From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Viro Date: Wed, 23 Sep 2020 14:49:17 +0000 Subject: Re: [PATCH 3/9] iov_iter: refactor rw_copy_check_uvector and import_iovec Message-Id: <20200923144917.GM3421308@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit List-Id: References: <20200923060547.16903-1-hch@lst.de> <20200923060547.16903-4-hch@lst.de> <20200923141654.GJ3421308@ZenIV.linux.org.uk> <200cf2b9ce5e408f8838948fda7ce9a0@AcuMS.aculab.com> In-Reply-To: <200cf2b9ce5e408f8838948fda7ce9a0@AcuMS.aculab.com> To: David Laight Cc: Christoph Hellwig , Andrew Morton , Jens Axboe , Arnd Bergmann , David Howells , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "linux-mips@vger.kernel.org" , "linux-parisc@vger.kernel.org" , "linuxppc-dev@lists.ozlabs.org" , "linux-s390@vger.kernel.org" , "sparclinux@vger.kernel.org" , "linux-block@vger.kernel.org" , "linux-scsi@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-aio@kvack.org" , "io-uring@vger.kernel.org" , "linux-arch@vger.kernel.org" , "linux-mm@kvack.org" , "netdev@vger.kernel.org" , "keyrings@vger.kernel.org" , "linux-security-module@vger.kernel.org" , Linus Torvalds On Wed, Sep 23, 2020 at 02:38:24PM +0000, David Laight wrote: > From: Al Viro > > Sent: 23 September 2020 15:17 > > > > On Wed, Sep 23, 2020 at 08:05:41AM +0200, Christoph Hellwig wrote: > > > > > +struct iovec *iovec_from_user(const struct iovec __user *uvec, > > > + unsigned long nr_segs, unsigned long fast_segs, > > > > Hmm... For fast_segs unsigned long had always been ridiculous > > (4G struct iovec on caller stack frame?), but that got me wondering about > > nr_segs and I wish I'd thought of that when introducing import_iovec(). > > > > The thing is, import_iovec() takes unsigned int there. Which is fine > > (hell, the maximal value that can be accepted in 1024), except that > > we do pass unsigned long syscall argument to it in some places. > > It will make diddly-squit difference. > The parameters end up in registers on most calling conventions. > Plausibly you get an extra 'REX' byte on x86 for the 64bit value. > What you want to avoid is explicit sign/zero extension and value > masking after arithmetic. Don't tell me what I want; your telepathic abilities are consistently sucky. I am *NOT* talking about microoptimization here. I have described the behaviour change of syscall caused by commit 5 years ago. Which is generally considered a problem. Then I asked whether that behaviour change would fall under the "if nobody noticed, it's not a userland ABI breakage" exception. Could you show me the point where I have expressed concerns about the quality of amd64 code generated for that thing, before or after the change in question?