
>> On supported arches, it recovers from overflow in howmany(), but only if
>> the compiler doesn't optimize away the recovery.
>>
>> The first part can be done better:
>>
>> if (uio->uio_resid >= IO_SEQMAX * 16384)
>> fp->f_seqcount = IO_SEQMAX;
>>
>> Then for the second part, any version works since all values are small and
>> non-negative, but I prefer to restore the the version that I rewrote 15-20
>> years ago and committed 11 years ago (r175106):
>>
>> fp->f_seqcount += howmany(uio->uio_resid, 16384);
>> if (fp->f_seqcount > IO_SEQMAX)
>> fp->f_seqcount = IO_SEQMAX;
>> ...
> Bruce,
> Is this how you want it?
> Index: sys/kern/vfs_vnops.c
> ===================================================================
> --- sys/kern/vfs_vnops.c (revision 349977)
> +++ sys/kern/vfs_vnops.c (working copy)
> @@ -499,8 +499,13 @@
> * closely related to the best I/O size for real disks than
> * to any block size used by software.
> */
> - fp->f_seqcount += lmin(IO_SEQMAX,
> - howmany(uio->uio_resid, 16384));
> + if (uio->uio_resid >= IO_SEQMAX * 16384)
> + fp->f_seqcount = IO_SEQMAX;
> + else {
> + fp->f_seqcount += howmany(uio->uio_resid, 16384);
> + if (fp->f_seqcount > IO_SEQMAX)
> + fp->f_seqcount = IO_SEQMAX;
> + }
> return (fp->f_seqcount << IO_SEQSHIFT);
> }
That looks OK, except it is misformatted with tabs corrupted to other than
8 spaces.
I checked that uio_resid is checked by some callers to be >= 0, so we
don't have to check that here. (Callers between userland and here
start with size_t nbytes or an array of sizes for readv(), and have
to check that nbytes or the sum of the sizes fits in ssize_t, else
overflow would alread have occurred on assigment to uio_resid. Callers
that construct a uio that is not so directly controlled by userland
presumably don't construct ones with unusable sizes.)
Bruce
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